The Gravitational Assist Maneuver

On October 7, 1959, the Soviet space probe Luna 3 passed over Asia and transmitted a handful of blurry black-and-white photos back to Earth. These were humanity's first glimpse of the Moon's far side (see below). The probe was launched on a circumlunar trajectory three days prior and its initial orbit about the Earth did not take it back on a path from which it could successfully relay its valuable data. However, Luna did not propel itself onto the necessary path. This groundbreaking probe was also the first to accomplish another feat: the gravitational assist.

Since 1959, spaceflight, especially interplanetary spaceflight, has relied upon gravity assists to reach destinations all over the Solar System. Apart from Luna 3, all other examples we will consider involve using gravity assists to change orbits around the Sun, rather than some other object (such as the Earth in the case of Luna 3). The basic principle is as follows: changing orbits around the Sun requires changing spacecraft velocity relative to the Sun, known as heliocentric velocity. Conventionally, this is done using thrusters aboard the craft itself (e.g. with chemical rockets; see ion propulsion for another example). However, onboard thrusters always require ejecting mass, and heavier rockets are much more difficult and costly to launch.

Gravity assists take advantage of planets' orbital velocity and gravitational influence to alter a spacecraft's heliocentric velocity. Take the diagram below, which shows a probe flying by the planet Jupiter.

Passing close to Jupiter puts our probe on a curved trajectory about the planet. Moreover, as it falls "downhill" into the gravity well of the giant planet, it picks up speed (as indicated by the longer arrows). This gain in speed is short-lived, however, because it loses kinetic energy climbing out of the well as it departs. As a result, though the velocity vector is pointed in a different direction than before, it seems we've made no progress in increasing our probe's speed.

But this is not true: all velocities in the above diagram are relative to Jupiter, i.e. measuring the rate at which an observer on Jupiter would see the spacecraft traveling. What matters for its orbit though, is heliocentric velocity.

Suppose that, with respect to the Sun, our planet has an initial orbital velocity toward the left. Then the total heliocentric speed can increase for our spacecraft if the velocity relative to the planet is rotated to line up more closely with the planetary velocity (as indicated by the addition of arrows in the above diagram). Two of the greatest outer Solar System missions in history relied on this principle: the Voyagers.

Both Voyager 1 and 2 made use of a gravity assist at Jupiter to accelerate them to Saturn, and Voyager 2 did the same to reach Uranus and Neptune. While a slight change in direction at each planet is visible in the above diagram, the following graph better captures Voyager 2's changes in speed.

The blue curve graphs the magnitude of Voyager 2's heliocentric velocity at various distances from the Sun along its journey. Note that at each flyby of a giant planet, the probe's speed sharply increased for a short time (as it plunged into the planet's gravity well) but also was higher after each flyby than before, with the exception of after its final encounter with Neptune. The other curve shown is the Solar System escape velocity, that is, the velocity required at a specified distance from the Sun to ultimately escape its gravitational influence. Remarkably, Voyager 2 did not even have sufficient velocity to escape the Solar System until after its first flyby with Jupiter! Without gravity assists, the spacecraft would not be headed toward interstellar space today.

As indicated by Voyager 2's encounter with Neptune, gravitational assists can also reduce a spacecraft's heliocentric velocity. This is necessary for certain missions to the inner Solar System; objects launched from the Earth do not simply fall toward the Sun - they have to lose the angular momentum they inherit from our own planet!

In October 2018, the European Space Agency spacecraft BepiColombo on a mission to orbit Mercury in 2025. See here for an animation of the probe's seven-year trajectory. If the probe had been launched directly toward Mercury, the additional velocity acquired from falling into the Sun's gravity well would have made orbit impossible. Instead, the mission trajectory incorporated two Venus flybys and six Mercury flybys, all to slow down the spacecraft without the use of too much thrust.

Finally, some space missions even use encounters with planets to leave the plane of the Solar System! The Ulysses spacecraft, launched in 1990, had the goal of studying the Sun. In particular, it aimed to measure the solar wind (the flow of charged particles) and magnetic field emanating from the Sun. Unlike previous missions, Ulysses had the opportunity to study the Sun from above its poles in an orbit that was inclined 80.2° to the plane of the Solar System. A vast majority of solar system missions remain in the nearly flat plane of the Sun's equator in which the planets lie.

To achieve this unusual orbit, the probe went all the way to Jupiter just to flyby the giant planet. Jupiter's large mass allowed for a more effective gravity assist and its distance from the Sun meant that the probe was traveling slower there and the maneuver required a smaller change in velocity. Ultimately, Ulysses made a great deal of new discoveries, including that the solar magnetic field "flips" every 11 years.

Faced with the difficulties of efficient Solar System navigation, numerous space missions have utilized creative solutions involving gravity assists to reach their targets, providing another example of the spectacular innovations necessary to explore worlds beyond our own.

Sources: https://solarsystem.nasa.gov/missions/luna-03/in-depth/, https://solarsystem.nasa.gov/basics/primer/, https://www.researchgate.net/publication/228803791_Design_of_Lunar_Gravity_Assist_for_the_BepiColombo_Mission_to_Mercury, https://medium.com/teamindus/daring-gravity-assist-maneuvers-of-past-space-missions-411643cd3d55, https://solarsystem.nasa.gov/missions/ulysses/in-depth/

OSIRIS-REx

OSIRIS-REx is a NASA sample return mission to the near-Earth asteroid 101955 Bennu. It aims to collect a sample from an asteroid whose composition could reveal a great deal about the beginning of the Solar System and the formation and evolution of the Earth. The first asteroid sample return mission was Hayabusa, developed by the Japanese Aerospace Exploration Agency (JAXA). This probe returned about 1,500 microscopic grains from the asteroid 25143 Itokawa. OSIRIS-REx, however, was designed to obtain at least 60 grams of material in the form of macroscopic samples. In addition, Bennu differs enormously from Itokawa in that it is carbonaceous while the latter is siliceous. Further, there is evidence that it is rich in organic and volatile compounds. Bennu is also of interest because its orbit takes it very close to Earth. It was measured to have a small cumulative probability of 0.037% of striking the Earth sometime in the 22nd century. This is due to the present uncertainty as to whether Bennu with pass through a gravitational "keyhole" in its 2135 flyby of Earth that would set it on a collision course. This mission will allow more precise predictions of its trajectory.

Bennu's orbit is slightly larger than Earth's but also more elliptical. As a result, it crosses inside Earth's orbit with every revolution.

The spacecrafts's name stands for Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer. This unwieldy acronym contains the four main goals of the mission: to return a sample to Earth that will elucidate the Solar System's origins, to map the asteroid with spectroscopy to learn about its composition and formation, to investigate whether near-Earth asteroids such as Bennu could provide materials as resources for human development, and to discover what impact threat Bennu poses, if any. The word "regolith" describes the layer of loose material at the surface of an asteroid, from which OSIRIS-REx will obtain a sample.

On September 8, 2016, the OSIRIS-REx mission began with a launch from Cape Canaveral. Over a year later, the probe performed a gravity assist at Earth that set it on course to the asteroid Bennu. On June 27, 2018, the sister mission Hayabusa 2 (JAXA's second asteroid sample return) arrived at 162173 Ryugu, another near-Earth asteroid. The techniques and targets of these two missions were selected to complement one another.

During the next few months, the spacecraft made its final approach to the asteroid. Along the way, it performed several asteroid approach maneuvers (AAM's) to lower its relative speed to Bennu. It was essential to achieve an incredibly low relative speed because the asteroid is so small: only 0.3 miles in diameter. OSIRIS-REx was the first spacecraft to ever attempt orbit about such a small object. Therefore, a series of four AAM's in October and November were performed to reduce the relative speed from 1,100 mph down to a minuscule 0.10 mph, about the speed of a sloth!

Meanwhile, the spacecraft had come quite close to its orbiting distance. On November 16, the above image was taken of Bennu from a distance of just 85 miles. On December 3, 2018, OSIRIS-REx "arrived" at Bennu, an occasion marked by the firing of thrusters that left the craft within 5 miles of the asteroid.

Shortly after arriving, the probe's spectrometers indicated that clays on its surface were hydrated, meaning that they had likely interacted with water in the distant past. While no liquid water exists on the surface of Bennu today, this discovery painted an intriguing picture of the asteroid's history. On December 31, another brief thrust put OSIRIS-REx in orbit of Bennu, making the latter the smallest object ever orbited by spacecraft. The gravitational pull of the asteroid is minuscule, only a few millionths of Earth's.

The next phase of the mission was a detailed survey of the asteroid's surface via several close flybys to guide the future sample acquisition. However, it did not take long before scientists discovered surprising features of Bennu that would make the next mission stages challenging. For one thing, the terrain was more rugged than expected; Bennu's surface is densely packed with boulders in many places, meaning that higher accuracy would be necessary for sample collection.

Further, in January 2019, OSIRIS-REx discovered periodic particle ejections from the surface, captured by long-exposure photography above (the contrast of this photo is boosted to capture the debris). Investigations concluded that these events did not pose a significant threat to the spacecraft. In June 2019, the probe entered a second orbit at an altitude of about 2230 feet (680 meters) to obtain even higher resolution images of possible sample collection sites. By August, all but 4 potential sites had been eliminated.

The above image shows the site that was ultimately selected, nicknamed "Nightingale". The final decision took place in December 2019. The size of the spacecraft is superimposed on the image for scale. The site was ultimately chosen because of the fine material and lack of large boulders, maximizing the safety of the sample collection. In addition, Nightingale was located in a crater estimated to be relatively young, giving OSIRIS-REx access to a pristine crust sampled not covered by the debris of eons.

In January 2020, a close flyover and more detailed survey of the Nightingale site was completed. The next month, the probe also surveyed a backup site, nicknamed "Osprey". After that, rehearsals began for the sample collection itself. In April there was a "Checkpoint Rehearsal" to practice the first few stages of the Bennu descent sequence. During the exercise, OSIRIS-REx came within 246 feet (75 meters) of Bennu, the closest it had come thus far. The final "Matchpoint Rehearsal" took place in August, where in addition to the maneuvers previously practiced, the probe also underwent a "Matchpoint Burn". This burn matched the spacecraft's orbital rate with that of the asteroid, positioning it over the landing site at only 131 feet (40 meters) off the ground. After each rehearsal was complete, OSIRIS-REx returned to the higher "home" orbit.

On October 20, 2020, the main event took place.

The above image is a schematic of the spacecraft's touch-and-go (TAG) collection maneuver (click for full size). In the roughly five hour sequence, the spacecraft broke its small orbit, extended its sample arm, and made its way to the sample site. When it reached Nightingale, it ejected pressurized nitrogen at the surface to disturb rocks, and then made contact for 10 seconds to collect its sample. See here for a series of images of the probe's approach and sample collection. About a week later, after the mission team confirmed the successful collection, they went forward with the sample stowing. This involved transferring the sample to the sample return capsule (SRC) and then closing and latching it. The SRC had to be secure enough to withstand reentry in the Earth's atmosphere.

After leaving the asteroid in 2021, the spacecraft will then return the sample to Earth in 2023.

Sources: http://www.asteroidmission.org, http://www.space.com/33616-asteroid-bennu-will-not-destroy-earth.html, http://global.jaxa.jp/press/2010/11/20101116_hayabusa_e.html, http://science.nasa.gov/media/medialibrary/2012/05/04/OSIRIS-REx_--_Jason_Dworkin.pdf, http://science.nasa.gov/media/medialibrary/2012/05/04/OSIRIS-REx_--_Jason_Dworkin.pdf, https://www.asteroidmission.org/?latest-news=two-pieces-cosmic-puzzle-hayabusa2-osiris-rex, https://www.asteroidmission.org/?latest-news=nasas-osiris-rex-executes-fourth-asteroid-approach-maneuver, https://www.asteroidmission.org/?latest-news=nasas-newly-arrived-osiris-rex-spacecraft-already-discovers-water-asteroid, https://www.asteroidmission.org/?latest-news=nasa-mission-reveals-asteroid-big-surprises, https://www.asteroidmission.org/?latest-news=nasas-osiris-rex-successfully-stows-sample-of-asteroid-bennu https://www.asteroidmission.org/?latest-news=one-step-closer-to-touching-asteroid-bennu

ExoMars Mission

ExoMars, or Exobiology on Mars, is a mission jointly run by the European Space Agency (ESA) and the Russian Federal Space Agency (Roscosmos) to investigate possible traces of life on the planet Mars. The mission includes two launches: one in 2016 and one in 2020, with the first delivering an orbiter and a lander to Mars and the second the ExoMars rover.

The first launch took place on March 14, 2016 in Kazakhstan using a Russian-built launch vehicle. Both the Trace Gas Orbiter (TGO) and the Entry, Descent, and Landing Demonstrator Module (EDM) arrived in the Martian system in October 2016.

On October 16, 2016, the two components separated as planned, with the TGO performing a maneuver shortly after to remain in orbit. The primary mission of the TGO, as the name suggests, was to refine our measurements of the scarcer components of the Martian atmosphere, including methane and water vapor. From an orbit about 250 miles above the surface of the red planet, the orbiter was positioned to obtain information orders of magnitude more accurate than any previous results. Methane in particular is generated by specific geological and organic processes. While the Trace Gas Orbiter could not identify the cause of gaseous emissions by itself, it did have the capability to pinpoint the sources geographically, aiding in the selection of the ExoMars rover landing site. The orbiter itself was constructed by the ESA while the Russian agency contributed several of its instruments.

Meanwhile the EDM lander (also called Schiaparelli after the Italian astronomer Giovanni Schiaparelli) was built to demonstrate crucial techniques for landing on the Martian surface shortly after the first spacecraft arrives at Mars. Weighing over 1300 pounds, the lander required a controlled landing to reach the Martian surface safely, just like the Curiosity rover.

The probe used a heat shield and parachutes to slow its descent and a liquid propulsion braking system to control its final touchdown on Mars. However, an error in the autonomous landing system led to the parachutes being deployed too early, when the lander was still several kilometers above the surface. As a result, the lander was torn to pieces, and did not function after impact. These components, along with the lander itself, were captured in an image on October 25 by NASA's Mars Reconnaissance Orbiter. While Schiaparelli was unable to survive landing, it still provided valuable data to guide future missions.

The orbiter's mission proved more successful. After spending several months in its initial highly elliptical orbit and calibrating its science equipment, a long period of aerobraking began in March 2017. This process involved using the friction of the Martian atmosphere at the spacecraft's closest approaches to take momentum away from the spacecraft and gradually lower the furthest point on the orbit from tens of thousands of miles away from the planet to just 250 miles. Due to the tenuousness of the Martian atmosphere, this was a very slow and delicate process, and was not completed until February 2018. The probe began its science mission in April.

The above image is one of the first taken by the ExoMars Trace Gas Orbiter. It shows Korolev crater, a location in Mars's north polar region. The bright material on the crater's rim is ice.

In November 2018, the Oxia Planum region on Mars was selected as the landing site for the ExoMars 2020 rover. The landing site was chosen to lie at low altitude to maximize the amount of Martian atmosphere available for slowing the rover's descent via parachute. It is a geologically rich region lying near the equator where liquid water likely existed billions of years ago. Oxia Planum was also chosen for its flat and easily navigable terrain. Below is a black and white image of the region.

Meanwhile, after a year of gathering data, the first results from the TGO were released in April 2019. Shortly after beginning its science mission, the orbiter had observed a major dust storm on the planet's surface and measured its effect on the distribution of water vapor and "semi-heavy water vapor" (in which one of the two hydrogen atoms in H₂O is the deuterium isotope with one neutron and one proton) in the atmosphere.

The image above (click to enlarge) shows that dust storms increase the atmospheric content of both types of water vapor at many altitudes. These concentrations were measured using solar occultation, in which the orbiter analyzed sunlight shining through the Martian atmosphere. The way that sunlight is scattered/absorbed reveals what gases are present. Other initial results included a surprising lack of methane in the atmosphere that disagreed with some previous measurements and a higher resolution mapping of where water-rich minerals are present on the Martian surface.

The second launch will occur sometime in 2020 (it was moved back from its original 2018 launch in 2017), carrying the European-built ExoMars rover and a surface platform on which it will land, contributed by Roscosmos. The spacecraft will arrive at Mars in early 2020 at a landing site chosen with help from the 2016 mission's data. The same technology demonstrated in the first landing will allow the second module to perform a soft touchdown on the surface of Mars. After landing, the surface platform will deploy ramps, off of which the rover will exit to begin its exploration of the surface.

The rover's mission will last at least six months. Its primary mission will be to search for organic substances on the Martian surface. Since the harsh conditions of the surface may have obliterated traces of chemicals, the ExoMars rover will have the ability to bore holes as deep as two meters to obtain better preserved samples. After collecting samples, the rover will transfer them to its onboard laboratory for chemical analysis. With its careful site selection and dedicated exobiology instruments, the ExoMars mission has perhaps the best opportunity yet of discovering definitive biosignatures on Mars. It also would accomplish the technological objective of honing the ability to make soft, precision landings on the red planet. Finally, the mission paves the way for the holy grail of Martian exploration: returning a sample from the red planet back to Earth.

Sources: http://exploration.esa.int/mars/, http://exploration.esa.int/mars/47852-entry-descent-and-landing-demonstrator-module/, http://exploration.esa.int/mars/58557-schiaparelli-crash-site-in-colour/, http://exploration.esa.int/mars/58888-exomars-science-checkout-completed-and-aerobraking-begins/, http://exploration.esa.int/mars/59184-schiaparelli-landing-investigation-completed/, http://exploration.esa.int/mars/60235-exomars-images-korolev-crater/, http://exploration.esa.int/mars/60914-oxia-planum-favoured-for-exomars-surface-mission/, http://www.esa.int/Science_Exploration/Human_and_Robotic_Exploration/Exploration/ExoMars/First_results_from_the_ExoMars_Trace_Gas_Orbiter

Applications of Ion Propulsion

This is the second part of a two-part post. The first post, describing the function of ion propulsion engines, may be found here.

Ion thrusters have many advantages over other forms of propulsion. In comparison with traditional chemical propellent, they are roughly 10 times more efficient, lightening the load of space-traveling craft and saving massive amounts of fuel for launch. This efficiency originates in part from the higher exhaust velocity of the xenon propellant particles, which are ejected from the spacecraft at speeds of 20-50 km/s! In addition, the electric power required to run ion engines is relatively small, on the order of a few kilowatts. In comparison, a typical microwave oven consumes 1.1 kW, and the power consumption is significantly less than that of a typical automobile. Solar panels can meet this power demand during flight, allowing ion thrusters to create smooth and continuous acceleration over the entire duration of a mission.

However, this efficiency comes at a price: ion propulsion produces very small thrusts. The first model of ion engine actually used in spaceflight, the NSTAR engine, produced a thrust of around 90 millinewtons. This is the same force that your hand would experience from a single piece of paper on Earth by gravity, a barely detectable force! However, this minute force can operate continuously, adding up to a significant acceleration over time in the frictionless environment of space. In comparison, space probes which operate on chemical propellant may exert thrusts in the hundreds or thousands of newtons (the equivalent of a couple hundred pounds at Earth's surface), but only for very short times.

The small thrust of ion engines pales even more in comparison to that of rockets that launch payloads from Earth's surface. To escape Earth's gravity, such rockets must exert a force greater than the force of gravity on the often huge rockets. For example, the Saturn V rocket (shown above) that launched humans to the Moon generated an astounding 34,500,000 newtons of thrust at launch. For this reason, ion engines cannot be used to launch spacecraft.

The idea of electric rock propulsion dates to the 1930's and the first test of ion engines in space came during the 1960's. However, no operational mission utilized ion thrusters until NASA's Deep Space 1 probe, launched in 1998. This spacecraft performed a flyby of the asteroid 9989 Braille and the comet 19P/Borrelly. To meet the acceleration requirements of the mission extension to comet Borrelly, Deep Space 1 changed its velocity by 4.3 km/s using less than 74 kilograms of xenon. The Hayabusa probe, launched by the Japan Aerospace Exploration Agency (JAXA) in 2003, was another important demonstration of ion thruster technology. This probe used ion thrusters on its mission to land on the near-Earth asteroid 25143 Itokawa, collect samples, and return them to Earth for analysis. In 2010, the probe successfully returned the sample to Earth, completing the first ever asteroid sample return mission. Hayabusa 2, another asteroid sample return mission, launched in 2014 with similar objectives.

An even greater demonstration of ion propulsion technology began in 2007 with the launch of the Dawn spacecraft. This spacecraft carried three ion engines, operating alternately throughout the mission. Thrusting frequently throughout its eight-year journey, Dawn followed a spiral outward from the Earth, past Mars, into orbit of the asteroid Vesta, and subsequently into orbit of the asteroid Ceres.

The above image shows Dawn's outward spiral as well as the intervals during which one of its ion engines was in operation. With the exception of the gravitational assist at Mars, Dawn thrusted almost continuously, moving outward under its own power. Its total velocity change exceeded 10 km/s, the greatest yet for a spacecraft under its own power. When it successfully entered orbit around Ceres in April 2015, Dawn became the first spacecraft to orbit two different extraterrestrial targets. This feat would not have been possible using traditional methods of chemical propulsion.

Meanwhile, advances continued in ion thruster technology. NASA's Evolutionary Xenon Thruster (NEXT) performed a 48000 hour test in a vacuum chamber lasting from 2004 to 2009 to demonstrate its successful operation. Using power at a higher rate but also providing somewhat greater thrust, the NEXT engine (shown above) is at least 30% more efficient than its predecessors and can operate for a longer time. Continued development of ion propulsion technology promises to provide the foundation necessary for more ambitious interplanetary space missions.

Sources: http://www.extremetech.com/extreme/144296-nasas-next-ion-drive-breaks-world-record-will-eventually-power-interplanetary-missions, http://www.nasa.gov/centers/glenn/about/fs08grc.html, https://www.nasa.gov/audience/foreducators/rocketry/home/what-was-the-saturn-v-58.html#.VV8XomCprzI, http://darts.isas.jaxa.jp/planet/project/hayabusa/index.htmlhttp://science.nasa.gov/science-news/science-at-nasa/1999/prop06apr99_2/, http://alfven.princeton.edu/papers/sciam2009.pdf, http://dawn.jpl.nasa.gov/mission/

MAVEN

MAVEN (Mars Atmosphere and Volatile EvolutioN) is a NASA space mission primarily focused on studying the Martian atmosphere. Previous Mars missions have already given us hints into Mars's past, including numerous signs of past liquid water and various geographical and chemical signatures of a planet that was habitable many, many years ago. MAVEN will approach the problem of elucidating Mars's past from a different angle, by examination of its upper atmosphere.

Before MAVEN, our hypothesis as to the demise of Mars's habitability was as follows: several billion years ago, when the Solar System and its planets were still young, Mars had, like Earth, a molten core.

Earth's molten core consists of metals such as iron that conduct electricity. The circulation of these metals throughout the core, induced by the rotation of the Earth, generates a magnetic field (as in the illustration above). This magnetic field, in turn, protects us from the charged solar wind by deflecting it towards the poles. Mars is theorized to have had a similar core and magnetic field. However, being farther from the Sun and smaller, Mars's core cooled over time, and eventually solidified. The planet's magnetic field then weakened, and solar wind blew away a majority of Mars's atmosphere, leaving it with a rarefied layer of mostly carbon dioxide. Such a thin atmosphere would have been inhospitable for liquid water, and thus any bodies of water dried up, likely killing any life, if it had developed there.

MAVEN's observations did not alter this basic understanding, but will provided much more detailed information. For example, by observing how solar wind interacts with Mars's current atmosphere, the spacecraft provided data concerning the exact mechanisms involved in atmospheric loss (see below). To this end, MAVEN carried several instruments that measure solar wind and its impact on Mars. Also, by observing the current rate of gas loss, the mission allowed scientists to extrapolate backward and infer a more precise timeline of Mars's climate evolution. In order to accomplish this goal, MAVEN included instruments that detect gaseous ions escaping from the Martian atmosphere. These are the "volatiles" to which the name refers. Finally, the probe also had a device known as a mass spectrometer, which can measure the abundance of different isotopes of certain chemical elements. Since heavier isotopes are less likely to be ejected from Mars simply due to their slightly greater mass, the ratio of isotope abundance in Mars's atmosphere versus that in Earth's, for example, can indicate exactly how much has been lost and thus how dense the atmosphere was in the past.

Artist's rendering of MAVEN

MAVEN launched on November 18, 2013. After about a ten month cruise, the probe entered orbit of Mars on September 21, 2014. A few weeks after, the spacecraft assumed its science orbit, a highly elliptical orbit. The low point of its orbit brings MAVEN within 100 miles of the surface, allowing it to easily sample the atmosphere, and the high point brings it to more than 3000 miles from Mars's surface, so that the spacecraft can also take global observations.

Late in 2014, MAVEN discovered a mechanism by which ions in the solar wind penetrated deeper into the atmosphere than previously thought possible. Also, to extend its range further, MAVEN periodically dipped its orbit even farther down toward the surface, reaching a minimum altitude of 78 miles instead of 93 miles. The first of these "deep-dip" campaigns took place in February 2015.

During March, MAVEN encountered multiple unexpected atmospheric phenomena. The first was the presence of an aurora in ultraviolet wavelengths at a relatively low altitude over Mars's northern hemisphere (the geographic locations are indicated in the image above). Also, the orbiter detected a dust cloud between 93 and 190 miles above the surface of Mars, which could not be explained by any atmospheric mechanism known at the time. On April 3, MAVEN completed its 1000th orbit of Mars.

MAVEN achieved one of the basic goals of its mission in November 2015; in that month, NASA announced that the solar wind rips gas away from the Martian atmosphere at a rate of about 100 grams per second. This rate varies significantly with solar activity and is believed to have been greater billions of years ago. This result, when combined with other Mars missions, finally allowed a comprehensive understanding of Mars's loss of carbon dioxide (the main component of the Martian atmosphere).

Mars used to have a great deal of carbon dioxide in a thick atmosphere, far more than in the rarefied blanket of gas surrounding the red planet today. Some of this carbon dioxide was trapped in mineral carbonates (as shown) or is cycled between the atmosphere and ice caps as they melt and refreeze. In addition, MAVEN's atmospheric loss measurement shows how a process called "sputtering" allows some gas to escape. However, orbital spacecraft indicate that the amount trapped in carbonates is not enough to explain the loss. Further, while the "sputtering" process favors the escape of the heavier isotope carbon 13 over carbon 12, the preference is only slight: this process alone cannot explain the higher isotope ratio measured on the ground by the Curiosity rover. Instead, the discoveries of MAVEN suggest that another mechanism is primarily responsible, namely the interaction of solar ultraviolet radiation (denoted "hv" in the image) with carbon dioxide molecules that causes dissociation. After this process, carbon 12 has a much higher chance of escaping the atmosphere than carbon 13, explaining the observations.

In October 2016, after observing Mars for more than a total Martian year, MAVEN had a fairly fleshed out account of how water escapes Mars. More precisely, it measured escaping hydrogen in the upper atmosphere that resulted from disassociation of water vapor (H₂O) in the lower atmosphere. Before MAVEN, it was believed that this loss of water vapor occurred at a relatively constant rate. However, the spacecraft found that the rate of hydrogen escape is larger by a factor of 10 between when Mars is closest to the Sun compared to when it is farthest. This suggested that the amount of water vapor in the Martian atmosphere varies significantly throughout its year and helped to elucidate the process of its escape.

It was suspected that the magnetic field environment surrounding Mars may be a complex hybrid of two main paradigms found elsewhere in the Solar System. The first is exemplified by the Earth: our planet generates its own magnetic field through internal dynamics. Venus has no field whatsoever of its own, representing the second type. Mars had a magnetic field billions of years ago, and parts of the surface still give off small magnetic fields. In October 2017, it was announced that MAVEN's data confirmed this hypothesis, revealing also an unexpectedly complex "tail" on the magnetic field where solar wind interacted with surface fields.

In 2018, MAVEN discovered a new type of aurora on Mars that is uncommon on Earth. Aurorae are caused by energetic charged particles emitted from the Sun hitting the atmosphere. On Earth, it is typically fast-moving electrons in the solar wind that are responsible for these phenomena. However, MAVEN's ultraviolet and solar wind instruments suggested that solar protons also cause ultraviolet aurorae on Mars. Researchers were faced with a mystery as to how the protons passed through the magnetic "bow shock" to enter the atmosphere. Further study concluded that these protons joined up with electrons to form neutral hydrogen before passing through the bow shock and re-ionizing (as indicated by the graphic above).

Early in 2019, MAVEN performed a braking maneuver to shrink its orbit around the red planet. This was done not for the probe's own mission, but to allow it to serve as a better relay satellite for the planned Mars 2020 rover launching the following year. During the adjustment, the orbit was shrunk from 3,850 to 2,800 miles (6,200 to 4,500 kilometers).

During the summer of 2020, MAVEN data revealed a curious phenomenon. Portions of the Martian atmosphere glow, exactly three times per night, in ultraviolet radiation. Moreover, this only occurs in spring and fall seasons. The pulsing glow is invisible to the naked eye since it is entirely ultraviolet, but is comparable in brightness to Earth's aurorae. It is postulated that the cause is waves in the middle Martian atmosphere. As with sound waves on Earth, these waves consist of alternating regions of denser and more rarified gas. The mixing of gases and higher pressure in the dense regions sets off chemical reactions, generating the glow. MAVEN was also able to observe how these waves interacted with Mars's tall mountains.

Sources: http://www.nasa.gov/mission_pages/maven/main/index.html#.Uf03vZIsmG0, http://www.nasa.gov/sites/default/files/files/MAVENFactSheet_Final20130610.pdf, http://lasp.colorado.edu/home/maven/science/, http://www.ipgp.fr/~aubert/Julien_Aubert,_Geodynamo,_IPG_Paris/Home.html, http://mars.nasa.gov/news/whatsnew/index.cfm?FuseAction=ShowNews&NewsID=1869, https://mars.nasa.gov/news/8282/nasas-maven-mission-finds-mars-has-a-twisted-tail/, https://www.nasa.gov/press-release/goddard/2018/mars-proton-aurora, https://mars.nasa.gov/news/8410/nasas-maven-spacecraft-shrinking-its-mars-orbit-to-prepare-for-mars-2020-rover/, https://mars.nasa.gov/news/8731/nasas-maven-observes-martian-night-sky-pulsing-in-ultraviolet-light/

Mars Science Laboratory

Mars Science Laboratory (MSL) is a NASA mission to Mars, whose goal is to search for organic material and microbial life on the Martian surface. To do this, the mission landed the rover Curiosity on Mars, whence it will navigate the surface and conduct scientific experiments.



The spacecraft successfully launched from Cape Canaveral, Florida on November 26, 2011. After leaving the Earth's atmosphere, it began a cruise stage that lasted until MSL approaches Mars in the summer of 2012. It successfully landed on the red planet on August 6, 2012.

This mission makes use of innovative technologies essential for any future landings on Mars. At over 2000 pounds, Curiosity is by far the largest Mars rover ever to be constructed, five times the size of the rovers Spirit and Opportunity of the early 2000's. In order to land intact on the Martian surface, a precision landing was necessary.

Previous rovers used inflatable airbags to cushion their landings, and simply bounced until settling to their destination. This did not allow high precision in landing. However, Curiosity is too large for such techniques, and made use of a more complicated landing sequence.



The landing procedure that was used to lower the Curiosity rover to the surface (click to enlarge). In the atmosphere, parachutes and braking thrusts were used to decelerate the craft. Then, a device known as a  sky crane lowered the rover to the ground on cables to ensure proper orientation.  Once Curiosity was safely on the surface, the crane detached and propelled itself away, as to not interfere with the rover.

After the landing, the first images sent back from Curiosity confirmed its position.



One of the first images from Curiosity showing the Martian surface. The venue from which the rover explored its environment was Gale Crater. This crater was selected due to the exposed sediment along its banks, which hold millions of years of Martian geologic history.

Over the next few days, the rover, remaining stationary, tested its scientific equipment, checking all of its instruments and cameras in preparation for its first motorized movement on the surface of Mars, which occurred on August 29.

On August 19, Curiosity performed its first sample analysis, using its on-board laser and spectral analyzer to determine its composition. In the following months, the rover conducted numerous experiments involving samples, both of soil and of atmosphere. In early November 2012, the atmosphere was found to contain an unusual concentration of heavy isotopes of its constituent elements (mainly carbon and oxygen, forming CO₂. This indicates that the lighter isotopes were lost to space in the distant past, and this could explain the thinness of the Martian atmosphere.

During the first few months of the mission, the rover also took weather data, identifying some meteorological events on Mars. Most changes were related to dust storms and whirlwinds, and in fact the dust was discovered to catalyze a convective process in the Martian atmosphere: the dust on the side of Mars facing the Sun is lifted by wind into the atmosphere where it warms it, causing a greater differential in temperature between one half of the Martian atmosphere and the other (where it is night). This causes a flow of air from the cool to the warm side, restarting the process.

In January 2013, Curiosity imaged rocks at night, illuminating them with lights on the spacecraft. Using ultraviolet lamps, the rover searched for fluorescent minerals during the Martian night, at which time they would be visible (see image below).



In early February 2013, the rover completed its first drilling, obtaining a sample from several inches below the surface of bedrock. After obtaining a good number of rock samples and performing numerous other tests at Gale Crater, the rover prepared in June to begin a journey to its next destination, Mount Sharp. By the time Curiosity had spent one year on Mars in August 2013, it had already transmitted over 70,000 images, and had traveled about a mile along Mars's surface.

In October 2013, Curiosity performed a detailed analysis of the argon present in the Martian atmosphere, identifying the abundance of different isotopes. From this result, scientists deduced that some meteorites on Earth do indeed have their origin on Mars. Also, the prevalence (relative to on Earth) of a heavier isotope of argon suggests that Mars did undergo massive atmospheric loss earlier in its history. Early that December, scientists, using data collected by Curiosity announced that the rover had discovered evidence of an ancient lake (which last existed about 3.7 billion years ago) in Gale crater. Remarkably, this lake would have had very low salinity, and so, unlike most other similar findings, would have been nearly freshwater. This is significant as freshwater lakes could have supported a wider variety of life.

In March 2014, Curiosity began moving toward an interesting geological site known as "the Kimberley" (shown above). This gave the rover its first opportunity to conduct geological investigations of the several sandstone varieties (rather than mudstone, which it had been previously examining) to be found at the new site. It arrived at the site at the beginning of April and began analysis, including a new drilling in May.

On June 24, 2014, Curiosity completed scheduled its primary mission of one Martian year (687 Earth days). During this time, Curiosity systematically investigated the soil composition, radiation exposure, and abundance of organic molecules. At the time of the primary mission's completion, the evidence gathered by the rover was enough to confirm that the environment of Mars had once been favorable for "supporting microbial life".

Later in June, the rover moved out of its landing area into new terrain. It ultimately arrived at the base of Aeolis Mons (also called Mount Sharp), the mountain at the center of Gale Crater, in August. Exploration of the mountain is the primary goal for MSL's first mission extension. The 5.5 km (18,000 ft) high mountain was captured in the image below, taken by the rover:

Data gathered on the lower slopes of Mount Sharp in late 2014 included a series of sediment deposits which indicated the presence of a large lake at Gale Crater early in Mars's history that lasted tens of millions of years or more. This was the first major evidence for such a long-lasting, stable body of water on the red planet.

By early 2015, Curiosity had moved out of the bottom 33 feet of altitude of Mount Sharp and had entered a region with prominent mineral veins (as shown in the image above taken on March 18, 2015, which includes a scale). Such mineral veins forms when fluids move through cracks in existing rock and leave deposits. The light and dark minerals indicate a variety of fluid compositions.

In April 2015, the rover made another exciting discovery. Data from Curiosity indicated that water vapor condenses into liquid water in the Martian soil every night and reevaporates in the morning. Even though the Martian night temperatures are well below the normal freezing point of water (they may drop to -100°F), perchlorate salts in the soil reduce the freezing point of water (just as salting roads prevents them from freezing) enough that liquid water can form. This unexpected discovery suggests that a great deal more water could exist on Mars than previously thought.

The rover spent several months investigating sand dunes as it continued its journey, learning a great deal concerning the wind patterns on the planet's surface through the inspection of sand dune "ripples" (see below). The appearance of the sand dunes was comparable in appearance to those found on Earth.

After the sand dune investigation, Curiosity crossed the Naukluft Plateau towards the upward slopes of the mountain. This journey encompassed much of the first half of 2016, during which time the rover analyzed a few additional rock samples. During its investigation of Mount Sharp, Curiosity aimed to determine what geological environments are most suitable for the preservation of organic compounds and to identify geological layers and transitions. As well as being informative in their own right, these new objectives will guide future Mars missions.

On October 1, 2016, a second extension of two years to the mission began, allowing the rover to travel further up Mount Sharp. Later that month, Curiosity made an interesting discovery: a golf-ball sized meteorite on the Martian surface.

Laser spectrometry of this darkly colored rock indicated that it was primarily composed of iron, along with some nickel and phosphorus. This type of meteorite is usually formed from the core of asteroids. Further, the study of Mars meteorites allows the comparison between its population of impacting bodies and Earth's, revealing a great deal about how the inner Solar System evolved over time.

Early in 2017, an analysis of Curiosity data brought a curious paradox into focus by not making a particular expected discovery. While investigating what is believed to be an ancient lake floor, the rover did not discover significant carbonate minerals. It was expected that in Mars's early days, an atmosphere with more carbon dioxide allowed the trapping of heat by the greenhouse effect to melt ice into liquid lakes. Were this the case, some of the CO₂ would have dissolved in water, resulting in carbonates. However, no such discovery was made. This led scientists to seek alternate mechanisms for the presence of liquid water on early Mars.

In June 2017, a comprehensive study of Curiosity's findings at Gale Crater were assembled into a more complete picture of the ancient lake that existed there billions of years ago.

Of particular interest is the stratification of this lake, suggested by the differing mineral compositions of the "shallow" and "deep" walls of the crater. The stratification of lakes in this way is a phenomenon seen on Earth, and may have been conducive to different types of microbial life.

The next month, the rover began its investigation of Vera Rubin ridge, a geologic layer of Mount Sharp particularly rich in the iron oxide mineral hematite. The above image shows the location of this ridge as well as the rover's July 2017 position. Since hematite can form under wet conditions, analyzing the ridge revealed clues to Mars's ancient environments. Drilling at this site was difficult for Curiosity because an important part of the drill used to stabilize it as it pulverized rock stopped functioning back in December 2016. On the ridge, engineers experimented with a new drilling technique that did not require the stabilizers. This yielded results by early 2018, when the rover was able to obtain new samples.

In March 2019, Curiosity had the opportunity to capture Mars's small moons, Phobos and Deimos, as they passed in front of the Sun. This event is analogous to a solar eclipse on Earth, but the small size of Phobos and Deimos means that they cannot entirely block the Sun's disk. Therefore, they are known as transit events. The above animation shows a series of photographs of the Phobos transit. The photos are fascinating in their own right, but also help to refine our knowledge of the moons' orbits.

For years, the rover had made regular measurements of the concentrations of different gases in the Martian atmosphere in Gale Crater. As was known from previous missions, Mars's atmosphere is primarily (95%) CO₂, with trace amounts of nitrogen, argon, and oxygen gas. By analyzing variations over several revolutions around the Sun, it uncovered a seasonal pattern: the amount of nitrogen and argon fluctuated throughout the Martian year. These variations matched scientists' predictions, as they occurred in response to large amounts of carbon dioxide being frozen and unfrozen in the polar ice caps each year. However, the concentration of oxygen defied this pattern.

The above chart shows the expected seasonal variation in oxygen concentration as well as the observations of Curiosity at Gale Crater. It reveals an unexpected rise in oxygen during late spring and early summer, and an equally unexpected decline in winter. This behavior suggests a chemical process involving Martian soil, but it is so far unexplained.

In the summer of 2020, Curiosity left the "clay-bearing unit," the sedimentary layer it had been exploring for more than a year, and began to ascend even further to the "sulfate-bearing unit" above. Characterized by sulfate minerals such as gypsum, the rocks in this layer were likely formed through evaporation. To reach this new layer, however, the rover had to navigate around a mile-wide patch of sand. The composite image above (made from 116 individual photos) helped it to chart a course.

Sources: Mars Science Laboratory, Wikipedia, http://marsprogram.jpl.nasa.gov/msl/, http://mars.jpl.nasa.gov/msl/, http://www.npr.org/blogs/thetwo-way/2013/12/09/249760330/curiosity-finds-evidence-of-ancient-fresh-water-lake-on-mars?utm_content=socialflow&utm_campaign=nprfacebook&utm_source=npr&utm_medium=facebook, http://upload.wikimedia.org/wikipedia/commons/6/65/673885main_PIA15986-full_full.jpg, http://mars.nasa.gov/files/msl/2014-MSL-extended-mission-plan.pdf, http://www.nasa.gov/press/2014/december/nasa-s-curiosity-rover-finds-clues-to-how-water-helped-shape-martian-landscape/#.VItLjov4tFL, http://www.vox.com/2015/4/13/8384337/mars-water-liquid-curiosity, http://mars.nasa.gov/msl/news/whatsnew/index.cfm?FuseAction=ShowNews&NewsID=1912, http://mars.nasa.gov/news/2016/mars-rock-ingredient-stew-seen-as-plus-for-habitability&s=2, https://mars.nasa.gov/news/2017/nasas-curiosity-rover-sharpens-paradox-of-ancient-mars&s=2, https://mars.nasa.gov/news/curiosity-peels-back-layers-on-ancient-martian-lake/, https://mars.nasa.gov/news/curiosity-mars-rover-begins-study-of-ridge-destination/, https://mars.nasa.gov/news/8425/curiosity-captured-two-solar-eclipses-on-mars/, https://mars.nasa.gov/news/8704/curiosity-mars-rovers-summer-road-trip-has-begun/?site=msl

Juno

Juno is a NASA spacecraft whose mission is to orbit Jupiter and gain further insight to its composition and formation. It is named for the goddess Juno, wife of Jupiter in Roman mythology.

The spacecraft launched on August 5, 2011 to start its six year mission, culminating in a Jupiter arrival in 2016. The probe's trajectory included a flyby of Earth designed to conserve fuel. Unlike previous missions to the outer Solar System, Juno's energy will come only from solar panels, despite the relative dimness of the Sun at Jupiter's orbit.

Juno's trajectory from launch in 2011 to arrival at Jupiter in 2016.

In 2012, Juno executed several deep-space-maneuvers that prepared the probe for its flyby of Earth. Next, in October 2013, Juno completed its Earth flyby, assuming a trajectory directly toward Jupiter.

On July 4, 2016, the spacecraft executed an engine burn that inserted it into orbit around Jupiter. The probe assumed a highly elliptical orbit that took it past the north and south poles of Jupiter with every revolution.

The image above shows Juno's orbits around Jupiter over time, beginning with the orbital insertion on July 4.

This image, Juno's first acquired from orbit, shows the gas giant as well as three of the four Galilean moons, Io, Europa, and Ganymede (from left to right).

After its initial insertion burn, the Juno spacecraft spent over two months completing an elongated orbit that took it far away from the Solar System's largest planet. The first of 37 science flyby took place on August 27 and brought Juno over the north pole of Jupiter, capturing the first ever image of this polar region (see below).



The polar region is very different in appearance than the midlatitudes and equatorial region of Jupiter. The latter regions have characteristic colored bands of red, white, and orange, as well as prominent storm features. The poles are bluer, and lack these storm features. Juno's initial orbit was 53.4 days in duration. At its second closest approach to Jupiter on October 19, a maneuver was planned that would reduce the orbit to 14 days. However, the spacecraft entered safe mode just before the flyby when the onboard computer found conditions to be awry and neither data collection nor orbital maneuvering occurred on the 19th. Juno was later found to be functioning normally.

After two more successful flybys on December 11, 2016 and February 2, 2017, mission directors decided to not risk the reduction maneuver and maintain Juno in its 53-day orbit indefinitely. The main impediment to the function of the probe was the radiation belts near Jupiter's poles, which would gradually deteriorate Juno's functioning with every flyby. Since this radiation is only significant at closest approach, the longer orbit will not prevent the spacecraft from making the planned number of flybys. However, it did reduce their frequency by a factor of almost 4. Originally, 33 total orbits were planned in less than 1.5 years. The adapted budget plan covered only 12 orbits through July 2018, a span of two years.

Nevertheless, valuable data and images continued to pour in. The image below is a color-enhanced view of Jupiter's south pole, highlighting the massive swirling storms circling the pole.

In addition to images of the top of the atmosphere, Juno's instruments provided clues about deeper layers of the Jovian clouds. By the middle of 2017, enough Data had been collected from the Juno Microwave Radiometer, which detects thermal radiation from different depths in the atmosphere, to conclude that Jupiter's equatorial belts penetrated down to a great depth. In contrast, belts and storms at higher latitudes are relatively "shallow," with other structures appearing at increasing depth.

Nevertheless, the "weather layer" of Jupiter, which contains all the belts and cyclones, penetrates much further in depth than the analogous atmospheric layer on Earth. That is, the patterns of atmospheric movement (e.g. the spinning of Jupiter's great storms) persist down from the top portion of the atmosphere for a few thousand miles. In March 2018, four papers were published concerning Jupiter's atmospheric structure using Juno data. Among the results were the discovery of persistent circumpolar cyclones, as shown below.

The above two images are (false-color) computer generated composites of data from Juno's Jovian Infrared Auroral Mapper (JIRAM) instrument. The top image is the north polar region, showing a central cyclone surrounded by eight satellite cyclones. The south pole is similar, but has only five surrounding cyclones. Despite being in close proximity, these storms are much more persistent through time than those seen on Earth.

Another result published in this set of papers analyzed how Jupiter rotates below the weather layer. Precise gravitational measurements from the spacecraft indicate that a few thousand miles down into the atmosphere, the planet orbits approximately as a rigid body. That is, any deviations from steady rotation (such as the jet streams, belts, and storms) have a much, much smaller magnitude in this deeper layer.

In June 2018, NASA approved an extension of Juno's mission through 2021. Later in the year, Juno completed its global mapping of Jupiter with its 16th flyby of the planet. Each flyby had taken place at a different longitude (separated from the previous by 22.5°), allowing imaging of the entirety of the giant planet. By this time, 16 additional passes had been planned, offset from the first set to provide a composite total picture with finer resolution. During the first half of 2019, the Juno team had compiled enough measurements of Jupiter's magnetic field to determine that the field had changed measurably with time. Indeed, the measurements differed slightly but significantly from those of the Pioneer and Voyager spacecraft decades before. Nowhere other than the Earth had a changing magnetic field previously been detected. Experts expect that the variations stem from Jupiter's strong atmospheric winds, which move around material even in the deep layers containing molten metal. These inner layers drive the magnetic field just as on Earth.

A few months later, Juno executed a creative solution to a long-foreseen but very serious problem. As Juno explored different regions of Jupiter, it stood to reason that one flyby or another would bring the spacecraft into the giant planet's shadow. This posed a major problem for a solar-powered spacecraft, however, because only the batteries would prevent the temperature of various instruments from dropping too low. An unmodified orbital path on November 3, 2019, would have taken Juno into Jupiter's shadow for 12 hours, long enough to drain the batteries and possibly compromise the mission. Therefore, on September 30, the probe executed a 10.5 hour-long burn of its engines that allowed it to "jump the shadow" during the flyby a month afterward.

On its way to the December 26, 2019 flyby of Jupiter, Juno took images of Ganymede's north pole, shown below. The polar orbit of Juno let it see the poles more completely than previous space missions. At these poles were ice formations shaped by impacting charged particles directed toward the poles by Ganymede's magnetic field.

Sources: https://www.missionjuno.swri.edu/news/juno_spacecraft_in_orbit_around_mighty_jupiter, https://www.nasa.gov/feature/jpl/nasa-s-juno-spacecraft-sends-first-in-orbit-view, http://www.nytimes.com/2016/07/05/science/juno-enters-jupiters-orbit-capping-5-year-voyage.html?_r=0, https://www.nasa.gov/feature/jpl/jupiter-s-north-pole-unlike-anything-encountered-in-solar-system, https://www.nasaspaceflight.com/2016/09/juno-closest-approach-jupiter-readies-for-primary-science-mission/, https://www.nasa.gov/press-release/nasa-s-juno-mission-to-remain-in-current-orbit-at-jupiter, https://www.nasa.gov/press-release/a-whole-new-jupiter-first-science-results-from-nasa-s-juno-mission, https://www.nasa.gov/feature/jpl/nasa-juno-findings-jupiter-s-jet-streams-are-unearthly, https://www.nature.com/articles/nature25775, https://www.nasa.gov/feature/jpl/nasas-juno-mission-halfway-to-jupiter-science, https://www.missionjuno.swri.edu/news/Juno-Finds-Changes-in-Jupiters-Magnetic-Field, https://www.missionjuno.swri.edu/news/jun_prepares_to_jump_jupiters_shadow, https://www.nasa.gov/feature/jpl/nasa-juno-takes-first-images-of-jovian-moon-ganymedes-north-pole

Rosetta

Rosetta was a space probe launched by the European Space Agency whose mission is to orbit the comet 67P/Churyumov-Gerasimenko (comets are named as a combination of their catalogue number and their discoverers) and study it to learn more about the Solar System's origins.

This space probe consisted of two parts: Rosetta, the main spacecraft, and Philae, the lander that landed on the comet.

Rosetta was originally planned for launched in 2003, to study a totally different comet. But the launch was delayed, and a new trajectory was conceived. Rosetta was launched by the European Space Agency on March 2, 2004, beginning its hugely complicated series of flybys in space. No rocket could propel the spacecraft directly to the comet, so Rosetta's trajectory had to be long and complex. The first encounter was an Earth flyby, about a year after launch on March 4, 2005, where Rosetta tested its instruments and received a gravitational pull which sent it on orbit that would take it close to Mars. On February 25, 2007, Rosetta made a successful flyby of Mars, and it was set towards the Earth once again. As Rosetta approached the Earth for the flyby of November 13, 2007, an astronomer mistook the spacecraft for a small asteroid, about 75 feet in diameter, and predicted that it would pass within only a few thousand miles of the Earth. This caused momentary panic, until its track was recognized as Rosetta's.

After its second Earth flyby, Rosetta flew by by an asteroid, 2867 Steins, but did not use many instruments on it, to save most of its memory for the comet encounter. The asteroid was only about 2.5 miles across and Rosetta approached within 480 miles of the body on September 5, 2008. Although not much information was gathered on the asteroid, its orbit was verified more accurately than ever before.

Rosetta then made its last flyby of Earth on November 13, 2009.

On July 10, 2010, the probe encountered the asteroid 21 Lutetia. At its closest approach Rosetta was only 1960 miles away, and a significant amount of data was gathered. The ongoing analysis of this data will determine the composition of the asteroid. In addition, the exact orbit characteristics, mass, and rotation have been obtained, as well as many photographs.

Rosetta's image of the asteroid 21 Lutetia at closest approach.

After the second asteroid encounter, Rosetta shut off its systems in deep space in order to save its energy for the comet encounter. This stage is known has "Deep-Space Hibernation" and it began in May 2011. In total, this stage lasted for almost three years and ended on January 20, 2014. On this date, Rosetta sent a signal indicating that the hibernation had been successful and the spacecraft's systems were functioning normally. By this time, the comet had passed its apogee and it was once again approaching the Sun, so that Rosetta was close enough to power itself on the star's energy.

Even then, though, both the spacecraft and the comet were traveling relatively slowly. Rosetta began observations in January of that year as it approached the comet. In May, Rosetta began a series of maneuvers to prepare to orbit the comet. At the same time, a halo began to develop around the comet as it approached the Sun. The following image from early May shows this halo, or coma. This coma forms from sublimating ice and the release of gas trapped inside the comet's nucleus. In late June 2014, Rosetta measured that about 2 glasses of water a second were released from the comet in the form of vapor. This rate increased as the comet approached the Sun.

On July 14, Rosetta was close enough to obtain images of the body of the comet, revealing an unusual two-lobed structure (above). Images continued to increase in resolution as Rosetta approached.

Through July and early August 2014, the spacecraft continued to use fuel to decrease its speed relative to 67P/Churyumov-Gerasimenko, since orbit around such a small body ultimately required Rosetta's relative speed to the comet to be only 1 m/s (a typical walking speed)! In addition, Rosetta took its first temperature and surface readings. On August 6, 2014, Rosetta became the first spacecraft to ever enter orbit around a comet.

Rosetta had moved within 60 miles of the comet by this time, and began to map the surface with high-resolution imagery (such as the image above) to identify possible landing sites. By the end of August, 5 possibilities for a landing site had been chosen. During September, Site J, which the white plus sign marks on the image below, was selected for landing. In early November, this site was renamed "Agilkia" after an island on the Nile river.

On the days leading up to landing, Rosetta altered its orbit to release trajectory. During the morning of November 12, separation of Rosetta and Philae was confirmed. Seven hours later, the lander successfully touched down at the Agilkia site, but its harpoons did not activate, and the lander bounced twice, ultimately landing about a kilometer away from its original intended site.

Philae took images, such as the above, showing the comet from 40 m above during its initial descent.

Upon its final descent, Philae unfortunately landed on its side, with most of its solar panels facing the ground or in shadow. On November 15, after using its battery for 57 hours, the lander entered hibernation. However, during its 57-hour period of activity, Philae successfully used all 10 of its scientific instruments. This included a drill, which chemically analyzed a sample from the comet's surface and relayed the data to Earth via Rosetta. Notably, Philae did detect the presence of organic molecules on the comet's surface.

Important observations continued to pile up from the Rosetta orbiter over the next several months. During December 2014, Rosetta measured the composition of water vapor released from the comet as it approached the Sun. This vapor had more deuterium (the isotope of hydrogen with one neutron) than the water found on Earth. The measurement suggested that comets such as 67P/Churyumov-Gerasimenko (theoretically originating in the Kuiper Belt near Pluto) may not have been the primary source of water for Earth's oceans, which were likely filled by impacting solar system bodies.

In February 2015, as the comet continued to approach the Sun and grow more active, Rosetta departed its near circular orbit and positioned itself for several close flybys to collect gas released from the comet. The first of these flybys took place on February 14 at a distance of less than 4 miles!

Rosetta took the above image of the two-lobed comet on March 14, 2015. It has been enhanced to indicate the increasing volume of dust and gas streaming from the comet as it approached perihelion.

The same month, Rosetta also completed the first ever detection of molecular nitrogen at a comet. This offered crucial information as to the comet's origin, because molecular nitrogen can only become trapped within a comet's ice at very low temperatures. Therefore, the measurement supported the theory that 67P/Churyumov-Gerasimenko originated in the Kuiper Belt. During the spring, researchers also used data from both Rosetta and Philae to conclude that the comet was not significantly magnetized, the first such measurement for a comet. This was significant because iron was a major component of protoplanetary dust and some models indicated that assembled objects would have been magnetized in the early Solar System. The result gives another way to test theories of the formation of the Solar System.

During June, communication with the lander Philae was sporadically established as its solar panels were illuminated. However, no more investigations were able to be made. Later that month, the ESA officially granted an extension to the Rosetta mission through mid-2016.

The above images show the higher comet activity as it approached perihelion. The comet's closest approach to the Sun did not take it inside Earth's orbit, but it reached a minimum distance of 1.28 AU on August 13, 2015.

The mission continued to yield new results after perihelion had passed. For instance, using data from July 2015, it was able to measure and investigate the size of the diamagnetic cavity surrounding the comet's nucleus. This cavity is a region where the gas streaming off of the comet deflects incoming charged particles from the solar wind. Assuming the comet itself is not magnetized (which Rosetta determined it wasn't), this region should be free of magnetic fields. However, it only becomes appreciable in size near perihelion, when more material is ejected from the comet.

The above chart shows the magnetic field measured by Rosetta during a period of a few hours on July 26, 2015. The blue highlighted region (in which the field is nearly 0) corresponds to when the spacecraft passed through the diamagnetic cavity along its orbit. Rosetta passed into the edges of the cavity many times, allowing for detailed measurements of its size.

Continued analysis of data from near perihelion in August indicated that certain organic substances were present on Rosetta, including glycine, the simplest of the amino acids. Though the discrepancy in isotope ratios discussed above indicates that comets of this exact type did not transport these compounds to Earth in its early history, it supports the general claim that glycine can originate from comets, a conclusion hinted at by earlier study but never unambiguously verified.

Data from the probe indicated that the comet "breathed out" primordial oxygen near perihelion; that is, heating up the body of 67P released gases that had been trapped there for billions of years. While other mechanisms had been proposed for the molecular oxygen found around the comet, the Rosetta mission verified that a majority must have appeared via this process.

On September 5, 2016, as Rosetta began its final approach toward the comet, it spotted the lander Philae from a distance of 2.7 km after its precise location had not been known for two years! The image below has an astonishing resolution of 5 cm/pixel, and shows the lander in the bottom right, wedged in a crack. This explains why Philae was not able to operate under solar power after its landing.

Late on September 29, 2016, Rosetta's thrusters fired for the last time, sending it on a collision course with the comet. The spacecraft continued to transmit valuable data and images during its descent, taking advantage of its only opportunity to collect information from such a low altitude above the comet. The last image transmitted from Rosetta, shown below, was taken at an altitude of only about 20 meters, just before impact. The final impact occurred on September 30, ending communication with the probe.

The 12-year Rosetta mission provided invaluable knowledge about the structure, composition, and evolution of the comet 67P, and through it a better understanding of the formation of our Solar System billions of years ago. Comets are among the most pristine of time capsules for investigating the beginning of the Solar System as well as possible sources of water and other important molecules on Earth. The Rosetta mission will continue to shape our conception of our origin for years to come.

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