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.

Sources: (Photos from ESA) ESA-Rosetta, http://en.wikipedia.org/wiki/Rosetta_(spacecraft),http://www.bbc.com/news/science-environment-27498534, http://www.astronomy.com/news/2014/07/the-twofold-comet-comet-67pchuryumov-gerasimenko, http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_arrives_at_comet_destination, http://www.esa.int/var/esa/storage/images/esa_multimedia/images/2014/09/philae_s_primary_landing_site/14819792-1-eng-GB/Philae_s_primary_landing_site.png, http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_s_comet_contains_ingredients_for_lifehttp://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_and_Philae_find_comet_not_magnetised, http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_finds_magnetic_field-free_bubble_at_comet,http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_s_comet_contains_ingredients_for_life, http://www.esa.int/Our_Activities/Space_Science/Rosetta/Mission_complete_Rosetta_s_journey_ends_in_daring_descent_to_comet