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