Juno (spacecraft)

thumb|Juno in launch configuration

Juno is a NASA space probe orbiting the planet Jupiter. Built by Lockheed Martin and operated by NASA Jet Propulsion Laboratory, the spacecraft was launched from Cape Canaveral Air Force Station on August 5, 2011 UTC, as part of the New Frontiers program. Juno entered a polar orbit of Jupiter on July 5, 2016, UTC, to begin a scientific investigation of the planet. After completing its mission, Juno was originally planned to be intentionally deorbited into Jupiter's atmosphere, It will continue to explore Jupiter to study Jovian rings and inner moons area which is not well explored; this phase will also include close flybys of the moons Thebe, Amalthea, Adrastea, and Metis.

Juno mission is to measure Jupiter's composition, gravitational field, magnetic field, and polar magnetosphere. It also searches for clues about how the planet formed, including whether it has a rocky core, the amount of water present within the deep atmosphere, mass distribution, and its deep winds, which can reach speeds up to .

Juno is the second spacecraft to orbit Jupiter, after the RTG-powered Galileo orbiter, which orbited from 1995 to 2003. Unlike all earlier spacecraft sent to the outer Solar System and beyond—which used radioisotope thermoelectric generators for power—Juno is powered by solar panels, more commonly used by satellites orbiting Earth and working in the inner Solar System.

As of February 2026, Juno remained operational and in contact with Earth through the NASA Deep Space Network.

Naming

A NASA compilation of mission names and acronyms referred to the mission by the backronym Jupiter Near-polar Orbiter. However the project itself has consistently described it as a name with mythological associations and not an acronym. The spacecraft's current name is in reference to the Roman goddess Juno. but is not to be confused with New Horizons 2, a proposed but unselected New Frontiers mission.

Overview

thumb|Animation of Juno trajectory from August 5, 2011

Juno was selected on June 1, 2005, as the next New Frontiers mission after New Horizons. The desire for a Jupiter probe was strong in the years prior to this, but there had not been any approved missions. The Discovery Program had passed over the somewhat similar but more limited Interior Structure and Internal Dynamical Evolution of Jupiter (INSIDE Jupiter) proposal,

Juno completed a five-year cruise to Jupiter, arriving on July 5, 2016. The spacecraft was designed to orbit Jupiter 37 times over the course of its mission. This was originally planned to take 20 months. The spacecraft performed an orbit insertion burn to slow it enough to allow capture. It was expected to make three 53-day orbits before performing another burn on December 11, 2016, that would bring it into a 14-day polar orbit called the Science Orbit. Because of a suspected problem in Juno main engine, the burn scheduled on December 11, 2016, was cancelled and Juno remained in its 53-day orbit until the first Ganymede encounter of its Extended Mission. This extended mission began with a flyby of Ganymede on June 7, 2021. Subsequent flybys of Europa and then Io decreased the orbital period to 33 days by February 2024.

During the science mission, infrared and microwave instruments will measure the thermal radiation emanating from deep within Jupiter's atmosphere. These observations will complement previous studies of its composition by assessing the abundance and distribution of water, and therefore oxygen. This data will provide insight into Jupiter's origins. Juno will also investigate the convection that drives natural circulation patterns in Jupiter's atmosphere. Other instruments aboard Juno will gather data about its gravitational field and polar magnetosphere. The Juno mission was planned to conclude in February 2018 after completing 37 orbits of Jupiter, but now has been commissioned through 2025 to do a further 42 additional orbits of Jupiter as well as close flybys of Ganymede, Europa and Io. The probe was then intended to be deorbited and burnt up in Jupiter's outer atmosphere The vehicle coasted for about 30 minutes, and then the Centaur was reignited for a second firing of 9 minutes, placing the spacecraft on an Earth escape trajectory in a heliocentric orbit. When it reached the Jovian system, Juno had traveled approximately .

File:Atlas V with Juno on CCAFS SLC-41 (PIA14416).jpg|Atlas V on launch pad

File:Juno Lifts Off.jpg|Lift-off

File:Launch of Juno 2011.ogv|Launch video

</gallery>

Deep space maneuvers and flyby of the Earth

After traveling for about a year in an elliptical heliocentric orbit, Juno performed two deep space maneuvers (DSMs), firing its engine twice near aphelion (beyond the orbit of Mars) to change its orbit and return to pass by the Earth at a distance of 559 kilometers in October 2013. helped Juno slingshot itself toward the Jovian system in a maneuver called a gravity assist. The spacecraft received a boost in speed of more than , and it was set on a course to Jupiter. The flyby was also used as a rehearsal for the Juno science team to test some instruments and practice certain procedures before the arrival at Jupiter. and changed its trajectory from a hyperbolic flyby to an elliptical, polar orbit with a period of about 53.5 days. The spacecraft successfully entered Jovian orbit on July 5, 2016, at 03:53 UTC. Originally, Juno was expected to complete 37 orbits over 20 months before the end of its mission. Due to problems with helium valves that are important during main engine burns, mission managers announced on February 17, 2017, that Juno would remain in its original 53-day orbit, since the chance of an engine misfire putting the spacecraft into a bad orbit was too high. In June 2018, NASA extended the mission through July 2021.

The orbits were carefully planned in order to minimize contact with Jupiter's dense radiation belts, which can damage spacecraft electronics and solar panels, by exploiting a gap in the radiation envelope near the planet, passing through a region of minimal radiation. The Juno Radiation Vault, with 1-centimeter-thick titanium walls (three times as thick as the Galileo spacecraft body's), also aids in protecting Juno electronics by reducing the incoming radiation by a factor of 800. Despite the intense radiation, JunoCam and the Jovian Infrared Auroral Mapper (JIRAM) were designed to endure at least eight orbits, while the Microwave Radiometer (MWR) was made to endure at least eleven orbits. All instruments are operational as of perijove 71. Although the flux of electrons close to Jupiter is about ten times as high as it is around Jupiter's moon Europa, Juno will still receive a lower total dose of radiation in its polar orbit (20 Mrad through end of mission) than the Galileo orbiter received in its equatorial orbit. Galileo subsystems were damaged by radiation during its mission, including an LED in its data recording system.

Orbital operations

thumb|Animation of Juno trajectory around Jupiter from June 1, 2016, to October 1, 2028

thumb|upright|[[Ganymede (moon)|Ganymede, photographed on by Juno during its extended mission]]

The spacecraft completed its first flyby of Jupiter (perijove 1) on August 26, 2016, and captured the first images of the planet's north pole.

On October 14, 2016, days prior to perijove 2 and the planned Period Reduction Maneuver, telemetry showed that some of Juno helium valves were not opening properly. On October 18, 2016, some 13 hours before its second close approach to Jupiter, Juno entered into safe mode, an operational mode engaged when its onboard computer encounters unexpected conditions. The spacecraft powered down all non-critical systems and reoriented itself to face the Sun to gather the most power. Due to this, no science operations were conducted during perijove 2.

On December 11, 2016, the spacecraft completed perijove 3, with all but one instrument operating and returning data. One instrument, JIRAM, was off pending a flight software update. Perijove 6 took place on May 19, 2017.

Although the mission's lifetime is inherently limited by radiation exposure, almost all of this dose was planned to be acquired during the perijoves. , the 53.4 day orbit was planned to be maintained through July 2018 for a total of twelve science-gathering perijoves. At the end of this prime mission, the project was planned to go through a science review process by NASA's Planetary Science Division to determine if it will receive funding for an extended mission.

Extended missions

In January 2021, NASA extended the mission operations to September 2025. In this phase Juno began to examine Jupiter's major moons, Ganymede, Europa and Io. A flyby of Ganymede occurred on June 7, 2021, 17:35 UTC, coming within , the closest any spacecraft has come to the moon since Galileo in 2000. A flyby of Europa took place on September 29, 2022, at a distance of .

Juno's second extended mission (EM2) began in October 2025 with a planned three-year duration. Principal Investigator Scott Bolton noted "The new mission provides opportunities for Juno to unexplore new regions in the Jovian system, and to follow up on Juno's discoveries made during its prime and 1st extended missions. [...] During EM2, Juno will dive deep within Jupiter's inner radiation belts where the rings and inner moons reside. EM2 provides an opportunity for a thorough investigation of these components and their complex interaction, providing a unique data set to compare with other giant planet ring systems, including the ice giants."

Planned deorbit and disintegration

NASA originally planned to deorbit the spacecraft into the atmosphere of Jupiter after completing 32 orbits of Jupiter. The controlled deorbit was intended to eliminate space debris and risks of contamination of possible life-bearing moons (especially Europa) by surviving terrestrial microorganisms onboard the spacecraft in accordance with NASA's planetary protection guidelines. NASA has since extended the mission twice, first to September 2025 and again to September 2028 and no deorbit is planned.

Cost

Juno was originally proposed at a cost of approximately US$700 million (fiscal year 2003) for a launch in June 2009 (equivalent to US$ million in ). NASA budgetary restrictions resulted in postponement until August 2011, and a launch on board an Atlas V rocket in the 551 configuration. the mission was projected to cost US$1.46 billion for operations and data analysis through 2022.

Scientific objectives

thumb|Jupiter imaged using the VISIR instrument on the [[Very Large Telescope, 2016. These observations helped to plan Juno's mission.]]

The Juno spacecraft's suite of science instruments will:

Measure the orbital frame-dragging, known also as Lense–Thirring precession caused by the angular momentum of Jupiter, and possibly a new test of general relativity effects connected with the Jovian rotation.

Scientific instruments

The Juno mission's scientific objectives are being achieved with a payload of nine instruments on board the spacecraft:

Microwave radiometer (MWR)

thumb|Microwave Radiometer

The microwave radiometer comprises six antennas mounted on two of the sides of the body of the probe. They will perform measurements of electromagnetic waves on frequencies in the microwave range: 600 MHz, 1.2, 2.4, 4.8, 9.6 and 22 GHz, the only microwave frequencies which are able to pass through the thick Jovian atmosphere. The radiometer will measure the abundance of water and ammonia in the deep layers of the atmosphere up to pressure or deep. The combination of different wavelengths and the emission angle should make it possible to obtain a temperature profile at various levels of the atmosphere. The data collected will determine how deep the atmospheric circulation is. The MWR is designed to function through orbit 11 of Jupiter.

Jovian Infrared Auroral Mapper (JIRAM)

thumb|Jovian Infrared Auroral Mapper

The spectrometer mapper JIRAM, operating in the near infrared (between 2 and 5 μm), conducts surveys in the upper layers of the atmosphere to a depth of between where the pressure reaches . JIRAM will provide images of the aurora in the wavelength of 3.4 μm in regions with abundant H3+ ions. By measuring the heat radiated by the atmosphere of Jupiter, JIRAM can determine how clouds with water are flowing beneath the surface. It can also detect methane, water vapor, ammonia and phosphine. It was not required that this device meets the radiation resistance requirements. The JIRAM instrument is expected to operate through the eighth orbit of Jupiter.

Magnetometer (MAG)

thumb|MAG

The magnetic field investigation has three goals: mapping of the magnetic field, determining the dynamics of Jupiter's interior, and determination of the three-dimensional structure of the polar magnetosphere. The magnetometer experiment consists of the Flux Gate Magnetometer (FGM), which will observe the strength and direction of the magnetic field lines, and the Advanced Stellar Compass (ASC), which will monitor the orientation of the magnetometer sensors.

Jovian Auroral Distributions Experiment (JADE)

thumb|JADE

The energetic particle detector JADE will measure the angular distribution, energy, and the velocity vector of ions and electrons at low energy (ions between 13 eV and 20 KeV, electrons of 200 eV to 40 KeV) present in the aurora of Jupiter. On JADE, like JEDI, the electron analyzers are installed on three sides of the upper plate which allows a measure of frequency three times higher.

Jovian Energetic Particle Detector Instrument (JEDI)

thumb|JEDI

The energetic particle detector JEDI will measure the angular distribution and the velocity vector of ions and electrons at high energy (ions between 20 keV and 1 MeV, electrons from 40 to 500 keV) present in the polar magnetosphere of Jupiter. JEDI has three identical sensors dedicated to the study of particular ions of hydrogen, helium, oxygen and sulfur.

Radio and Plasma Wave Sensor (Waves)

thumb|Radio and Plasma Wave Sensor

This instrument will identify the regions of auroral currents that define Jovian radio emissions and acceleration of the auroral particles by measuring the radio and plasma spectra in the auroral region. It will also observe the interactions between Jupiter's atmosphere and magnetosphere. The instrument consists of two antennae that detect radio and plasma waves. It was anticipated that it would operate through only eight orbits of Jupiter ending in September 2017 during Juno's 47th orbit, the imager began showing hints of radiation damage. By orbit 56, nearly all the images were corrupted; the cause was identified as a damaged voltage regulator. By annealing the camera at a temperature of 25 °C (77 °F), the camera was brought back to operations. Junocam undergoes this operation periodically and as of July 2025 remains in operation.

Operational components

Satellite bus

Juno satellite bus, its main electronics and propulsion box, is a hexagonal prism. Once in orbit around Jupiter, Juno receives only 4% as much sunlight as it would on Earth, but the global shortage of plutonium-238 at the time, as well as advances made in solar cell technology over the past several decades, makes it economically preferable to use solar panels of practical size to provide power at a distance of 5 AU from the Sun.

The Juno spacecraft uses three solar panels symmetrically arranged around the spacecraft. Shortly after it cleared Earth's atmosphere, the panels were deployed. Two of the panels have four hinged segments each, and the third panel has three segments and a magnetometer. Each panel is providing of active cells – the largest on any NASA deep-space probe at the time of launching. The solar panels will remain in sunlight continuously from launch through the end of the mission, except for short periods during the operation of the main engine and eclipses by Jupiter. A central power distribution and drive unit monitors the power that is generated by the solar panels and distributes it to instruments, heaters, and experiment sensors, as well as to batteries that are charged when excess power is available. Two 55 Ah lithium-ion batteries that are able to withstand the radiation environment of Jupiter provide power when Juno passes through eclipse.

Telecommunications

thumb|Juno high-gain antenna dish being installed

Juno uses in-band signaling ("tones") for several critical operations as well as status reporting during cruise mode, but it is expected to be used infrequently. Communications are via the and antennas of the NASA Deep Space Network (DSN) utilizing an X-band direct link.

Due to telecommunications constraints, Juno will only be able to return about 40 megabytes of JunoCam data during each 11-day orbital period, limiting the number of images that are captured and transmitted during each orbit to somewhere between 10 and 100 depending on the compression level used. The overall amount of data downlinked on each orbit is significantly higher and used for the mission's scientific instruments; JunoCam is intended for public outreach and is thus secondary to the science data. This is comparable to the previous Galileo mission that orbited Jupiter, which captured thousands of images despite its slow data rate of 1000 bit/s (at maximum compression level) due to the failure of its high gain antenna.

The communication system is also used as part of the Gravity Science experiment.

Propulsion

Juno uses a LEROS 1b main engine with hypergolic propellant, manufactured by Moog Inc in Westcott, Buckinghamshire, England. It uses approx. of hydrazine and nitrogen tetroxide for propulsion, including available for the Jupiter Orbit Insertion plus subsequent orbital maneuvers. The engine provides a thrust of 645 newtons. The engine bell is enclosed in a debris shield fixed to the spacecraft body, and is used for major burns. For control of the vehicle's orientation (attitude control) and to perform trajectory correction maneuvers, Juno utilizes a monopropellant reaction control system (RCS) consisting of twelve small thrusters that are mounted on four engine modules. The plaque depicts a portrait of Galileo and a text in Galileo's own handwriting, penned in January 1610, while observing what would later be known to be the Galilean moons. The figurines were produced in partnership between NASA and Lego as part of an outreach program to inspire children's interest in science, technology, engineering, and mathematics (STEM). Although most Lego toys are made of plastic, Lego specially made these minifigures of aluminum to endure the extreme conditions of space flight.

Scientific results

Among early results, Juno gathered information about Jovian lightning that revised earlier theories. Juno provided the first views of Jupiter's north pole, as well as providing insight about Jupiter's aurorae, magnetic field, and atmosphere.

In 2021, analysis of the frequency of interplanetary dust impacts (primarily on the backs of the solar panels), as Juno passed between Earth and the asteroid belt, indicated that this dust, which causes the Zodiacal light, comes from Mars, rather than from comets or asteroids that come from the outer solar system, as was previously thought.

Juno made many discoveries that are challenging existing theories about Jupiter's formation. When Juno flew over the poles of Jupiter it imaged clusters of stable cyclones that exist at the poles. It found that the magnetosphere of Jupiter is uneven and chaotic. Using its Microwave Radiometer, Juno found that the red and white bands that can be seen on Jupiter extend hundreds of kilometers into the Jovian atmosphere, yet the interior of Jupiter is not evenly mixed. This has resulted in the theory that Jupiter does not have a solid core as previously thought, but a "fuzzy" core made of pieces of rock and metallic hydrogen. This peculiar core may be a result of a collision that happened early on in Jupiter's formation.

In April 2020, Juno detected a meteor impact on Jupiter, with estimated mass of 250–5000 kg.

Results from Juno on storms suggests that they are far taller than expected, with some extending 60 miles (100 kilometers) below the cloud tops and others, including the Great Red Spot, extending over 200 miles (350 kilometers). With Juno traveling low over Jupiter's cloud deck at about 130,000 mph (209,000 kph) Juno scientists were able to measure velocity changes as small as 0.01 millimeter per second using a NASA's Deep Space Network tracking antenna, from a distance of more than 400 million miles (650 million kilometers). This enabled the team to constrain the depth of the Great Red Spot to about 300 miles (500 kilometers) below the cloud tops. The new results show that the cyclones are warmer on top, with lower atmospheric densities, while they are colder at the bottom, with higher densities. Anticyclones, which rotate in the opposite direction, are colder at the top but warmer at the bottom.

Timeline

{| class="wikitable"

!Date (UTC)

!Event

!Latitude (centric)

!Longitude (Sys. III)

|August 5, 2012, 06:57:00

|rowspan=2|Deep Space Maneuvers (total dV: 345 m/s + 385 m/s)

|September 3, 2012, 06:30:00

|October 9, 2013, 19:21:00

|Earth gravity assist (from ) — Gallery

|4°

|100°

|October 19, 2016, 18:10:53

|Perijove 2: Planned Period Reduction Maneuver, but the main engine's fuel pressurisation system did not operate as expected.

|5°

|350°

|December 11, 2016, 17:03:40

|Perijove 3

|6°

|10°

|February 2, 2017, 12:57:09

|Perijove 4

|7°

|270°

| March 27, 2017, 08:51:51

|Perijove 5

|9°

|50°

|September 1, 2017, 21:48:50

|Perijove 8

|10°

|320°

|October 24, 2017, 17:42:31

|Perijove 9

|11°

|230°

|December 16, 2017, 17:56:59

|Perijove 10

|12°

|300°

|February 7, 2018, 13:51:49

|Perijove 11

|23°

|70°

|February 17, 2020, 17:51:36

|Perijove 25

|29°

|300°

|June 8, 2021, 07:46:00

|Perijove 34: Ganymede flyby, coming within of the moon's surface. Orbital period reduced from 53 days to 43 days.

|29°

|300°

|September 2, 2021 22:42:51

|Perijove 36

|May 1, 2026, 15:01:51

|Perijove 83: Thebe flyby at 5,000km.

File:Ganymede infrared NASA Juno JIRAM.jpg|Infrared view of Ganymede during the anniversary flyby by Juno

File:Tros Crater, Ganymede - PJ34-1 - Detail - Map Projected.png|Tros Crater on Ganymede.

File:Europa in natural color.png|View of Europa taken during Junos flyby on 29 September 2022

File:Europa - Perijove 45 (cropped).png|High resolution image of Europa by Juno.

</gallery>

File:Io by JunoCam, processed by Roman Tkachenko.png|Low resolution view of Io captured by JunoCam (September 2017)

File:Io seen by JunoCam.png|Io, as recorded by JunoCam (2 September 2017)

File:189401-JupiterMoon-Io-PlumeNearTerminator-Juno-20181221.jpg|Plume near Io's terminator (21 December 2018)

File:PIA26234-JupiterMoonIo-Volcanos-20231015.jpg|Io, viewed by JunoCam Several Volcanos (15 October 2023)

File:PIA26235-JupiterMoonIo-Plume-20231015.jpg|Io, viewed by JunoCam Volcanic plume (15 October 2023)

File:Io imaged by Juno spacecraft.png|Io, taken by the JunoCam instrument during Junos flyby (30 December 2023)

File:PIA26751.jpg|Thebe as seen by Juno on May 1, 2026.

</gallery>

Notes

References

External links

Juno mission at Southwest Research Institute
Juno Mission at NASA's Solar System Exploration
, Bill Nye discussing the science behind NASA's Juno mission to Jupiter
JunoCam image processing web site
Animation of perijove 15 flyby by Gerald Eichstädt (see channel for more)
Animation of perijove 16 flyby by Gerald Eichstädt and Seán Doran (see albums 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 20 21, 23, 24 and 25 for more)
Juno image album by Kevin M. Gill

Naming

Overview

Deep space maneuvers and flyby of the Earth

Orbital operations

Extended missions

Planned deorbit and disintegration

Cost

Scientific objectives

Scientific instruments

Microwave radiometer (MWR)

Jovian Infrared Auroral Mapper (JIRAM)

Magnetometer (MAG)

Jovian Auroral Distributions Experiment (JADE)

Jovian Energetic Particle Detector Instrument (JEDI)

Radio and Plasma Wave Sensor (Waves)

Operational components

Satellite bus

Telecommunications

Propulsion

Scientific results

Timeline

See also

Notes

References

External links