After a near five year cruise through the solar system, NASA’s Juno spacecraft has become the second craft to permanently enter orbit of the largest planet in our solar system. The mission to Jupiter is anticipated to last 20 months in a polar orbit orientation to study Jupiter’s formation, composition, gravity field, magnetic field, and polar magnetosphere.

Mission background:

Originally proposed in fiscal year 2003, the Juno mission is part of NASA’s New Frontiers program and was designed as a follow-on mission to examine elements of the Jovian system that the Galileo mission of the 1990s either could not examine or returned intriguing results of.

Under its original proposal, the name Juno was chosen to compliment the god Jupiter in Greco-Roman mythology as Juno was Jupiter’s wife and was able to see behind the veil of secrecy Jupiter cast around himself to reveal his true nature.

Like this connection to Greco-Roman mythology, the Juno spacecraft is designed to peer through the veil of Jupiter’s clouds to examine the internal characteristics of the largest planet in our solar system, with the hope of illuminating information on how Jupiter formed as well as whether or not it has a rocky core and the amount of water present in its deep atmosphere.

Upon its official selection as a mission, Juno was tasked with 7 primary mission objectives, including a determination of the ratio of oxygen to hydrogen in Jupiter’s atmosphere.

This mission objective will help measure the abundance of water in Jupiter and help solidify some of the prevailing theories as to the initial formation of the planet 4.5 billion years ago and how its formation is linked to the overall formation of the solar system.

Additionally, Juno is tasked with obtaining a better estimate of Jupiter’s core mass, which will also help scientists understand how the planet formed.

During its 20 month stay at Jupiter, Juno will also help map the planet’s massive gravitational and magnetic fields to further reveal how Jupiter’s mass is distributed throughout its interior as well as the origin and structure of its magnetic fields.

Moreover, Juno will create a global map of the variation in atmospheric composition, structure, cloud opacity, and temperature at pressures in excess of 100 bars.

Another of Juno’s objectives is to reveal more about the three-dimensional structure of Jupiter’s polar magnetosphere and associated auroras while also measuring the Lense-Thirring precession.

Lense-Thirring precession is an aspect of the general theory of relativity that is a relativistic correction to the precession of a gyroscope near a large rotating mass.

Juno will attempt to measure this phenomenon that’s caused by the angular momentum of Jupiter.

Spacecraft design:

To accomplish these mission objectives, Juno is outfitted with nine scientific instruments, including a Microwave radiometer, the Jovian Infrared Auroral Mapper, a Magnetometer, a Gravity Science suite of instruments, the Jovian Auroral Distribution Experiment, the Jovian Energetic particle Detector Instrument, a Radio and Plasma Wave Sensors, Ultraviolet Imaging Spectrograph, and JunoCam.

The microwave radiometer (MWR) carries six antennas mounted on two sides of the main body of the Juno spacecraft that are individually capable of performing electromagnetic wave measurements on frequencies in the microwave range of 600 MHz, 1.2 GHz, 2.4 GHz, 4.8 GHz, 9.6 GHz, and 22 GHz, respectively.

With this range of frequency measurements, MWR will be able to measure the quantity of water and ammonia in the deep layers of Jupiter’s atmosphere at depths of 500 to 600 kilometers and in atmospheric pressures of up to 200 bars.

MWR measurements will also provide a temperature profile of Jupiter’s atmosphere at different altitudes as well as a determination of how deep atmospheric circulation exists within the planet.

Meanwhile, the Jovian Infrared Auroral Mapper (JIRAM) will provide images of Jovian auroras in the 3.4 μm wavelength in regions of the atmosphere that are specifically abundant in H3+ ions.

JIRAM will also monitor the near infrared frequency ranges at depths between 50 and 75 kilometers during multiple surveys of the upper atmosphere.

This monitoring will allow scientists to better understand how clouds with water flow beneath the surface of Jupiter as well as provide potential detections of methane, water vapor, ammonia, and phosphine within the upper layers of the planet’s atmosphere.

In combination with JIRAM, the Jovian Auroral Distribution Experiment (JADE) will allow for the detection of energetic particles in Jupiter’s auroras and will measure the angular distribution, energy, and velocity vectors of ions and electrons at low energy present in the auroras.

Conversely, the Jovian Energetic particle Detector Instrument (JEDI), will measure the angular distribution, energy, and velocity vectors of ions and electrons at high energy present in the polar magnetosphere of Jupiter.

Moreover, the fourth component of the auroral experiment package is the radio and plasma wave sensor (Waves) which will identify regions of auroral currents that define Jupiter’s radio emissions as well as the acceleration of auroral particles.

This will be determined by a measurement of radio and plasma spectra in the auroral region.

Last but not least, the fifth component of the auroral equipment package is the Ultraviolet Imaging Spectrograph (UVS), which will capture spectral images of UV auroral emissions in the polar magnetosphere and measure their wavelengths, positions, and arrival times during spectrograph viewing opportunities.

For the expansive and encompassing investigation of Jupiter’s magnetic field, Juno’s magnetometer (MAG) instrument will be influential in mapping the magnetic field, determining the dynamics of Jupiter’s interior, and determining the three-dimensional structure of the polar magnetosphere.

To this end, the magnetometer instrument is composed of two experiments: the Flux Gate Magnetometer (FGM) and the Advanced Stellar Compass (ASC).

The FGM will measure the strength and direction of magnetic field lines while the ASC determines the orientation of the magnetometer sensors.

While MAG focuses on mapping Jupiter’s massive magnetosphere, the Gravity Science (GS) instrument will be busy measuring the planet’s gravity via radio waves.

The GS experiment will help better determine the distribution of mass inside Jupiter and will help detect small gravity variations from Jupiter that create small changes in velocity on the relatively small Juno spacecraft.

Moreover, GS will be able to detect the Doppler Effect on radio broadcasts from Juno to Earth in the Ka and X Band frequencies.

Finally, the ninth and final instrument aboard the craft is JunoCam.

A visible light camera and telescope, JunoCam is only expected to last seven orbits before it succumbs to the damaging radiation and magnetic environments of the planet.

During its seven orbits, however, it is anticipated that the camera will dramatically help with NASA’s education and public outreach initiatives about the Juno mission and the importance of robotic exploration of the solar system.

To power all of these instruments, Juno is equipped with three solar panels arranged in a symmetrical pattern around the spacecraft.

Juno is the first mission to Jupiter to use solar panels instead of Radioisotope Thermoelectric Generators (RTGs), and advancements in solar cell technology in the past several decades made solar panels preferable to this mission given its operational distance of 5 AU from the sun and its overall objective and timescale.

Use of solar panels on Juno make it the farthest solar-powered mission in the history of space exploration, as all other vehicles to travel to this distance have used RTG technology.

Given the distance and the fact that Juno will only receive approximately 4% as much sunlight as it would in orbit of Earth, each of its three solar panels is 2.7 m (9.8 ft) in width by 8.9 m (29 ft) in length for a total power generation capability at Jupiter of 486 W, dropping to 420 W at the end of the mission due to radiation degradation.

For communications, the spacecraft will use the 70 meter antenna of the Deep Space Network with an X band direct link from an onboard computer operating at 50 Mbit/s of instrument throughput.

Additionally, attitude control for Juno is provided through a series of twelve thrusters in a monopropellant reaction control system while a bipropellant LEROS 1b main engine from Westcott, UK, provides the propulsion needed for all major post-launch burns.

Launch and cruise to Jupiter:

Initially planned to launch in 2009, the mission underwent a two-year launch delay due to funding constraints from the US federal government.

After final assembly and shipment to the Kennedy Space Center, Juno was encased in a five-meter payload fairing before being hoisted atop an Atlas V rocket at the Cape Canaveral Air Force Station.

After a countdown delayed by a leak in ground equipment and a boat in the exclusion range, the Atlas V 551 thundered off SLC-41 on 5 August 2011 at 12:25 EDT to send Juno on its way.

Following a propulsive combination of five strap on solid rocket boosters, an RD-180 engine, and a single-engine Centaur upper stage, Juno was delivered into an initial parking orbit of Earth, where the spacecraft remained for 30 minutes before the re-ignition of the Centaur upper stage for a nine minute firing sequence to place Juno on an Earth Escape Trajectory.

Once in this escape trajectory, the Centaur upper stage fired its reaction engines to impart a 1.4 RPM spin onto Juno.

Then, 54 minutes after launch, Juno successfully separated from the Centaur upper stage and extended its solar panels – which reduced its overall spin rate by two-thirds.

With its solar panels deployed, Juno began its five-year, 19 AU journey to Jupiter with an outbound-from-the-inner-solar-system trajectory.

The probe crossed the orbital plane of Mars in early 2012 before performing two deep space maneuvers on 30 August and 3 September 2012 to begin a swing back into the inner solar system.

Juno once again crossed the orbital distance of Mars in early 2013, before crossing the orbital plane of Earth and swinging closer to the Sun than its home planet before encountering Earth on 9 October 2013.

This encounter with Earth was a planned gravity assist flyby to impart extra velocity onto the spacecraft without the need for the craft to carry the propellant to obtain such a velocity change.

The encounter with Earth increased Juno’s speed by more than 3.9 km/s (8,800 mph) and fine-tuned Juno’s course toward Jupiter.

Juno’s control in science teams also used the Earth encounter as a rehearsal for the craft’s arrival at Jupiter and as an opportunity to test some of the instruments and practice certain procedures before arrival in the Jovian system.

After its encounter with Earth, Juno was placed into hibernation mode for much of its journey to Jupiter, with just a few wake up commands to ensure that all systems were functioning properly and to additionally fine-tune the craft’s approach profile to Jupiter.

Orbit insertion and planned mission:

With fine-tuning complete, Juno crossed the termination shock into Jupiter’s magnetosphere on 24 June and continued its cruise toward the lower density of the Jovian magnetosphere throughout 25 June.

On 30 June, NASA sent the ji4040 command to Juno at 15:15 EDT to place the spacecraft into autopilot mode for its arrival at Jupiter.

The signal took 48 minutes to cover the 860 million km (534 million mile) distance between the Deep Space Network antenna in Goldstone, California, and the Juno spacecraft.

“Ji4040 contains the command that starts the Jupiter Orbit insertion sequence,” said Ed Hirst, mission manager of Juno from NASA’s Jet Propulsion Laboratory in Pasadena, California.

“After the sequence executes, Juno is on autopilot. But that doesn’t mean we get to go home. We are monitoring the spacecraft’s activities 24/7 and will do so until well after we are in orbit.”

Arriving on 4 July 2016 after a journey of 2.8 billion km (1.74 billion miles), the NASA-led mission became the second interplanetary craft to arrive at its destination on the United States’ Independence Day, with the first being the successful landing of the Pathfinder mission’s Sojourner rover on Mars in 1997.

With Juno in autopilot mode, the spacecraft’s onboard computers began the 35-minute Jupiter Orbit Insertion (JOI) burn at 22:30 EDT – with confirmation of JOI burn commencement arriving through the Deep Space Network at 23:18:19 EDT after a travel time of 48 minutes 19 seconds across a distance of 869 million km (540 million miles).

Juno’s computers completed the JOI burn at 23:05 EDT, with confirmation of a successful burn and insertion into Jupiter orbit arriving through the Deep Space Network at 23:53:19 EDT.

This means, because of the distance the signals must travel, that confirmation of the JOI burn’s commencement was actually received approximately 13 minutes after the burn itself concluded.

With the burn is complete, Juno will settle into its initial 53-day orbit of Jupiter, which it will remain in for 106 days (two orbits) before performing another burn on 19 October to adjust its orbit to that of just 14 days.

This 14 day primary science orbit is different than it was when Juno was launched.

In 2011, the probe was initially targeting an 11 day primary science orbit trajectory.

The orbit was changed to 14 days to allow Juno to build maps of Jupiter’s magnetic and gravity fields to provide a global perspective of the planet earlier in the mission than originally planned.

The original plan would have required 15 orbits to map these global forces, with 15 more orbits filling in gaps to make the map complete.

In the revised plan, Juno will now obtain basic mapping coverage in just eight orbits, with a new level of detail added with each successive doubling of that number, at 16 and 32 orbits.

The revised plan lengthens Juno’s mission to 20 months instead of the original 15 and increases the number of orbits to 37 instead of 30.

However, the extra time does not represent additional science for the mission.

Instead, it will simply take Juno longer to collect the data it’s tasked with measuring.

After 37 orbits and 20 months at Jupiter, NASA plans to perform a final burn of Juno’s engine to send the probe into a destructive entry of Jupiter’s atmosphere on 20 February 2018.

At that point, Juno will have travelled more than 560 million km (348 million miles) in Jovian orbit for a total distance travelled since launch of 3.39 billion km (2.106 billion miles).

Like Cassini will have done at Saturn a few months prior, Juno will undergo a destructive end of mission plunge into Jupiter to protect the various Jovian moons that contain the possibility of harbouring life.

Unlike its predecessor, Galileo, Juno will not have the opportunity to enjoy mission extensions due to the amount of propellant it carries within its tanks and the need to ensure that Jupiter’s moons are not accidentally contaminated by an uncontrolled end of mission event.

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