As the majority of Americans celebrated July 4th—American Independence Day—NASA, the Jet Propulsion Lab, and space geeks all around the world celebrated something entirely different. On that day, 8:53 PM EDT, confirmation was received that the spacecraft Juno had successfully entered orbit around Jupiter after a five-year voyage. The spacecraft had fired its rocket for 35 minutes, successfully putting it on a 53-day highly-elliptical orbit. According to principle investigator Scott Bolton, they “just did the hardest thing NASA’s ever done.”
Juno, named after the Roman chief god Jupiter’s wife (and sister), is the second spacecraft to orbit Jupiter. The first spacecraft to do so was Galileo, named after the Italian astronomer who first spotted the “Galilean moons” of Io, Europa, Ganymede, and Callisto. The Galileo spacecraft performed wonderful science and took spectacular photographs of comet Shoemaker-Levy 9 as it crashed into Jupiter on the far side of the planet from Earth. It also suffered from a number of radiation-related problems including chronic faulting into safe mode. These radiation problems have been taken very seriously by the Juno team, who have designed the entire mission around it.
The radiation challenge for both Galileo and Juno are caused by the intense magnetosphere of Jupiter. The magnetic field around Jupiter is much more intense than around Earth, accelerating electrons in the region nearly to the speed of light. However, the radiation this causes is not evenly distributed. Specifically, there is substantially less radiation near Jupiter’s poles. To reduce the amount of radiation Juno is exposed to, it is on a highly-elliptical polar orbit. This means that over the course of an orbit, Juno dives down towards the planet from the poles, avoiding the radiation it would experience from a more equatorial orbit. It speeds quickly over the planet’s surface, getting as close as 5000 km above the top of the clouds, then exits out the other polar region. The spacecraft then spends most of its time far from Jupiter, beyond the radiation belts, where it transmits back the data collected from its close approaches.
A second radiation-anticipating measure put in by the Juno team is a radiation vault. This is a 180 kg, one cm-thick titanium cage that contains all of the important scientific and controls electronics, substantially reducing the amount of radiation they receive. Rocket science being what it is, weight is almost always the limiting factor and every extra kilogram of payload costs thousands of dollars in fuel; this radiation vault is a huge expenditure.
Another curiosity in Juno’s design is its three massive solar panels, which total 66 square metres. All other outer-solar system missions including New Horizons, Voyager 1 and 2, and Galileo use radioisotope thermoelectric generators (RTGs) since Jupiter is so far from the sun. Unfortunately, NASA has been increasingly short on the Plutonium-238 required for RTGs, which has become rarer since the introduction of various nuclear arms-limiting treaties. Without Pu-238, and with substantial advancements in solar power technology, it has become more economical to use solar panels in outer-solar system missions.
Juno will next fly close to the surface of Jupiter on August 27 (a point known as periapsis, or closest approach). At that time, all of its science instruments will be turned on, and it will start collecting data on Jupiter’s magnetic field, gravitational field, and composition of the interior. Among the many questions that Juno is anticipated to shed light on is how, where, and when Jupiter formed in the early days of the solar system. This is a very pressing question since humanity’s expanding search for extra-solar planets is revealing an ever-growing list of gas giant planets that are orbiting in unexpected places. These observations suggesting that our ideas of planet formation may be very far from correct, and Juno may give us some new insight.
On its third approach to Jupiter—October 19—Juno will perform a burn, putting it on a shorter 14-day orbit and bringing its periapsis to 5000 km above the clouds. This is its “science orbit” where it will collect most of its scientific data. After 37 orbits, Juno will perform one final burn to put it into a death dive. This death dive prevents the possibility that Juno will lose contact with Earth due to the pressures of the constant radiation attack. If that were to occur, Juno could eventually crash into Europa, contaminating the pristine and potentially life-harbouring sea beneath the surface.
The Juno team has said that it hopes to release its first science data on September 3rd. Exciting new pictures and interesting findings should start to appear shortly thereafter.
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