Engineers at Nasa say Mission Juno to Jupiter is the most challenging the American space agency has ever accomplished. After travelling 2.8 billion km in five years the spacecraft had to perform a precise manoeuvre inside a time frame of 1.2 seconds to enter an orbit without getting its electronics fried by radiation.
Juno will repeat versions of that manoeuvre 37 times over the next 20 months as it flies complicated orbits around the largest planet. At its closest, Juno will be about 5,000 km from what counts as the surface. The mission is scheduled to end in February 2018 when it will be deliberately flown into Jupiter.
The $1.1-billion Mission Juno was launched on August 5, 2011. Juno took a complicated route to reach Jupiter on July 4, 2016. About two years after its launch, it returned to Earth for a slingshot gravity-assist. That boosted the speed to a record 265,000 kmph. When it reached Jupiter, it slowed to 208,000 kmph - a record speed for orbit insertion.
One key issue worried engineers. Jupiter has huge magnetic fields and radiation belts, which could destroy all electronics. But the exact location, shape and intensity of radiation belts are unknown. The belts have "holes" near the planetary poles and the orbit was plotted to go through a polar "hole" of a few tens of kilometres. The spacecraft has an array of equipment for nine different experiments. Those instruments are shielded from radiation within a titanium box.
The main engine was fired for 35 minutes to set up the orbit. There was no power to spare as it successfully "threaded the needle" in that 1.2 seconds. Hence, all scientific instruments were switched off. The equipment is now being redeployed, apparently with no damage. Experiments will commence in late August after everything is online again. Juno will track the distribution of high-energy electrons, look for water, check the planet's mass distribution and map the auroras and magnetosphere.
The spacecraft has an unusual design. The main module is basketball-shaped. It is connected to three solar arrays of 9 metres each. The solar panels are covered in extra thick glass as radiation shielding. The array is supposed to provide stability in the Jovian atmosphere as well as power. There is a main engine and 12 reaction-control thrusters to give it motive power.
The outer planets, Jupiter Saturn, Uranus and Neptune are believed to be balls of gas. One hypothesis is that Jupiter is totally gaseous and that it was formed very early in the solar system's history. Jupiter rotates fast. Its days last nine hours and 56 minutes. It has vast, permanent storms with wind speeds of over 600 km per hour. It has a huge magnetic field, detectable at a distance of 100 Jupiter radii (Jupiter's radius is nearly 11 times more than Earth's). In comparison, Earth's magnetic field extends out just 10 Earth radii. There is also radiation from volcanic activity on Jupiter's moon, Io.
Jupiter has huge auroras, which must be caused by charged particles. One possibility is that Io puts charged volcanic dust into the Jovian atmosphere. But this is not known for sure. There is also a chance that Jupiter does have a solid core.
Juno should find evidence one way or another and that will reveal more about Jupiter and the solar system as well. Juno will also get data on the magnetic and gravitational fields, rotational patterns and weather systems. In fact, just by measuring the Doppler Shift in radio signals from Juno, scientists will get a sense of whether Jupiter is entirely gaseous.
The first two orbits will take 53.5 Earth days each. But by October, Juno will be pushed into a faster 14-Earth-day orbit. This will still be highly elliptical (egg-shaped) and pass over the poles. Juno must always contort through complex orbits to avoid the radiation belts.
There is some information about the atmosphere due to the Galileo probe, which spent eight years in the Jovian system. Jupiter has a deep atmosphere, mainly consisting of hydrogen and helium. It lacks a solid surface - the atmosphere thickens into liquid. At its closest, Juno will be just 5,000 km above that liquid surface, well below the clouds masking the planet.
Juno will complete one orbit before it starts experiments. It is already sending back stunning pictures from its visible-light camera, JunoCam. The mission carries a plaque in Galileo's handwriting from January 1610 when he discovered three of the moons.
Juno also carries three bespoke Lego figurines representing Galileo, and the Roman god, Jupiter and his consort, Juno. Roman myth depicted Jupiter as a philanderer, who often disguised himself to have affairs. Juno was very suspicious and very good at penetrating those disguises. Nasa is hoping this Juno lives up to the mythical forbear's reputation.
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