After Five Years in Space, a Moment of Truth
Juno’s engines need to fire at just the right moment
and in just the right direction for just the right amount of time. The
spacecraft will have to perform this tricky move – called the Jupiter Orbit Insertion – entirely on its own.
ORBITING JUPITER
Once Jupiter’s gravity captures Juno, the spacecraft begins its carefully designed orbit that allows it to closely examine the planet while avoiding its harmful radiation. And because Juno relies on solar power, the orbit has to keep the spacecraft exposed to sunlight for its entire mission.
Once Juno reaches Jupiter in 2016, it will follow a special orbit called a polar orbit, which takes the spacecraft over Jupiter’s poles, traversing the planet in a north-south direction. Because polar orbits are best for mapping and monitoring a planet, many satellites that study the earth follow a similar path. This type of orbit has never been tried around Jupiter.
Juno takes 11 days to complete a revolution while Jupiter takes only 10 hours to spin around once. Mission planners designed the trajectory so that the spacecraft passes over a different section of Jupiter during each orbit. After completing the mission’s 33 planned orbits, Juno will have covered the entire surface of Jupiter.
To ensure that it collects the best possible data – especially in making accurate measurements of the gravitational and magnetic fields – it has to get as close as it can to Jupiter. On each orbit, Juno comes within 5,000 kilometers (3,100 miles) of the planet’s cloud tops. That might still sound like a great distance, but if Jupiter were the size of a basketball, the equivalent distance would be about 0.8 cm (0.315 inches).
The three hours before and after closest approach is the most important time for Juno’s science instruments. During the rest of its orbit, mission controllers are focused on navigating, steering, and sending data and status reports back to Earth.
Juno’s close orbit also enables it to avoid Jupiter’s harmful radiation, which is concentrated in a belt that loops around the planet’s equator. In this region, tiny particles – ions and electrons – zip around nearly at light speed. Even though they’re small, they pack quite the punch, and they can destroy a spacecraft’s electronics.
Juno’s polar orbit takes it between Jupiter and this danger zone, avoiding most of the radiation and thereby keeping the spacecraft functioning for at least a year. But as the mission continues, Juno’s orbit tilts, pushing the spacecraft closer to this treacherous region. In a year, the amount of radiation that’s expected to bombard Juno is equals about 60 million dental x-rays.
The radiation is so destructive that two of Juno’s instruments – the jovian infrared auroral mapper and the camera, JunoCam – are only planned to last through Orbit 8. The microwave radiometer is designed to last through Orbit 11.
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Polar Orbit
An orbit that takes Juno over Jupiter’s north and south poles is the best way to map the entire planet. Called a polar orbit, this path allows Juno to pass directly over any longitude, so as Jupiter rotates, Juno can eventually cover the entire surface. Many satellites that produce maps of Earth also follow polar orbits.
Jupiter is surrounded by a doughnut-shaped region of strong radiation (Earth has a similar radiation band called the Van Allen belt). This radiation can damage Juno’s electronics, but fortunately, its polar orbit enables it to duck under the belt and stay safe.
Previous spacecraft have only orbited Jupiter’s equator, so Juno will be the first to get a detailed look at the poles. -
Gravity Science Orbits
During each of Juno’s orbits, its closest approach to Jupiter lasts six hours. And it’s during this time that the spacecraft carries out the bulk of its scientific observations. On any given orbit, Juno is doing one of two main tasks: looking into the depths of Jupiter’s clouds or measuring the planet’s gravity.
When probing Jupiter’s gravitational field, Juno switches off its microwave radiometer instrument and turns on its gravity-science instrument.
Variations in Jupiter’s inner structure will have tiny effects on its gravitational field, which will ever so slightly alter Juno’s orbit. These subtle shifts in motion cause equally subtle shifts in the frequency of a radio signal received from Earth. Known as the Doppler effect, it’s the same type of frequency shift that happens when the pitch of an ambulance’s siren increases when speeding toward you and decreases when speeding away.
To measure these tiny shifts – and therefore probe Jupiter’s inner structure – Juno points its high-gain antenna at Earth to receive the radio signal. Juno has to point the antenna – the large dish sitting on top of the spacecraft – very precisely for an accurate measurement. Of the 33 total planned orbits, Juno uses its gravity-science instrument during orbits 4 and 9 through 32. -
Radiation
Radiation often carries a negative connotation, but in fact, it’s everywhere – and not all of it is dangerous. Radiation is just energy that travels through space, and it can take the form of electromagnetic waves or atom-sized particles that zip around really fast.
Relatively harmless examples of electromagnetic radiation include radio waves, infrared light, and even the visible light you see with your eyes. More dangerous forms of radiation include x-rays, which carry enough energy to damage tissue – even at low levels. That’s why medical x-rays should be used only when needed – and with proper protection. Similarly, high-energy radiation can wreak havoc on Juno’s electronics.
Jupiter’s magnetic field traps and accelerates electrons and protons to high speeds. Concentrated in a belt that circles the planet, these high-energy electrons are the dangerous kinds of radiation that pose the greatest threat to Juno.
But Juno’s polar orbit takes it under the belt, thus avoiding much of the harmful radiation. Still, Juno can’t avoid the belt completely, so mission engineers must keep a close eye on how much time Juno spends near and in the radiation belt. -
Radiometry Passes
During each of Juno’s orbits, its closest approach to Jupiter lasts six hours. And it’s during this time that the spacecraft carries out the bulk of its scientific observations. On any given orbit, Juno is doing one of two main tasks: looking into the depths of Jupiter’s clouds or measuring the planet’s gravity.
To explore Jupiter’s inner clouds, Juno turns on its microwave radiometer (MWR) – and turns off its gravity-science instrument – and orients itself so that the antennas point at Jupiter. Mounted on two of Juno’s six sides, the MWR antennas take continuous measurements while Juno spins. These so-called radiometry passes occur during orbits 3 and 5 through 8, out of a total 33 planned orbits. -
Capture Orbit
Six months before Juno arrives, it gets to work. It starts to take measurements of Jupiter and its magnetosphere – the huge bubble created by its magnetic field. In particular, scientists are interested in learning how the magnetosphere interacts with the solar wind – the blast of material from the Sun that streams outward at a million miles per hour.
A couple of weeks before being captured by Jupiter’s gravity, when it’s still about 15 million kilometers (9.3 million miles) away, Juno crosses the magnetosphere’s boundary and enters Jupiter’s domain.
On July 5, 2016, Juno finally reaches its destination and approaches Jupiter’s north pole. It fires its thrusters to spin faster, bumping its rate to five rotations per minute, which gives the extra stability it needs as it slams on the brakes. Juno fires its engines for 30 minutes to slow down enough to enter Jupiter’s orbit. As soon as the engine burn finishes, Juno reduces its spin to two rotations per minute, where it will remain for the rest of the mission.
Throughout this series of maneuvers, Juno is within full view of Earth, allowing mission controllers to monitor its radio signals and confirm the successful completion of each critical step.
Called a capture orbit, Juno’s first orbit is its longest, taking 107 days to circle Jupiter. After another engine burn slows it down even more, Juno will settle into an 11-day orbit.
