After Five Years in Space, a Moment of Truth
When Juno arrives at Jupiter, it will be whizzing faster
than any human-made object has ever gone. To slow down and enter Jupiter’s
orbit, it has to perform a delicate maneuver – or else it will fly off into
space, never to return.
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Interact with the hotspots below to learn more.
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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.
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