NASA is preparing for Juno to stick around Jupiter a lot longer than it had originally planned. The probe — which has been orbiting the gas giant since July — is going to stay in its 53-day orbit around the planet for a while, with no definitive plans at the moment to put the vehicle in a shorter orbit. Originally, NASA had hoped to have Juno in a two-week orbit by now, but ongoing engine troubles are delaying that move.
If Juno never goes into its shorter orbit, it’s not exactly bad news. NASA says it won’t diminish the amount of science Juno can do at Jupiter. The biggest difference is that the spacecraft will be swinging by the planet at a much slower rate, so the mission could conceivably last beyond 2019, instead of its previously scheduled end of February 2018.
The trouble for Juno started last month, right before the spacecraft was about to do its second swing by Jupiter. The vehicle doesn’t orbit the planet in a circle but takes a highly elliptical path in order to avoid as much of the radiation-filled environment around Jupiter as possible. Because of this, Juno gets super close to the surface of Jupiter for just a few hours each orbit. These close passes are known as Perijove passes, and they’re the times when Juno can gather the most data.
The most recent Perijove pass occurred on October 19th, but the mission team didn’t intend to do any science on that one. Instead, the plan was to ignite Juno’s main engine, putting the vehicle in the shorter 14-day orbit. The engine burn can only be done during a Perijove pass and none of the science instruments can be on when it happens. Leading up to the pass, NASA engineers found that a few engine valves were taking longer to open than they were supposed to. "That is something that is significant because it can affect how the engine operates," Rick Nybakken, Juno project manager at NASA’s Jet Propulsion Laboratory tells.
NASA decided to scrap the engine burn for the October 19th pass and take science measurements of Jupiter instead. But then a software glitch put Juno into safe mode. This mode turns off all of the vehicle’s instruments and prompts the probe to turn toward the Sun while it awaits instructions from Earth. NASA was able to resolve the glitch and bring Juno out of safe mode on October 24th, but at that point the Perijove pass was completed and no science data was gathered.
Now, NASA is focusing on the engine troubles. The mission team won’t be performing a burn on Juno’s upcoming Perijove pass, scheduled for December 11th, and they’re preparing for the possibility of never igniting the spacecraft’s engine again. "That’s the thing we’re looking at," says Nybakken. "We’re not going to do it if we can’t do it safely. And so we’re looking at different ways we can do the burn. Right now, it’s too early to say which way it’s going to go."
That means Juno may not de-orbit in February 2018 like NASA had expected. Instead, the mission could last beyond 2019, according to Nybakken. That’s because the spacecraft will be exposed to the worst parts of Jupiter’s radiation less frequently. Eventually, all those charged particles around the planet will slowly damage Juno enough that it can’t function anymore. But Juno only receives the bulk of the planet’s radiation when it does its flybys of Jupiter. So on a 53-day orbit, the vehicle gets pelted with the worst radioactive doses at a much slower rate. "Radiation accumulation is a function of the number of orbits and not a function of time," says Nybakken. "So it’s really hard to gauge impacts of radiation perspective other than to keep in mind, we accumulate on a full orbit basis not on a time basis."
The only real problem? An eclipse in mid-2019 that would put Juno in Jupiter’s shadow for six to 10 hours. During that time, the spacecraft would not get any sunlight on its solar panels and its temperature would drop significantly. Juno has gone through periods of no sunlight before, but never for such an extended period of time. So NASA’s navigators are trying to figure out a way to change Juno’s orbit to avoid the eclipse, in case the probe is still operating by then. "That looks like a significant obstacle that we’d have to overcome," says Nybakken, "but we have very creative navigators."