Juno: A near-death experience

For two years, NASA’s Jupiter-orbiting Juno spacecraft has been facing a mortal problem. When it arrived at the giant planet and braked into its preliminary orbit on 4 July 2016, all went smoothly.

But after a few preliminaries, the spacecraft was supposed to shift from that orbit (which swooped close by Jupiter once every 53 days) to a smaller one, with close-flybys every 14 days.

Not only would that have allowed the science – which occurs primarily during Juno’s close approaches to Jupiter – to be done nearly four times faster, but the mission’s planned 37 close-flybys would have been completed more than a year ago.

Then, the main engine malfunctioned, stranding Juno in its 53-day orbit.

Initially, that wasn’t an urgent problem. Juno’s primary wear-and-tear occurs during the close flybys, when it dives past Jupiter’s dangerous radiation belts. The spacecraft’s radiation exposure per orbit isn’t much different if it does so once every 53 days, rather than once every 14. 

“All the science is enabled by the close passes, so if you’re in a longer orbit, it’s pretty much the same,” the mission’s principal investigator, Scott Bolton of the Southwest Research Institute in San Antonio, Texas, said at the time.

But Juno was also inching toward an encounter with Jupiter’s shadow – one that would occur before its 3 November 2019 flyby – not a big concern in 2016, but a major one this year. 

In fact, Juno was heading for 12 hours of darkness that would probably cause its batteries to die and its temperature to plummet to a point where it would be unable to awaken when it finally moved back into sunlight.

“Pre-launch mission planning did not anticipate a lengthy eclipse that would plunge our solar-powered spacecraft into darkness,” says the mission’s project manager, Ed Hirst, of NASA’s Jet Propulsion Laboratory in Pasadena, California.

“Eclipses are not friends of solar-powered spacecraft,” Bolton adds.

To avoid this – but still wary of Juno’s main engine – mission controllers decided to use Juno’s thrusters. 

In an unprecedented manoeuvre spanning the night of 30 September to 1 October, they ran the spacecraft’s low-powered thrusters for 10½ hours, altering its velocity by 203 kilometers per hour – just enough to allow its orbit to “jump” across Jupiter’s shadow and remain in sunlight.

Critical as it was for the mission’s survival, Bolton says, the change in Juno’s orbit, from a larger perspective, was minor. “Our science plan is not affected,” he says. “Instead of worrying about freezing to death, I am looking forward to the next science discovery.”