There was much excitement when the Juno spacecraft successfully arrived at Jupiter in July after a five-year journey through the solar system. A perfect engine firing placed the solar-powered spacecraft into just the right orbit around the gas giant, with the promise of great discoveries to come.
Now, 150 days into the mission, Juno should have made six or seven close fly-bys of Jupiter (flying through the point of its orbit that is closest to the giant planet). It is at this point that the spacecraft makes most of its important scientific observations. But in reality, we have had just one science-intensive fly-by so far (in August), with another planned this month (December 11). So what happened?
Juno was originally injected into a 53-day orbit around Jupiter. The plan was to complete two of these long orbits while all the instruments were being checked, before firing the engine again in October to move the spacecraft closer to the planet in a 14-day orbit. However, shortly before the burn, the Juno team reported that two helium valves—which play a vital role in firing the main engine —weren’t operating properly. So instead of risking the spacecraft by firing the engine, the team decided to wait and analyze the issue in more depth. It’s always better having a healthy, working spacecraft than an uncontrollable one.
That’s not to say that Juno will never reach the 14-day orbit, but we now expect to stay in this 53-day orbit for at least the first half of 2017. But if we can’t figure out what’s going on with the valves, we could stay in this orbit indefinitely, as Juno doesn’t get any extra radiation exposure by doing this.
From a science perspective, this change just means we’ll be taking data more slowly—with 53 days between each fly-by rather than 14. Juno will still achieve its full scientific potential, but we scientists will have to be more patient than we’d originally planned, as well as reworking all our carefully laid plans for Earth-based support.
With the engine burn postponed, Juno’s science instruments were scheduled to provide complete coverage during the close fly-by on October 19. But Juno unexpectedly went into “safe mode” just 13 hours before the fly-by.
Safe modes are designed into software in case the computer encounters any glitches. If this happens, everything non-essential is turned off, the computer reboots, the spacecraft makes sure its solar panels are pointed at the sun to maximize its power, and it awaits further instructions from Earth. Unfortunately, this meant that no science data were obtained. It came out of safe mode five days later, and mission managers are now being cautious about the next close approaches to avoid it happening again.