NASA’s Juno mission team is delaying putting their spacecraft into a shorter orbit around Jupiter — a key maneuver that was supposed to happen on October 19th. But for some reason Juno’s main engine is acting up, so NASA is now looking to pull off the operation on December 11th. The team hopes that will give them enough time to figure out why Juno’s engine isn’t working as expected.

For some reason Juno’s main engine is acting up

Juno has been in orbit around Jupiter since July 4th, and its path around the planet is a strange one. The spacecraft is on what is known as a highly elliptical orbit, meaning it spends most of its time far out in space and then spends a few short hours up close to Jupiter’s surface. This orbit is meant to keep the spacecraft away from most of the intense radiation that surrounds the gas giant.

Right now, Juno’s orbit takes 53 days to complete, but that time period will eventually get shorter. After completing two of its longer orbits, Juno was supposed to ignite its main engine again and put itself into a 14-day orbit around Jupiter. However, NASA is a little nervous about doing this right now since parts of the main engine are not working as expected. Specifically, a pair of helium check valves, which help the main engine fire, didn’t work like NASA wanted them to during a command sequence on Thursday. "The valves should have opened in a few seconds, but it took several minutes," Rick Nybakken, Juno’s project manager, said in a statement. "We need to better understand this issue before moving forward with a burn of the main engine."

Juno's 14-day orbital paths. (NASA)

The best time for Juno to reduce its orbit is when it swings by the planet for a few hours in what is known as a perijove pass. That next pass is happening on Wednesday, but now NASA is going to wait for the following pass so that the mission team has more time to figure out what’s going on with the helium valves. That means Juno has at least one more 53-day orbit to complete before it can get into its shorter orbit.

The delay isn’t all bad news though

The delay isn’t all bad news though. Originally, the plan was to turn off all of Juno’s instruments on the 19th, so that nothing interfered with the engine burn. But since there’s no burn scheduled, the mission team is going to leave the instruments on while Juno passes close to Jupiter. Plus, the length of Juno’s orbit doesn’t affect the quality of the science the spacecraft can do. "The mission is very flexible that way," Scott Bolton, Juno’s principal investigator from the Southwest Research Institute in San Antonio, said in a statement. "The data we collected during our first flyby on August 27th was a revelation, and I fully anticipate a similar result from Juno's October 19th flyby."

But the shorter Juno’s orbit, the more perijove passes it can make and the more science it can do during its mission at Jupiter. Hopefully, 53 days is enough time for NASA to figure out what’s up with Juno’s engine and get it working again.

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