If it crashed into something. Jupiter does not possess the majestic rings of Saturn, but it does have a thin ring of debris orbiting it. Juno passed through a region that appeared clear, but that did not mean it actually was. Even a dust particle could cause significant damage, as Juno was going 130,000 miles per hour relative to Jupiter.

If it flew too close to Jupiter and was ripped to pieces. In one of NASA’s most embarrassing failures, the Mars Climate Orbiter spacecraft was lost in 1999 because of a mix-up between English and metric units. Climate Orbiter went far deeper into Mars’ atmosphere than planned. Juno traveled within 2,900 miles of Jupiter’s cloud tops, so a miscalculation could have been catastrophic.

If the computer crashed. On July 4 last year, the mission controllers of the New Horizons spacecraft that was about to fly by Pluto experienced some nervous moments when the spacecraft stopped talking to them. The computer on New Horizons crashed while trying to interpret some new commands and compressing some images it had taken, the electronic equivalent of walking while chewing gum.

The controllers put New Horizons back in working order within a few days, and the flyby occurred without a hitch. For Juno, the scientific instruments have been turned off for its arrival at Jupiter. “We turn off everything that is not necessary for making the event work,” said Dr. Levin, the project scientist. “This is very important to get right, so you don’t do anything extra.”

The intense barrage of radiation at Jupiter could have knocked out Juno’s computer, even though it is shielded in a titanium vault. Usually, when there is a glitch, a spacecraft goes into “safe mode” to await new instructions from Earth, but in this case, that would have been too late to save Juno. The spacecraft had been programmed to automatically restart the engine to allow it to enter orbit.

“If that doesn’t go just right, we fly past Jupiter, and of course, that’s not desirable,” Dr. Bolton said.

The Approach

Juno has been executing an automated sequence of actions since Thursday. A timeline of Juno’s arrival (all Eastern time):

9:13 p.m. Start of transmission of single frequency “tones” that will provide updates on the spacecraft’s condition.