A sort of followup to Are any software updates scheduled for Juno?

For the purpose of illustration, seeing that Juno has most recently encountered such an issue, I mention Juno here. However the questions at the end may apply universally.

So Juno got a gravity assist from Earth ... and then drew the veil.

A news website says:

... the Juno spacecraft unexpectedly entered ‘safe mode’ during the fly by maneuver and the mission teams are assessing the situation.

Let's assume a worst case condition

  • that it is a mechanical fault, say, caused by debris
  • that safe mode is here to stay

Going by this assumption

  • Can Juno Mission Control bypass safe mode?
  • Can Juno brake & enter Jupiter orbit in safe mode?
  • What studies may still be performed in safe mode?

2 Answers 2


Safe mode is a software state that is initiated by a detection of some inability of the system to do what it has been asked to do. There are many, many ways that can happen, such as insufficient power, inability to determine attitude, detection of a failed component, such as a reaction wheel, or a software fault or crash.

In safe mode, only the minimum activities are supported to keep the spacecraft power-positive and in communication with Earth. The communication with Earth is usually on a low-gain antenna at low rate to minimize the need for accurate pointing.

You can always command the spacecraft out of safe mode. But before you do that, you want to make sure that you figured out why it went into safe mode in the first place, and that you have fixed or worked around that problem so that it won't immediately drop back into safe mode again.

During critical events, such as a orbit insertion, a flyby, or landing where you only get one chance, many fault protection responses are disabled to avoid losing the mission due to going into safe mode. We sometimes call this "battle shorting", analogous to military equipment that can disable protections such as over current or over temperature when in battle, when you care more about other things than damage to the equipment. The original form of battle shorting was removing fuses and shoving pennies into the fuse sockets.


Most satellites have some sort of a safe mode, a mode the satellite enters when it is having difficulties with certain things. The exact nature of the safe mode varies from satellite to satellite, but in general, it is a lower power state, thermally stable and power positive, turns off science instruments, and relies less on absolute pointing.

I can say that most safe modes can be by-passed in some way or another. For instance, New Horizons will disable it's safe mode for a few weeks around the Pluto flyby, and if it doesn't succeed in doing that, the mission was pointless. However, safe mode did trip during the Jupiter flyby, which was deemed non-critical.

As far as what caused Juno's safe mode during flyby, I can't help but suspect it's related to this statement, as quoted here:

Juno's flyby was the first and only time it would enter a solar eclipse and be forced to rely on its battery for power

Bottom line, I suspect that the satellite determined it didn't have enough power to make it through the eclipse with all of it's instruments on full power, which is one of the most common safe mode triggers for solar-powered satellites. If this is the case, the probe should be up and running again in no time. This is the only eclipse of the mission, most likely the batteries aren't specified to last much longer than this eclipse, and perhaps either the batteries degraded with time, or else they weren't specified correctly. In either case, the spacecraft still is working and overall positive, so it should be fine.

Tildalwave provided me with the system requirements, which includes a nice section about safe mode. Essentially what happens is the data rate drops, the attitude may be compromised, and as a result, lower gain antennas are used for communication.

  • $\begingroup$ Insufficient battery power can't be the issue ... Reliance upon battery power during the solar eclipse seems perfectly normal. Besides the eclipse was for a short duration, and the craft panels before the eclipse (I may be wrong!) received much more sunlight than they might have out in orbit around Jupiter. $\endgroup$
    – Everyone
    Oct 10, 2013 at 13:16
  • $\begingroup$ @Everyone: This is the only time the spacecraft has ever had to depend on battery power, except for when it was on the ground during testing. My guess is, the batteries have degraded somewhat since then. It is true it will get more power near Earth than Jupiter, but you can't get power in elcipse. $\endgroup$
    – PearsonArtPhoto
    Oct 10, 2013 at 13:18
  • $\begingroup$ "...and only time it would enter a solar eclipse"... is this true? Does it avoid eclipsing completely while orbiting Jupiter? It would make sense, I suppose... $\endgroup$
    – user29
    Oct 10, 2013 at 15:41
  • 1
    $\begingroup$ @Chris: The plan is for it to be in a polar orbit, presumably one that does not enter the shadow of Jupiter. As it is a sunlight powered spacecraft, it seems likely that it can't support eclipse at Jupiter for some reason. $\endgroup$
    – PearsonArtPhoto
    Oct 10, 2013 at 16:06
  • 1
    $\begingroup$ Correction. Actually, although flattering for having been mentioned in your answer, that system requirements PDF was originally provided by @MarkAdler in another answer of his re Juno. ;) $\endgroup$
    – TildalWave
    Oct 10, 2013 at 16:56

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