When NASA is giving commands to a distant spacecraft like Juno, how is that done in terms of timing and protocol?

For example do they:

  1. give a command to burn for 15 seconds upon receiving the signal (in which case they'd need to account for the 45 minute light travel time)?

  2. give a command to burn for 15 seconds and leave the timing of it to the craft (e.g. perhaps at a certain location that the craft will determine)?

  3. something else?


1 Answer 1


Generally, option 3 is used: a sequence of commands is uploaded, and they're all tied to specific times. "burn for 15 seconds at date x time y".

Unlike your option 1 , this allows mission control to upload commands in advance. For example, the New Horizons Pluto flyby program contained thousands of commands that needed exact timing, and the spacecraft had to change its attitude often to point its instruments at their targets. That meant the main antenna wasn't pointing at Earth most of the time and couldn't receive commands in real time.

Spacecraft outside Earth orbit don't know where they are (position is determined by mission control, spacecraft don't have sensors that can accurately determine position), so that makes option 2 difficult.

Option 1 was used for a few missions, e.g. the Lunokhod Moon rovers were controlled in real time. Every mission since then has used onboard computers to store and execute commands.

Spacecraft and rovers can have some autonomy, e.g. the Mars rovers can be told "drive in direction X" and they'll drive to that point autonomously, going around obstacles, stopping for interesting science targets or when the rover can't find a safe path.

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    $\begingroup$ There is another option: Upload commands to start a burn at date x time y, and use onboard accelerometers to end the burn when the ∆V reaches z (i.e., burn duration is not prespecified). Some projects have budgets big enough to afford accelerometers. When they need a precise maneuver, they can use this method. $\endgroup$ Commented Aug 10, 2018 at 21:22
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    $\begingroup$ @TomSpilker, from my experience, most spacecraft use a timed burn instead of the accelerometers because of the sampling rate of accelerometers. In fact, if the sampling rate isn't high enough, or if you data bus is congested and the data doesn't reach the controller, then your burn may go on for much longer. This is especially important for pulse-like burns. However, I have seen flying designs where the accelerometer is used as primary and the timing as backup. $\endgroup$
    – ChrisR
    Commented Aug 11, 2018 at 19:23
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    $\begingroup$ @ChrisR Cassini could do either, timed or accelerometer-driven. But then look at the budget we had! We could afford the accelerometers and the internal data bus to handle high-rate sampling. $\endgroup$ Commented Aug 11, 2018 at 22:23
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    $\begingroup$ Notably, JAXA's Hayabusa used a disastrous combination of #1 and autonomous actions. They ordered the probe to descend close to asteroid Itokawa, then ordered it to release the Minerva lander. Before the release command arrived, Hayabusa made an altitude correction maneuver, and then released the Minerva probe while ascending, flinging it into space instead of dropping it onto the asteroid. $\endgroup$ Commented Aug 12, 2018 at 1:18
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    $\begingroup$ (That was actually the second attempt to deploy Minerva, the first was automatically canceled due the navigational cameras having a hard time tracking the asteroid. They might have switched to direct control to try to work around that problem.) $\endgroup$ Commented Aug 12, 2018 at 1:21

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