For various reasons, spacecraft are sometimes "put to sleep" for periods of time on long voyages. Many systems are shut down, and radio communications suspended. Tracking of spacecraft at very large distances usually requires the spacecraft to receive the weak signal, amplify it, and rebroadcast it over a narrow beam from a high gain antenna if possible, so I'm guessing that sleeping spacecraft at large distances can't be actively tracked, although their trajectories can be forward-calculated with high precision.

This answer started me thinking about the whole process, and I remembered the videos of the crowd at ESA going crazy when Rosetta woke up.

In the video below it looks like the Rosetta spacecraft was hibernating for about 2.5 years. Is this the longest time that a space craft has "gone to sleep" with no tracking, and then "woke up", on schedule and in the right place?

The reason I say "on schedule" is that I am interesting in how long mission planners have been comfortable loosing contact with a spacecraft in an intentional, scheduled manner. Was it really without communication for 2.5 years? Was there also no tracking for the whole time?

This and this answer are informative reading about Rosetta waking up for example, and some references on tracking (ranging) can be found in this nice answer


2 Answers 2


First of all, yes, Rosetta was indeed without any contact for 31 months. That is longer than New Horizons, which wasn't in hibernation for more than 202 days1. These are the only two spacecraft that I can find that meet this criteria.

1On Pluto's doorstep, New Horizons spacecraft awakens for encounter

  • 2
    $\begingroup$ Great! That's a definitive answer for sure. Also it's quite an accomplishment for ESA even taken by itself! ISEE-3 has quite a long history - roughly 35 years. But as you say it may not meet the criteria of being put to sleep with no contact followed by waking up on schedule. That's a long timeline to sort through though. $\endgroup$
    – uhoh
    Commented May 22, 2016 at 13:26
  • $\begingroup$ I've asked a follow-up question: WHY (actually) did Rosetta have to go into hibernation for 2.5 years? $\endgroup$
    – uhoh
    Commented May 22, 2016 at 13:59
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    $\begingroup$ ISEE-3 kept working the entire time, it was just too far away to receive its signals. $\endgroup$
    – Hobbes
    Commented May 22, 2016 at 16:07
  • $\begingroup$ Would you like to try for Pioneer 6 (admittedly they didn't schedule that). $\endgroup$
    – Joshua
    Commented May 17, 2017 at 18:10
  • $\begingroup$ And STEREO-Ahead, although it was nowhere near 2.5 years. It was put to sleep before it passed behind the sun. (STEREO-Behind, unfortunately, never came back, so it doesn't meet the "woke up on schedule" requirement) $\endgroup$
    – Joe
    Commented Dec 9, 2020 at 23:55

Was there also no tracking for the whole time?

There was no need for tracking. The satellite merely had to be in view of the antenna on the Earth and the Earth had to be in view of the antenna on the satellite. That leaves a very wide range for error in the estimated orbit prior to putting the satellite to sleep.

From the video, Rosetta was 45 light minutes from Earth at the time it woke up. The beamwidth of the 34 meter high gain antennas used by NASA's Deep Space Network is 0.077 degrees. (I would assume ESA's high gain antennas have a similar behavior.) At 45 light minutes, that corresponds to a one million kilometer cross track error. Precision orbit determination does much, much better than that (orders of magnitude better than that), even with a 31 month extrapolation.

This, by the way, is why angle measurements are pretty much useless for orbit determination of deep space vehicles.

  • $\begingroup$ That sounds certainly correct! But I am not sure how it applies here. I didn't mention angle. However, it sounds surprisingly like an answer to a question that I was thinking of asking in a few days. I don't think I posted it yet - wow! $\endgroup$
    – uhoh
    Commented May 22, 2016 at 13:13
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    $\begingroup$ @uhoh - It applies because you asked Was there also no tracking for the whole time? I updated my answer to reflect that. $\endgroup$ Commented May 22, 2016 at 14:01
  • $\begingroup$ Oh, I see. Did I misuse the word "tracking"? I meant to refer to some kind of measurements of the satellite's trajectory, probably of the ranging varieties (simple, velocity and ∆DOR) - but by my using the word "tracking" does it look like I'm asking about angle? If so I should definitely edit the question! $\endgroup$
    – uhoh
    Commented May 22, 2016 at 14:09
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    $\begingroup$ The primary purpose of tracking is to know where the spacecraft is and where it will be. This goes under the guise of precision (or precise) orbit determination and prediction. There are many, many reasons for doing this. One of them is knowing where to point the directional antennas on the satellite and on the Earth during the next scheduled communication period. $\endgroup$ Commented May 22, 2016 at 14:18
  • $\begingroup$ Thanks - I'm pretty sure I understand that. Luckily they could predict where to point the antenna 2.5 years after they stopped tracking - it's pretty amazing! $\endgroup$
    – uhoh
    Commented May 22, 2016 at 14:21

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