The ESA Spacecraft is going "too fast" to transmit pictures of Asteroid Stein?

Question

About photographing the Stein Asteroid, the news release stated "As planned, the spacecraft's signal was lost for about 90 minutes as engineers turned it away from the sun and because the craft was moving too fast for its antennas to transmit." How is a spacecraft traveling too fast to transmit?

Answer 1

Mine was a stupid answer - I agree with Stargaze - has to be a mechanical tracking problem.

Answer 2

Well, aren't they travelling at thousands of miles (if you can calculate miles in space) per hour?

Answer 3

My guess is that they turned the spacecraft towards the target, but that would have changed the antennas position (they would have been pointed in a direction other than Earth's.) The motors on the antennas were not fast enough to keep the antennas pointed at Earth while the craft was flying by, so they couldn't take images in real time. The data was probably saved and transmitted later when the antennas were once again pointed in our direction.

Yes the article wasn't very clear on explaining that.

Answer 4

The radio signal from a spacecraft is subject to Doppler frequency shifting.  In order to receive signals from so far away, very tight band-pass filters have to be used, and this requires us to know very accurately the frequency upon which to receive.

As the spacecraft does a flyby of an asteroid, its effective signal frequency may shift too rapidly to track.  That's because a flyby is really a kind of orbit, and the periapsis (closest point) is where the velocity is changing most abruptly.

Another possible explanation -- again, we're reading a poorly-written article -- is that the spacecraft is acquiring information too quickly to transmit in real time.  This happens a lot.  The bandwidth between Rosetta and Earth doesn't allow for a lot of data-carrying capacity.  So the spacecraft stores up the data it acquires until it's done with the acquisiting and can then transmit.

There is most likely a conflict of pointing constraints.  In any spacecraft, where you put stuff is largely dictated by mass distribution and thermal (and other forms of) protection.  That doesn't always mean you can do two things simultaneously.  Very often you can't point a camera at something and, at the same time, keep your high-gain antenna pointed at Earth.  But I agree that doesn't really merit "too fast."