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Image delays
Jul. 1st, 2004 08:13 amSo Cassini is in orbit around Saturn and seems to be doing all right. We should be shortly seeing some images taken very near Saturn, just after the engine burn.
You may notice that, even though Cassini has the fat pipe that comes from a functioning high-gain antenna (unlike Galileo, which had to store images on a tape recorder and trickle them back at a low bit rate because its main antenna never worked), it isn't streaming images back in near-real time like Voyager did. This is actually the result of a cost-cutting engineering decision: Cassini's cameras aren't on an articulated arm, but are bolted directly to the spacecraft frame. So the whole spacecraft has to turn to aim the cameras, store the images in its internal RAM, and then turn back to point the HGA at Earth. (I think I learned that from Spaceflight Now, which does good play-by-play commentary.) The turning can be done at little cost because, most of the time, the probe uses reaction wheels for attitude control rather than thrusters.
I seem to recall that the same strategy has been used with a number of less elaborate probes in recent years to keep the cost down; the NEAR mission to asteroid Eros had a similar design. What really makes it work is that solid-state storage is cheap these days.
You may notice that, even though Cassini has the fat pipe that comes from a functioning high-gain antenna (unlike Galileo, which had to store images on a tape recorder and trickle them back at a low bit rate because its main antenna never worked), it isn't streaming images back in near-real time like Voyager did. This is actually the result of a cost-cutting engineering decision: Cassini's cameras aren't on an articulated arm, but are bolted directly to the spacecraft frame. So the whole spacecraft has to turn to aim the cameras, store the images in its internal RAM, and then turn back to point the HGA at Earth. (I think I learned that from Spaceflight Now, which does good play-by-play commentary.) The turning can be done at little cost because, most of the time, the probe uses reaction wheels for attitude control rather than thrusters.
I seem to recall that the same strategy has been used with a number of less elaborate probes in recent years to keep the cost down; the NEAR mission to asteroid Eros had a similar design. What really makes it work is that solid-state storage is cheap these days.
