By Mike Deliman
This may sound a bit funny, but in the space industry, we're constantly playing catch-up. We're either looking at or for things that happened millions or hundreds of millions of years ago, sending rockets off to get to where something will have been just in time to take a picture of or bore a hole into it, or designing new rockets for flight 5 years from now with computer bits that would have been considered top-of-the-line 5 or 10 years ago. When we're recovering data and sending commands from and to deep space probes, we point our antennae to where the probe is supposed to be 30 minutes from now and start sending our data now; the idea is by the time the data actually gets there the craft will be where it was expected and receive the commands, and send it's data back to us.
This process I just described - of anticipating where a craft is, transmitting before it's there - and it transmitting back - is pretty much how the Deep Space network is currently used. It takes huge amounts of planning, all done in advance, to set up the multiple sessions that allow one successful exchange like that to work out. People consult tables of times when craft will be "visible", consult tables of one-way light distances to find transmission times. When they think they know when they need time, and how much data they expect to exchange (how much time they need), they contact the folks who run the big antennas. if the time slot is available, arrangements are made, and that one set of transmissions can take place. The folks who run the Deep Space Network and their customers do this sort of thing all the time - it's how they try to make the most efficient use of their giant antennae.
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