Radio signals can and do collide with other objects in space, and definitely do disperse between the spacecraft and the Earth, but they will reach the Earth for two reasons. The first; even though our solar system has a lot of stuff in it, the vast majority of it is still empty space. The second is that all spacecraft are equipped with finely tuned and precisely calibrated antennas, which allow them to aim their signal directly at the earth, rather than beaming its signal in every direction. Beaming signal in every direction would require a lot of power to produce, so it’s much more efficient to use a smaller amount of power to limit the signal to only direction you care about, i.e., in the direction of Earth.
Now, if you’re a spacecraft orbiting another planet, you can easily imagine that the planet will occasionally get in the way of you beaming information back to the Earth. Radio signals do not go through planets, and nor do they go on curved paths. So the spacecraft, in these times, must be capable of running itself for a while, before it comes back around from the far side of the planet (from an Earthly perspective). During these times the spacecraft are always in a communications blackout, and totally unable to communicate with us on Earth. All spacecraft are designed to operate this way – they have to be able to run a series of tasks without continual monitoring by someone on Earth.
Generally, though, if the spacecraft is cruising the solar system, there’s very little to get in between the Earth and a distant satellite, so the issues you run into are primarily making sure that your antenna is pointed in the exact right direction so that the signal you beam out does in fact intersect with the Earth, and that the signal’s power drops with distance. So you start out by trying to put a very efficient signal generator on your spacecraft, and then you listen with a very large antenna on Earth. We typically wind up listening with dishes that are 30 – 70 meters across (that’s 100 to 230 feet); these are large.
However, the largest of telescopes won’t help you if your spacecraft – for one reason or another – stops pointing its antenna at the earth. This is a great way to lose a spacecraft – once the antenna is no longer pointed at the Earth, it isn’t going to be telling you what’s wrong, and there’s no way to tell it to fix itself because it’s also not listening to signals coming from Earth. It’s only going to be able to listen to things from the direction its antenna is pointed.
Fortunately, we don’t additionally have the issue of not being able to always be listening on Earth – for spacecraft exploring other planets, NASA has set up the Deep Space Network (DSN), which has three listening stations – Madrid Spain, Canberra Australia, and Goldstone California. Between the three, there’s always one station that can listen for signals from distant satellites. You can watch which ones are listening to which spacecraft at any time if you like here. (Give it a minute to load up – it’s pretty fun.)
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