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How do unmanned space probes avoid running into things?

My question: How do Probes (like Voyager 1) navigate through Space, the Asteroid Belt and around Planets with out colliding with something?

I realize Space is a Vast place, but with the velocity everything has and the lag on communication how is it possible to avoid a rock.( Any size would be disastrous.) I assume most all rocks are too dark and cold to be visual. Do probes use radar for guidance? If so what power source with out draining the cells.

All unmanned space probes are steered from the ground. As you say, there's a lot of room in space, and it's not like there are any bumps in the road. :) We keep very close track of the spacecraft's position and velocity, and given those, we can predict very precisely where it is going, so running into large objects is not a problem. Generally, the trajectory of the spacecraft is planned out years in advance, and so "steering" it just a matter of commanding it to do the proper rocket burns at the proper times, and making tiny adjustments. And in fact, exactly the same thing happens in manned missions, except instead of commanding the onboard computer to do it, you just ask the pilot nicely. :)

We can track asteroids down to 50 km or so in size, and objects down to 1 cm in size in low Earth orbit. Of course, smaller, undetectable rocks are more numerous. However, even in the asteroid belt, they are spread out over such a large area that the likelihood of a collision with a rock large enough to end the mission is very, very small, and we must simply accept it as one of the risks of spaceflight.

Spacecraft are often hit by micrometeorites up to the size of a grain of sand. A few hits like this are considered just normal wear-and-tear. Another common source of trouble are hits by cosmic rays, which are high-energy radiation. These can cause small bugs in the on-board software, or, more rarely, damage electronics.

However, the must common reason for losing a mission is a component failure or human error. Spacecraft are lost when their engines blow up (like CONTOUR, which failed to phone home after a scheduled burn, and was later found along its planned orbit--in three pieces), when humans send them the wrong commands (like Mars Observer, where JPL thought that a subcontractor was giving them burns in metric units, and actually they were sending them in imperial units), or when a key system fails (like the landing system on Mars Polar Observer or the bad wiring in the oxygen tanks on Apollo 13). Space missions are extremely complicated, and the mission can be doomed by a tiny flaw in an any critical system, either in the design or from damage during launch or exposure to the temperature extremes and vacuum spacecraft must weather.

Often, spacecraft fail without any indication what went wrong. They just stop radioing back to Earth. Was there a problem with the software? Did a cosmic ray cripple the computer? Did a critical bit of hardware fail? Was it hit by a meteorite?

We can calculate the risk from the meteorite hits, and it is pretty low. Human mistakes and engineering failures, though, are much more unpredictable. :)

November 2003, Britt Scharringhausen (more by Britt Scharringhausen) (Like this Answer)

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