[chris (OP)]

Zachary is wondering:

How do astronauts on the international space station (ISS) know when a object is coming toward  then when they can not use sonar and radar because they need something to ripple in?

What do you think?

[Colin2B]

Sonar is sound waves which require air or water to 'ripple' in.
Radar doesn't need anything to ripple in and will work in hard vacuum, however, I don't believe ISS has any proximity or foreign object detection systems.

[SeanB]

They rely on the US satellite tracking system, that uses a few large ground based radar units to get information on all the objects over 10cm in diameter in orbit, which is stored as a time, position and speed, so you can calculate position of an object within a block of space with a fair degree of accuracy. This then is used as a computer simulation of objects that approach within 10km of the ISS, and these objects that are predicted to approach within these limits are then considered for further orbital path determination, so that they can get a better accuracy of the orbit.

These predictions are not too accurate long term, but are good for around 24 hours as a rough test if something will be in the same place as the ISS at the same time, so that they can plan if it will miss, will come close ( the accuracy is not that good to say it will hit) and thus they either will move the station or simply put the crew in safe confinement in the Soyuz capsules in case it hits.

In general there are multiple hits per orbit, mostly small flakes of paint from older missions, tiny things that were released in use as the plastics in spacecraft sunshields disintegrate and other stuff. Speed and relative energy of these particles varies from almost zero for stuff in the same orbit and for stuff shed from the ISS and the supply craft, to almost orbital velocity of 22kps for stuff in orthogonal orbits, which can cause some damage if they hit, but where the actual particle is very small, so only makes a scratch or a tiny dent.

But no, there is no detection on the ISS, just a lot of work on the ground.