ISS is at an altitude of about 400 km (approximately). But there will be some of space debris at that altitude (since some of the space debris orbits decay over time). So the debris has a probability of coming in contact (colliding) with the ISS. So, how does the ISS avoid those collisions?
I will assume you are asking about collision risk management. As usual, it comes in several guises:
See this reference: http://www.nasa.gov/mission_pages/station/news/orbital_debris.html
The key assets in Collision Avoidance are:
- the telescopes and radars and satellites in the U.S. Department of Defense Space Surveillance Network that help in detecting, classifying and estimating orbital parameters of space debris
- computers in DoD’s Joint Space Operations Center cranking through the large volumes of data obtained by the surveillance network and identifying dangerous stuff up there
- computers, qualified personnel and procedures at NASA Mission Control Center-Houston
- counterparts of the above in Russia (MCC-M, MSIC)
When Debris Avoidance Maneuvers cannot be performed due to late detection of the threat, risk avoidance procedures force the ISS crew to go for the boats (err, Soyuz vehicles).
Mitigation comes into play after the collision. As discussed elsewhere (Are there any safety procedures in place on the ISS in case of puncture?), if there is time to isolate leaking compartments, the crew may do so. However, repairing the station is considered to be the job of followup expeditions.
Broadly speaking, there are three other possible solutions to the problem that have not been implemented on the ISS:
- Prevention of debris generation (by responsible design)
- Debris collection
- Active defense (with kinetic interceptors or laser ablation)
A hat tip to osgx for references on ideas on installing a debris detection telescope and a laser on the ISS:
- Toshikazu Ebisuzaki, Mark N. Quinn, Satoshi Wada et al. Demonstration designs for the remediation of space debris from the International Space Station. Acta Astronautica, July 2015. DOI 10.1016/j.actaastro.2015.03.004
- Wolfgang O. Schall. Laser requirements for the removal of space debris from orbit. Proceedings of SPIE 3574 (1998) pp. 426–436. DOI 10.2514/2.3785 - the first paper proposing an ISS-based laser.