With over 9000 cataloged (known) objects in Earth orbit, sooner or later there will be close encounters (or even collisions) between two satellites.
The first publicly documented collision between two cataloged space objects occurred in July of 1996. A French military micro-satellite called Cerise ( International Designation 1995-033B / Satellite Catalog No. 23606) suddenly lost stability when it appeared that its stabilization boom was impacted. After analysis, it was concluded that the possible culprit was a piece of space debris from an Ariane booster (1986-019RF/18208). Controllers were able to reprogram the payload and regain attitude control.
Several news releases on Aug 12, 1997 noted that the CRISTA-SPAS satellite, flying free of STS-85, passed very close to an old rocket motor launched by the shuttle in 1984 (the payload, a Com sat, failed to reach the desired geo-synchronous orbit). The satellite in question was not identified in the news releases. A search of satellites from that shuttle mission indicated the objects in question were from the shuttle Challenger, launched in February of 1984.
Doyle Groves (a SeeSat-L subscriber) using a PC program that can calculate satellite encounters (called COLA) posted a message on SeeSat-L indicating that the unidentified satellite in question was most likely the PAM-D rocket body (14694/1984-011F). The close encounter occurred at 00:40:30.3 UTC on Aug 12 , 1997 at an approximate range of 3.1 km. The encounter occurred just west of Baha California (Mexico).
Jeremy Ollerenshaw ( a SeeSat-L subscriber), again using COLA, posted a message on SeeSat-L indicating that Mir was to have a close pass on Sept 15, 1997 at 20:33 UTC with a US sat MSTI-2 (23101/1994-028A). ABC News later reported on the close encounter (~500 meters) which took place over northern Japan. A NASA spokesperson said that the encounter and precautions taken by the Mir crew were not unusual and occur frequently on a monthly basis. The news report stated this was the closest encounter in the 11 year Mir history.
Rob Matson (a subscriber to SeeSat-L) reported on the expected close-encounter of Mir with another satellite rocket body on July 28, 1999 at 18:24:54 UT.
The USSPACECOM tracks all cataloged satellites for possible collision with the shuttle,ISS, and US government maneuvering sats. If it appears an object may come too close to the shuttle, then the shuttle maneuvers to avoid the object. This has happened with several missions (STS-44, 48, 53, 72, 82) in which at least 5 maneuvers were required.
The first known attempts to maneuver the ISS are documented in the SeeSat-L archives here and here.
The orbit of the ISS was successfully raised for the first time October 26, 1999 to avoid a possible close encounter with another satellite rocket body.
On March 28, 2002 at 20:44 UTC the ISS and a Delta 2 rocket body passed within 14 km of each other over New Guinea. It was decided beforehand that it would not be necessary to maneuver the ISS.
The Russians have their own tracking systems that include both radars and optical telescopes. Supposedly, Mir had never maneuvered to increase the distance between it and a possible encounter object. When a close encounter was predicted, all the Mir crew retired to the Soyuz vessel until the encounter time period passed. Of course, Mir had to be periodically maneuvered (usually by an attached Progress vessel) to maintain its desired altitude. Atmospheric drag slowly lowered the altitude of Mir.
The problem arises with those objects that can not be tracked because of their small size. Debris larger than 1 cm in diameter present a catastrophic hazard to orbiting payloads. In addition, there is no known shielding material available for debris of this type. It's believed, only the high powered, narrow angle field of view tracking radars can track objects down to 1 cm in diameter, up to altitudes of a 1000 km.
An example of how much kinetic energy can be obtained by a small amount of mass at orbital velocity follows. Aluminum has a density of about 2.66 grams/cubic-cm. A 1-cm diameter sphere of aluminum has a volume of 0.5236 cubic-cm, thus a mass of about 1.4 grams. Assuming a collision velocity of 10 km/sec (NASA says the average is 16 km/sec), the kinetic energy of the collision would be:
0.5 x (1.4x10^-3 kg) x (1.0x10^4 m/sec)^2 =70,000 kg-m^2/sec^=70 kJoules
This is the same energy as a 16-lb (7.3 kg) bowling ball impacting at 312 mph (520 km/hr).
More US optical tracking systems with higher detector resolution are being put into service to supplement the wide angle field of view radars that may have a resolution to detect objects tens of centimeters in diameter. The USAF GEODSS (Ground based Electro-Optical Deep Space Surveillance system) has large optical telescopes at Diego Garcia in the Indian Ocean, Maui (Hawaii) and Socorro (New Mexico).
The shuttle returns from every mission with minor damage caused by untracked man-made objects (debris) or natural space-born objects (micrometeoroids). Most of the damage has occurred on the thermal tiles. Damage has occurred on eight external panes of the multi-layer viewing windows for the shuttle vehicles warranting replacement of those external panes.
STS-70 (July 1995) took a hit on window #6 on the forward right side, nearest the pilots station. The ding was about 1/16 inch in size and occurred as the shuttle was in a wing velocity vector mode. It probably took place while the crew were asleep as they noticed it after they woke up. A photo (177k) originating from NASA-JSC clearly shows the impact area on the removed window.
Whenever the shuttle isn't required to be in a certain attitude, it will orbit with the tail section facing toward the direction of flight so as to minimize damage to critical components.
Further information on space debris can be found at:
Links: to the VSO Home Page, observing guide, satellite predictions.