======================================================================== Visual Satellite Observing F A Q Chapter-11 Miscellaneous Questions ======================================================================== ---- 11.0 Miscellaneous Questions ---- 11.1 Have satellites ever collided in orbit? How likely is it? Yes - maybe. It has been reported that the Cosmos 954 accident in 1978 may have been caused by a collision in space, though it is not known if the collision was with a man-made or natural object. See Shanebrook's report, "Treaty Needed to Counter Satellite Reactor Threat", available on the World Wide Web at: http://tardis.union.edu/me_dept/treaty1.html The probability of collision is quite small, though increasing with each new launch. The greatest problem lies in the geosynchronous belt of satellites. Except for the few satellites at the stable positions (75 degrees and 225 degrees longitude), all satellites with near-zero eccentricity and near-zero latitude, MUST be periodically boosted back to their position. This boosting is done with on-board fuel. All geostationary satellites carry a final reserve fuel load intended to boost the spacecraft to supersynchronous orbit when the satellite reaches its end of life. If the boost failed, the satellite would become an uncontrolled object, drifting toward one or the other of the stable longitudinal points. If conditions are right, the dead satellite will impact another geosynchronous satellite, creating debris in the process. It could get ugly. There have been several *intentional* impacts. In 1984, a missile launched from Kwajalein Atoll intercepted a Minuteman ICBM at more than 100 miles altitude destroying the Minuteman payload by unfolding a wire-mesh "umbrella". This impact certainly created debris which reentered the atmosphere relatively quickly. The United States Air Force "shot down" the DARPA P78-1 science satellite in 1985 at 556 km altitude using an ASAT rocket launched from an F-15 aircraft. (The satellite was still being monitored by scientists!) Since 1967, Russia has refined a satellite interceptor system. The intercept satellite makes orbit adjustments to "catch" another satellite until the distance between them is quite small. The intercept satellite then explodes, showering the target satellite with metal shards, "killing" the target satellite. This system was flight tested throughout the late 1960's and 1970's and presumably still exists. ---- 11.2 Is "Space Junk" a problem? Yes! What with antisatellite tests, humankind working outside spacecraft on Mir and the Shuttle Orbiter (both creating debris and vulnerable to debris), and the unavoidable flux of debris from the process of putting a spacecraft in orbit, space debris is a big problem. Debris in LEO essentially decays in a relatively short time -- days to months. But higher altitude debris can remain in orbit for decades or centuries. Estimates vary, but there is in excess of 3 million kilograms (8 million pounds) of dead spacecraft, spent rocket bodies and debris in orbit -- somewhere around 5 million individual pieces. The United States carefully attempts to locate and track every piece of space junk detectable, but objects smaller than a couple of centimeters across simply cannot be detected and tracked. It is very important to know where space junk and debris is orbiting and how long it will be present. No owner of a multi-million dollar satellite wants to launch his/her spacecraft only to impact with a piece of junk! The first known *unexpected/accidental* collision between a satellite and a piece of space junk occurred on July 24, 1996. The CERISE MICROSATELLITE (France) collided with a piece of space junk sending the CERISE tumbling. The impact apparently bent or severed a stabilization boom extending 6 m (18 ft) from the CERISE body. The spacecraft was restabilized and apparently suffered little loss of function, since on-board computers were programmed to compensate for the loss of the boom. ---- 11.2.1 Does space debris pose a risk to astronauts? Yes! Despite the serene appearance on TV video of astronauts in orbit, they are actually traveling in the neighborhood of 32,000 km/hr (20,000 mi/hr). Imagine impacting a bolt or sliver of metal traveling at 32,000 km/hr in the opposite direction! Can anything be done about it? Only by attempting to control the release of debris. Though much debris is release on every launch, only a tiny percentage actually achieves orbit, the bulk of it decays fairly quickly and burns up in the atmosphere. Nations must continue to thoroughly prepare launch vehicles to reduce/prevent extraneous matter from reaching orbit. Humans in space must be diligent not to release objects which become debris. ---- 11.3 Can a decaying or reentering satellite hit the ground? Yes! At least three have in recent years. Skylab decayed July 11, 1979 and spread debris in the Indian Ocean and across parts of Australia. Pieces as large as 373 kg (1,000 pounds) were recovered. No one was hurt! Cosmos 954 decayed in 1978 and dropped charred parts over some 78,000 sq km (30,000 sq mi) in Canada. The Chinese FSW-1 decayed in March 9, 1996, and is thought to have had parts survive to reach the ocean. It will happen again. ---- 11.4 Do reentering satellites ever spread radioactive material? Unfortunately, yes. There are some 60 known spacecraft carrying nuclear power sources launched by the United States and the former Soviet Union. Much of the debris from Cosmos 954, mentioned above, was radioactive material. The Galileo spacecraft carried aboard the Space Shuttle in 1989 carries an RTG (Radioisotope Thermoelectric Generator). It successfully achieved solar orbit and went on to Jupiter. A nuclear- powered navigation satellite launched by the USA in 1964 was not so lucky and failed to achieve orbit and is thought to have broken up and spread its radioactive materials into the atmosphere. In 1983, Cosmos 1402, carrying a nuclear reactor (RTG) disintegrated in the upper atmosphere over the South Atlantic Ocean and probably spread radioactive debris into the atmosphere. ---- 11.5 Can spy satellites actually read automobile license plates? Probably not. They most certainly can tell how many people are standing around a car, and perhaps what type of car it is, but actually reading the license plate is not an easy task. Look at the physics: Say the telescope has a 1 m (39 inch) primary and is orbiting 322 km (200 miles) altitude. The theoretical resolution of the telescope (i.e. the best possible) is 0.114 arcseconds (at visible wavelengths). At 322 km, the 7.6 cm (3 inch) tall license plate characters subtend an angle of about 0.048 arcseconds - less than half the size needed to be resolved. A spy satellite, under the *best* of conditions could tell the car *has* a license plate, but given the license plate is most likely being viewed obliquely, and probably at a range *greater* than the satellite's altitude and looking though the atmosphere, one quickly determines spy satellites cannot resolve a license plate. ======================================================================== This FAQ was written by members of the SeeSat-L mailing list, which is devoted to visual satellite observation. Members of this group also maintain a World Wide Web site. The home page can be found at the URL: http://www.satobs.org/ The information on the VSOHP web site is much more dynamic than that found in this FAQ. For example, the VSOHP site contains current satellite visibility and decay predictions, as well as information about current and upcoming Space Shuttle missions and Mir dockings. The VSOHP site also contains many images, equations, and data/program files that could not be included in this FAQ while maintaining its plain text form. This FAQ and the VSOHP web site are maintained asynchronously, but an effort is made to synchronize information contents as much as possible. The material in this FAQ chapter was last updated in February 1998. ========================================================================