From: United States Space Command
Posted: Thursday, March 1, 2001
Reentry Assessment describes the operational procedures by which USSPACECOM predicts the time and location of atmospheric reentry (not ground impact) of decaying man-made objects in space.
Space Surveillance is one component of USSPACECOM's Space Control mission. The Space Control mission is to ensure our ability to access space, to ensure our freedom of action within the space medium, and to ensure an ability to deny others the use of space, if required. Space Surveillance involves detecting, tracking, cataloging and identifying man-made objects orbiting Earth. Reentry Assessment provides a means of predicting when and where a decaying object will re-enter the Earth's atmosphere, and so avoid triggering a false alarm in missile-attack warning sensors of the United Sates and other countries.
The 1st Command and Control Squadron (1 CACS) of the U.S. Air Force Space Command, located insideCheyenne Mountain <Air Force Station in Colorado Springs, is responsible for tracking objects larger than ten centimeters orbiting Earth. Five eleven-person crews work around the clock, 365 days a year, to constantly track these objects. They task the Space Surveillance Network, a worldwide network of 19 space surveillance sensors (radar and optical telescopes, both military and civilian) to observe the objects. Then the crews use computers within the Cheyenne Mountain complex to match sensor observations to the more than 8300 orbiting objects, and update the position of each one. These updates form the Space Catalog, a comprehensive listing of the numbers, types, and orbits of man-made objects in space.
U.S. Space Command does not make landfall predictions. Current capabilities and procedures give us a limited ability to predict within a 30 minute, 6000-mile window when and where a particular object will re-enter the Earth's upper atmosphere.
Objects are tracked throughout their orbit life, with the results posted in the Space Catalog. When an object appears to be re-entering within seven days, orbital analysts in the Space Control Center (SCC) will increase sensor tasking (monitoring) and begin to project a specific re-entry time and location. At the four-day point, a monitor run is accomplished once a shift or three times a day. Messages indicating the calculated re-entry time and location are transmitted to forward users and customers (e.g., sensor operators that will be tracking, the Federal Emergency Management Agency, the U.S. Air Force's 14th Air Force, etc) at the four-, three-, two- and one-day points. Starting at the 24-hour point, the object is monitored at the highest level of scrutiny, with processing at the twelve, six and two-hour points. Again, ground traces and messages are transmitted. The object is monitored throughout re-entry.
The graphic above depicts the reentry of a
typical satellite in a Low-Earth orbit.
Figures may change depending on the orbital characteristics of space vehicles.
Re-entry Assessment is an "inexact science." It is virtually impossible to precisely predict where and when space debris will impact. This is due to limitations in our tracking system as well as environmental factors that impact on the debris. Most of our satellite tracking radar are located in the Northern Hemisphere, making continuous orbit coverage impossible. Consequently, a returning satellite could be outside sensor coverage for several hours.
Environmental factors acting on an object's orbit could include variations in the gravitational field of the land mass and ocean areas, solar radiation pressure and atmospheric drag. (Objects re-entering may skip off the Earth's atmosphere, much as a stone skipped across a pond, causing it to impact much further away than originally forecast.) Consequently, USSPACECOM does not give warning to civilian populations on point of impact for reentering objects.
We verify that an object has re-entered by three "No Show" sensor reports verifying the object is no longer in orbit. Once it is determined not to be in orbit, sensor tasking ends and the object is deleted from the "Active " catalog. The object remains in the inactive catalog for historical purposes.
The chances of someone being struck by a re-entering object are slight. The great majority of objects that re-enter disintegrate due to the intense heat created by re-entry into the Earth's atmosphere. Only a small percentage of objects ever re-enter over land since water comprises 75% of the Earth's surface. Only about 25% of the Earth's landmass is actually inhabited.
We know of no case in which space debris has been linked to personal injury.
Since tracking began with Sputnik, more than 17,000 man-made objects that U.S. Space Command tracked have re-entered the Earth's atmosphere. There are more than 8,300 objects currently orbiting the Earth. U.S. Space Command has tracked approximately 26,000 objects in its space catalog.
We do not maintain data on objects once they reenter the earth's atmosphere, and have no knowledge of the number of objects that might have survived reentry. Unless an object is actually found and returned to NASA or any other agency, we would have no knowledge of whether or not an object survived re-entry. For example, we know of two instances where objects have survived reentry. The first object is a small piece of the Lunar Module from Apollo 5. (Catalogue # 3107, International Designator 68-007B) It was launched on Jan. 22, 1968 and recovered in a farmer's field in Colombia on Feb. 12, 1968. The second object is a piece of a Soviet Gas Bottle from COSMOS 482. (Catalogue #5921, International Designator 72-036C) It was launched on Mar. 31, 1972 and recovered Apr. 2, 1972 from a farmer's field in New Zealand. Both objects are on display in the Space Control Center in Cheyenne Mountain.
// end //