Space surveillance is a critical part of USSPACECOM's mission and involves detecting, tracking, cataloging and identifying man-made objects orbiting Earth, i.e. active/inactive satellites, spent rocket bodies, or fragmentation debris. Space surveillance accomplishes the following:
- Predict when and where a decaying space object will re-enter the Earth's atmosphere;
- Prevent a returning space object, which to radar looks like a missile, from triggering a false alarm in missile-attack warning sensors of the U.S. and other countries;
- Chart the present position of space objects and plot their anticipated orbital paths;
- Detect new man-made objects in space;
- Produce a running catalog of man-made space objects;
- Determine which country owns a re-entering space object;
- Inform NASA whether or not objects may interfere with the space shuttle and Russian Mir space station orbits.
The command accomplishes these tasks through its Space Surveillance Network (SSN) of U.S. Army, Navy and Air Force operated, ground-based radar's and optical sensors at 25 sites worldwide.
Space Surveillance Network
The SSN has been tracking space objects since 1957 when the Soviets opened the space age with the launch of Sputnik I. Since then, the SSN has tracked more than 24,500 space objects orbiting Earth. Of that number, the SSN currently tracks more than 8,000 orbiting objects. The rest have re-entered Earth's turbulent atmosphere and disintegrated, or survived re-enty and impacted the Earth. The space objects now orbiting Earth range from satellites weighting several tons to pieces of spent rocket bodies weighing only 10 pounds. About seven percent of the space objects are operational satellites, the rest are debris. USSPACECOM is primarily interested in the active satellites, but also tracks space debris. The SSN tracks space objects which are 10 centimeters in diameter (baseball size) or larger.
The SSN uses a "predictive" technique to monitor space objects, i.e., it spot checks them rather than tracking them continually. This technique is used because of the limits of the SSN (number of sensors, geographic distribution, capability, and availability). Below is a brief description of each type of sensor.
Phased-array radars can maintain tracks on multiple satellites simultaneously and scan large areas of space in a fraction of a second. These radar's have no moving mechanical parts to limit the speed of the radar scan - the radar energy is steered electronically.
Conventional radars use immobile detection and tracking antennas. The detection antenna transmits radar energy into space in the shape of a large fan. When a satellite intersects the fan the energy is reflected back to the antenna, triggering the tracking antenna. The tracking antenna, then, locks its narrow beam of energy on the target and follows it in order to establish orbital data.
The Ground-Based Electro-Optical Deep Space Surveillance System (GEODSS) consists of three telescope sensors linked to a video camera. The video cameras feed their space pictures into a nearby computer which drives a display scope. The image is transposed into electrical impulses and recorded on magnetic tape. This is the same process used by video cameras. Thus, the image can be recorded and analyzed in real-time.
Combined, these types of sensors make up to 80,000 satellite observations each day. This enormous amount of data comes from SSN sites such as Maui, Hawaii; Eglin, Florida; Thule, Greenland; and Diego Garcia, Indian Ocean. The data is transmitted directly to USSPACECOM's Space Control Center (SCC) via satellite, ground wire, microwave and phone. Every available means of communications is used to ensure a backup is readily available if necessary.
Space Control Center
The SCC in Cheyenne Mountain Air Station is the terminus for the SSN's abundant and steady flow of information. The SCC houses large, powerful computers to process SSN information and accomplish the space surveillance and space control missions.
The NAVSPACECOM provides the site and personnel for the Alternate SCC (ASCC). The ASCC would take over all operations in the event the SCC could not function. This capability is exercised frequently.
Orbital Space Debris
USSPACECOM tracks about 8,000 man-made space objects, baseball-size and larger, orbiting Earth. The space objects consist of active/inactive satellites, spent rocket bodies, or fragmentation. About seven percent are operational satellites, 15 percent are rocket bodies, and about 78 percent are fragmentation and inactive satellites.
Most debris (about 84 percent) is out approximately 800 kilometers - roughly twice the normal altitude of the space shuttle which orbits at about 300 kilometers. Only a small amount of debris exists where the shuttle orbits.
The likelihood of a significant collision between a piece of debris (10 centimeters or larger) and the shuttle is extremely remote. The statistical estimate is one chance in 10,000 years, in the worst case. The probability is higher for objects smaller-than-baseball size which currently cannot be tracked with available sensors.
Although 8,000 space objects seems like a large number, in the 800 kilometer band there are normally only three or four items in an area roughly equivalent to the airspace over the continental U.S. up to an altitude of 30,000 feet. Therefore, the likelihood of collision between objects is very small.
Through the SSN, the command tracks and catalogs all space objects orbiting Earth which are 10 centimeters or larger. During shuttle missions, the center computes possible close approaches of other orbiting objects with the shuttle's flight path. NASA is also advised of space objects which come within a safety box that measures 10 by 10 by 50 kilometers of the orbiter.
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