Today, more than 8,000 man-made items circle the globe
in space. Only some seven percent of these space objects
are active satellites. The remainder, called debris,
comprises inactive satellites, spent rocket bodies,
and pieces that have broken off from spacecraft. Much
of the debris reenters the atmosphere and burns up.
Some falls back to Earth. Other debris can and does
strike satellites and space shuttles.
Dots shown here are not drawn to scale. In this rendering,
each appears to be about the size of Rhode Island.
US space surveillance units currently track 8,168 space
objects that range in size from that of a baseball
to the Mir space station. An inactive satellite can
weigh several tons, while pieces of rocket bodies may
weigh only ten pounds. Smaller fragments about the
size of paint chips have nicked windows on the space
shuttle but are too small to track with present-day
sensors.
The US started tracking satellites and debris in 1957,
when the USSR launched Sputnik 1. Since then, the US
Space Surveillance Network (SSN), now numbering twenty
sites around the world, has tracked more than 23,700
space objects.
Eighty-four percent of space debris can be found approximately
800 kilometers out, well beyond the space shuttle's
orbit of 300 kilometers above Earth, according to US
Space Command officials. Under current conditions,
they say, the shuttle is likely to collide with a significant
piece of debris no more than once in 10,000 years.
Nevertheless, USSPACECOM's Space Control Center (SCC),
Cheyenne Mountain AS, Colo., uses powerful computers
to process 70,000 observations of space objects daily.
Air Force Space Command's 1st Command and Control Squadron,
located within the SCC, compiles and analyzes the data,
creating a running catalog.
During space shuttle missions, the SCC and the squadron
calculate the possibility of objects in orbit passing
close to the shuttle's planned flight path. The center
notifies NASA about any space debris that may come
within twenty-five kilometers of the shuttle. The catalog
also provides data for collision avoidance as new satellites
are launched.
The cataloging process also ensures that the US and
other countries don't mistake falling debris for evidence
of an unfolding missile attack. To missile-warning
radars, space debris returning through Earth's atmosphere
gives the appearance of a live reentry vehicle. By
knowing where each significant piece is, and by predicting
its fall, USSPACECOM can provide data to prevent a
false alarm in missile-attack warning sensors of the
US and other countries.
The lion's share of the space surveillance work load
falls to the forty-six-member 1st Command and Control
Squadron, which manages the tracking duties of USAF's
twenty worldwide SSN radar and sensor sites. It maintains
positional data on more than 97.3 percent of the man-made
objects in orbit.
