Reentry Assessment – US Space Command Fact Sheet
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.
