by Mark Garcia
September 26, 2013
Last Updated July 27, 2016
More than 500,000 pieces of debris, or "space junk," are tracked as
they orbit the Earth.
They all travel at speeds up to 17,500
mph, fast enough for a relatively small piece of orbital debris to
damage a satellite or a spacecraft.
The rising population of space debris increases the potential danger
to all space vehicles, but especially to
the International Space Station,
space shuttles and other spacecraft with humans aboard.
NASA takes the threat of collisions with space debris seriously and
has a long-standing set of guidelines on how to deal with each
potential collision threat.
These guidelines, part of a larger body
of decision-making aids known as flight rules, specify when the
expected proximity of a piece of debris increases the probability of
a collision enough that evasive action or other precautions to
ensure the safety of the crew are needed.
Space debris encompasses both natural (meteoroid) and artificial
Meteoroids are in orbit about the
sun, while most artificial debris is in orbit about the Earth.
Hence, the latter is more commonly referred to as
Orbital debris is any man-made object in orbit about the Earth which
no longer serves a useful function. Such debris includes
nonfunctional spacecraft, abandoned launch vehicle stages,
mission-related debris and fragmentation debris.
There are more than 20,000 pieces of debris larger than a softball
orbiting the Earth.
They travel at speeds up to 17,500 mph,
fast enough for a relatively small piece of orbital debris to damage
a satellite or a spacecraft. There are 500,000 pieces of debris
the size of a marble or larger. There are many millions of pieces
of debris that are so small they can't be tracked.
Even tiny paint flecks can damage a spacecraft when traveling at
In fact a number of space shuttle
windows have been replaced because of damage caused by material that
was analyzed and shown to be paint flecks.
"The greatest risk to space missions
comes from non-trackable debris," said Nicholas Johnson, NASA
chief scientist for orbital debris.
With so much orbital debris, there have
been surprisingly few disastrous collisions.
In 1996, a French satellite was hit and damaged by debris from a
French rocket that had exploded a decade earlier.
On Feb. 10, 2009, a defunct Russian satellite collided with and
destroyed a functioning U.S. Iridium commercial satellite. The
collision added more than 2,000 pieces of trackable debris to the
inventory of space junk.
China's 2007 anti-satellite test, which used a missile to destroy an
old weather satellite, added more than 3,000 pieces to the debris
The Department of Defense (DoD) maintains a highly accurate
satellite catalog on objects in Earth orbit that are larger than a
NASA and the DoD cooperate and share responsibilities for
characterizing the satellite (including orbital debris) environment.
Space Surveillance Network tracks
discrete objects as small as 2 inches (5 centimeters) in diameter in
low Earth orbit and about 1 yard (1 meter) in geosynchronous orbit.
Currently, about 15,000 officially
cataloged objects are still in orbit.
The total number of tracked objects
exceeds 21,000. Using special ground-based sensors and inspections
of returned satellite surfaces, NASA statistically determines the
extent of the population for objects less than 4 inches (10
centimeters) in diameter.
Collision risks are divided into three categories depending upon
size of threat.
For objects 4 inches (10
centimeters) and larger, conjunction assessments and
collision avoidance maneuvers are effective in countering
objects which can be tracked by the Space Surveillance
Objects smaller than this
usually are too small to track and too large to shield
Debris shields can be effective
in withstanding impacts of particles smaller than half an
inch (1 centimeter).
and Reacting to Debris
NASA has a set of long-standing guidelines that are used to assess
whether the threat of such a close pass is sufficient to warrant
evasive action or other precautions to ensure the safety of the
These guidelines essentially draw an imaginary box, known as
the "pizza box" because of its flat, rectangular shape, around the
space vehicle. This box is about a mile deep by 30 miles across by
30 miles long (1.5 x 50 x 50 kilometers), with the vehicle in the
When predictions indicate that the
debris will pass close enough for concern and the quality of the
tracking data is deemed sufficiently accurate, Mission Control
centers in Houston and Moscow work together to develop a prudent
course of action.
Sometimes these encounters are known well in advance and there is
time to move the station slightly, known as a "debris avoidance
maneuver" to keep the debris outside of the box.
Other times, the tracking data isn't
precise enough to warrant such a maneuver or the close pass isn't
identified in time to make the maneuver.
In those cases, the control centers may
agree that the best course of action is to move the crew into the
Soyuz spacecraft that are used to transport humans to and from the
This allows enough time to isolate those
spaceships from the station by closing hatches in the event of a
The crew would be able to leave the
station if the collision caused a loss of pressure in the
life-supporting module or damaged critical components. The Soyuz act
as lifeboats for crew members in the event of an emergency.
Mission Control also has the option of taking additional
precautions, such as closing hatches between some of the station's
modules, if the likelihood of a collision is great enough.
Spacecraft to Avoid Orbital Debris
NASA has a set of long-standing guidelines that are used to assess
whether the threat of a close approach of orbital debris to a
spacecraft is sufficient to warrant evasive action or precautions to
ensure the safety of the crew.
Debris avoidance maneuvers are planned when the probability of
collision from a conjunction reaches limits set in the space shuttle
and space station flight rules.
If the probability of collision is
greater than 1 in 100,000, a maneuver will be conducted if it will
not result in significant impact to mission objectives. If it is
greater than 1 in 10,000, a maneuver will be conducted unless it
will result in additional risk to the crew.
Debris avoidance maneuvers are usually small and occur from one to
several hours before the time of the conjunction. Debris avoidance
maneuvers with the shuttle can be planned and executed in a matter
Such maneuvers with the space station
require about 30 hours to plan and execute mainly due to the need to
use the station's Russian thrusters, or the propulsion systems on
one of the docked Russian or European spacecraft.
Several collision avoidance maneuvers with the shuttle and the
station have been conducted during the past 10 years.
NASA implemented the conjunction assessment and collision avoidance
process for human spaceflight beginning with shuttle mission STS-26
Before launch of the first element of
the International Space Station in 1998, NASA and DoD jointly
developed and implemented a more sophisticated and higher fidelity
conjunction assessment process for human spaceflight missions.
In 2005, NASA implemented a similar process for selected robotic
assets such as the
Earth Observation System satellites
low Earth orbit and
Tracking and Data Relay Satellite System
In 2007, NASA extended the conjunction assessment process to all
NASA maneuverable satellites within low Earth orbit and within 124
miles (200 kilometers) of geosynchronous orbit.
DoD's Joint Space Operations
is responsible for performing conjunction assessments for
all designated NASA space assets in accordance with an
established schedule (every eight hours for human
spaceflight vehicles and daily Monday through Friday for
JSpOC notifies NASA (Johnson
Space Center for human spaceflight and Goddard Space Flight
Center for robotic missions) of conjunctions which meet
JSpOC tasks the Space
Surveillance Network to collect additional tracking data on
a threat object to improve conjunction assessment accuracy.
NASA computes the probability of collision, based upon miss
distance and uncertainty provided by JSpOC.
Based upon specific flight rules
and detailed risk analysis, NASA decides if a collision
avoidance maneuver is necessary.
If a maneuver is required, NASA
provides planned post-maneuver orbital data to JSpOC for
screening of near-term conjunctions.
This process can be repeated if the
planned new orbit puts the NASA vehicle at risk of future collision
with the same or another space object.