Space Debris or Orbital debris or Space junk or Space Waste is nothing but a collection of defunct objects in orbit around Earth. The space debris includes all the materials like spent rocket stages, old satellites, fragments from disintegration and collisions. As we know that orbits overlap with launch of new spacecrafts, the space debris may collide with the newly launched and operational spacecrafts.
There are more than 19,000 pieces of space debris larger than 5 cm. which are tracked and another 3,00,000 pieces smaller than 1 cm below 2000 Km altitude around the earth,. Most of the space debris is less than 1 cm. which include dust from solid rocket motors, paint flakes etc.
Safety from debris over 10 cm (3.9 in) comes from maneuvering a
spacecraft to avoid a collision. If a collision occurs, resulting
fragments over 1 kg (2.2 lb) can become an additional collision risk. Here are some important facts about Space debris.
What is Space Debris?
There are two types of Space debris - Natural and Artificial.
- Natural space debris consists of small pieces of cometary
and asteroidal material called meteoroids. We see these as
meteors when they travel through the Earth's atmosphere.
- Artificial space debris is any non-functional man-made object in
space (usually orbiting the Earth).
Where Does Artificial Space Debris Come From?
- Satellites which have reached the end of their life
- Failed Satellites and spacecrafts
- Rocket stages that have used to launch satellites into space
- Nose cones, payload covers, shrouds, bolts and other launch hardware
- Solid propellant slag
- Space activity cast-aways (accidental or deliberate), eg wrenches, human waste
- Deterioration fragments, eg peeling paint
- Fragments from exploding batteries, fuel tanks (not totally empty), etc
- Fragments from collisions, both accidental and deliberate
When was the First Piece of Artificial Space Debris Created?
The first piece of artificial space debris was created on 4th October, 1957 when the last stage of the rocket which launched Sputnik-1 remained in orbit.
Is Space Debris a Problem?
The main worry about space debris is a possible collision with active
or functioning satellites or spacecrafts.
How Likely is a Collision with Space Debris?
Prior to 1957, many astronomers warned that natural space debris
(ie meteoroids) would pose such a high collision risk that man's use
of space might be severely compromised. Fortunately, this turned out
to be not so. The collision probability from meteoroids is
very low, although not negligible. The larger the spacecraft and
the longer it remains in space, the greater the chance that it will
suffer a collision.
At the present time, only a handful of dangerous collisions with
artificial space debris are known to have occurred. However, the
production of this type of debris is increasing at such a rate that
it gives concern for the future.
What Other Problems are Due to Space Debris?
- Small pieces of space debris (less than 1/10 mm) are prolific enough
to cause erosion of optical surfaces. This is like sandblasting, and
can ruin telescope mirrors, and decrease the efficiency of solar cells.
- Particles such as paint flakes (under 1 mm) can cause small craters
in walls and windows. Almost 100 Space Shuttle windscreens have had to
be replaced (as of 2008) due to pits caused by such impacts.
What is the Minimum Size for Dangerous Damage?
It is believed that any fragment of space debris larger than 1 cm.
will penetrate the walls of existing satellites/spacecraft.
Why are Space Debris Impacts so Dangerous?
It may be surprising to know that a paint flake can cause a crater in a wind screen. Yes it us true. The reason is the velocity of space debris
impacts. A typical impact occurs at a closing velocity of 10 km/sec
or 36,000 Kms per hour!
Such impacts are called hyper velocity impacts to indicate their
extreme nature. At these velocities a piece of debris has more
kinetic energy (energy due to its motion) than an equal mass of
high explosive. An impact with a one kilogram object will thus
cause more damage than the explosion of one kilogram of TNT.
Can You Express a Space Debris Collision in Everyday Terms?
- A 2 mm space debris fragment colliding at 10 km/s is like being
hit with a cricket ball at 100 km/hour
- A 10 mm fragment at the same speed is like being hit by a large
motorbike at 100 km/hour
Why are Collision Velocities so High?
Velocities of objects in space are determined by the laws of physics
and the gravitational field of the body around which the objects orbit. An object has to move with a velocity of 7 Km per second around Earth to remain in the Earth orbit. The objects are in many different orbits around the Earth, some traveling in opposite direction to others. With these high velocities, it is natural that the collisional velocities will be very high.
In the case of natural space objects (meteoroids) that orbit around
the Sun, these travel at even greater velocities. To remain in its
orbit around the Sun, the Earth has to move at 30 km/sec. Other
objects that come near the Earth may have relative velocities
ranging anywhere from 11 km/sec to 72 km/sec. The average collisional
velocity between a meteoroid and a satellite is about 20 km/sec.
How Much Debris is in Space Now?
As of 2008 the number of pieces of artificial space debris in orbit
around the Earth is estimated to be:
| Size Range |
| Number of Fragments |
| 1 - 10 mm |
| 50,000,000 |
| 10 - 100 mm |
| 300,000 |
| > 100 mm |
| 12,000 |
Is Anything Being Done About Space Debris?
In June 2007 the United Nations General Assembly adopted a set of 7
orbital debris mitigation guidelines for member states (countries)
to follow. However, these are legally non-binding under international
law.
Most space-faring countries realise that space debris is a problem and
have their own programs to try and reduce the creation of more space
debris in future space activities. NASA has the most proactive
program in this area.
The problem is that these programs do not reduce the amount of debris
that is currently in orbit.
Can Satellites be Protected from Space Debris?
There are two ways that satellites and spacecraft may be protected from
orbital space debris impacts:
- Computer programs can search for possible collisions between large
space debris objects and high value spacecraft. When they detect the
likelihood of such a collision, the spacecraft is manuevred (by small
thruster rockets) out of harm's way. Such manuevres are now being
carried out for large spacecraft such as the International
Space Station and the Space Shuttle. However, these operations are
expensive and can disturb delicate experiments. Also, not every
satellite has the ability to maneuvre.
Space tracking networks can only track space objects larger than about
100 mm. Since even a 10 mm object can severely damage a satellite
it is obvious that collision avoidance will never be 100% effective.
- Debris shields can be designed to provide additional protection for
a spacecraft. One obvious way is simply to increase the thickness
of the vehicle walls. However, this adds a lot of mass to the craft
and makes it a lot more expensive to launch it into space.
Specially designed shields take advantage of the fact that two thin
walls separated by a space are more resistant to debris penetration
than a single thicker wall. This type of design is called a Whipple
shield after the astronomer Fred Whipple who came up with the idea
in the 1950's. The outer wall absorbs a lot of the debris energy so
that the inner wall is not punctured. This type of shield and modifications
to this design are currently installed on the International Space
Station. However, once again this does not offer 100% protection.
What About Debris Already in Orbit?
Although the probability of collision with a piece of orbital space
debris is currently very low, it will not remain so forever, even if
no more debris is put into orbit.
The reason for this is that collisions between objects in space will
slowly but surely increase the number of dangerous debris fragments.
These in turn will produce more debris and so on in a chain reaction.
In a few hundred years, the amount of debris will be so great that
space operations will be severely limited.
In 1989 science fiction writer Frederick Pohl predicted a similar situation
in his novel "Homegoing". Set many years in the future, mankind had
generated so much space debris that he was confined to the surface of
the Earth.
What can be Done to Remove Debris from Space?
A number of solutions have been proposed to this problem:
- Objects in low altitude orbits (below about 500 km) are affected
by atmospheric drag. This lowers their orbit until they re-enter
the atmosphere and are thus naturally removed from orbit. The
lower the orbit the faster it decays.
- Space "tugs" could be employed to "catch" large space debris objects
and either lower their altitudes for natural decay, or bring them
back to Earth.
- Giant "sponge" like objects could be deployed to "catch" or "soak up"
small debris pieces. After a time, the sponge would be removed from
orbit.
- Attach tails or tethers to large pieces of space debris to increase
the drag they experience and lower their orbits.
- Use large ground based lasers to "push" small pieces of debris
into lower orbits.
All of these schemes will be very expensive and use technology that is
still to be developed. The laser scheme looks as if it might be the
most promising. However, political problems might outweigh technical
problems.
Is Anyone Actively Involved in Debris Reduction?
There is currently no agency involved in active removal of existing space
debris from orbit.
However, an Australian firm based in Canberra (EOS) has been given a
Federal Government research grant to investigate laser removal of
space debris.
What Happens to Space Debris Reentering Earth's Atmosphere?
Most pieces of space debris will burn up as they enter the Earth's
atmosphere. This ablation process starts around a height of 100 km
and is usually complete by the time the object has descended to
about 20 km.
Very heavy or refractory pieces (and occasionally very light pieces)
may not burn up completely, and some part of the object may make it
down to the ground. However, even these objects have lost most of
their "space" velocity and hit the ground at no more than 100 metres
per second.
Are Returning Objects Hazardous?
It has been estimated that one piece of space debris may make it down
to the Earth's surface almost every day. Most of these are very small
and most will fall into the ocean or an unpopulated region of the Earth's
surface. Very few are ever recovered, much to the disappointment of
reentry scientists.
The probability of being hit by a piece of space debris is extremely
low and a lot less than the probability of your being hit by a car
while crossing a road.
Occasionally a large reentering derelict satellite may contain
hazardous cargo. Two cases of radioactive debris have actually
made it to the ground (these were from radioactive power sources).
Such reentries are known well in advance (although the exact point
of landfall can never be known precisely), and trained teams can
be sent to secure the site and ensure no danger to any people in
the area. Australia has a contingency plan set up to deal with
such an event.
Can Anything be Done to Reduce Reentry Risk?
If a satellite has maneuvring capability and still has remaining fuel
at the end of its life, it can be set up so that it reenters over a large
area of ocean. NASA carries out a reentry survivability analysis for
all large spacecraft, and if this indicates that a significant fraction of
the object may survive the reentry process, they will attempt to
control the reentry of such objects into a safe area.
If a large object is non-controllable, it may be possible to destroy
the object through collision with an anti satellite missile launched
from the ground. Such a collision produces many thousands of pieces
of space debris, each of which will burn up when it reenters the
atmosphere. However, as long as this is done at a sufficiently low
altitude (below 300 km), the additional space debris produced will
all decay from orbit (through atmospheric drag) within a month or
two. However, the cost of such an operation is very expensive,
and it could be argued whether this approach is worthwhile, considering
the very low probability of damage or injury.
How Long Does it Take for Space Debris to Decay?
By "decay" we mean a reduction in orbital height due to atmospheric drag.
The decay lifetime of a space object depends on its altitude, the level
of solar activity, and its mass to cross-sectional area.
Objects with a large mass to area ratio will remain in orbit longer as
they are less affected by drag. High solar activity increases the density of the atmosphere in low
Earth orbits and reduces satellites' decay lifetimes.
On the average a satellite in an initial 300 km high orbit will have
a decay lifetime of only a few months. One in a 500 km orbit has a
lifetime of around 10 years, and one at 1000 km altitude will stay in
orbit for thousands of years.
Are there any Other Future Problems with Space Debris?
Some astronomers have been worried about orbital space debris proliferation
because of two concerns:
- Increased numbers of medium to large scale space debris will cause
light trails across astronomical images, decreasing their scientific
and asthetic value. The trails also may confuse automatic computer
analysis of large numbers of images.
- Extremely large numbers of very small objects (fractions of a millimetre)
are expected to increase the background or ambient night-time sky glow, limiting the extent to
which astronomers can see faint objects (eg very distant galaxies).
