Time to Clear Away Space Junk

Although the U.S. space shuttle always gives the impression of orbiting calmly in space, the truth is somewhat different. When in space, the shuttle has to constantly check on the orbits of space debris that might hit it. Such junk floating around in space would cause enormous damage.

To avoid collisions with space debris, space shuttle missions follow a basic rule: If the probability of colliding with debris with a diameter of 10 centimeters or more exceeds 1/10,000 over the subsequent 36 hours, the shuttle's orbit must be changed.

Shuttles have now made more than 100 flights and on several occasions, they have been forced to change their orbits to avoid such collisions.

The items of debris include the obsolete remains of artificial satellites and rocket parts jettisoned from spacecraft.

It is estimated that about 2,000 tons of space junk exists at altitudes of up to 2,000 kilometers. Space debris is especially concentrated in the orbit used by stationary satellites--at an altitude of 36,000 kilometers--or in the orbit used by satellites for global positioning systems--an altitude of 20,000 kilometers--or where there are observation satellites--an altitude of 1,000 kilometers

Scientists are concerned about the destructive power of space junk. The power is determined by the mass and speed of the debris. Such fragments of junk often travel at about eight kilometers per second--20 times faster than the speed of a bullet.

"The destructive power of a three-millimeter fragment of aluminum debris is equivalent to that of a bowling ball moving at a speed of 100 kph, Seishiro Kibe of the National Aerospace Laboratory said.

The U.S. space shuttle is fitted with eight windows of tempered glass. However, during each mission one window, on average, gets cracked and needs to be replaced. The cracks are believed to be made by tiny fragments of space debris with a diameter of only a few millimeters.

In 1996, a French military satellite was struck by a piece of broken rocket with a diameter measured in tens of centimeters. In the accident, the satellite's altitude control system was damaged despite its robust design. Obviously, the accident caused great concern among experts in military and space development around the world.

Space debris varies in size. Scientists estimate there are about 8,900 fragments of debris with diameters of 10 centimeters or more.

The North American Aerospace Defense Command, which used to be deployed for monitoring the nuclear missiles of the former Soviet Union during the Cold War, is currently monitoring the orbits of space debris.

It is estimated that there are 35 million fragments of space debris with diameters between one millimeter and one centimeter and 100,000 fragments between one centimeter and 10 centimeters.

The international space station, which is being jointly built by 16 countries, including Japan, the United States and Russia, is scheduled to be completed in 2006. Scientists believe the station, with its altitude of 400 kilometers, will also be threatened by collisions with space debris several times a year.

In response to such a scenario, scientists improved the strength of the station's outer shell design after conducting experiments involving high-speed collisions of objects.

As a result, the station is built with a multilayered structure that includes a layer of aluminum plates and reinforced fiber. The station is believed to be capable of containing the impact of space debris up to a diameter of about one centimeter.

But, critics have pointed out that the station itself, with a total weight of 450 tons, may become the largest piece of space junk in the future. Kibe said, "Now we have to pay the price for not considering methods for the decommissioning of artificial satellites."

The amount of space debris is still increasing at a rate of 3 percent to 5 percent a year.

'Cleanup satellite'

Amid concern over the danger of space junk, scientists have been trying to develop a satellite that will dispose of the debris.

They plan to install robot arms on the "cleaning satellite." According to the scientists, the arms will be able to capture decommissioned satellites in space and tow them into orbits that will not affect other active satellites. Another planned method of disposing of obsolete satellites is to force them down into the atmosphere so that they burn up.

Cleaning satellites are being developed at several institutes including the U.S. National Aeronautics and Space Administration and the National Aerospace Laboratory in Japan. The organizations hope to have developed the satellites by 2010.

However, the retrieval of space junk is considered extremely difficult. Fragments of debris are not only moving at high speed; in many cases they are rotating at the same time.

If the cleaning satellite approaches space debris without adequate preparation, it might be damaged by the debris. The satellite, therefore needs a robust design and a highly accurate system for controlling its altitude and thrust.

Scientists are also working on measures to prevent the proliferation of space debris. Specifically, they hope to come up with designs for constructing satellites that will avoid the later scattering of small pins and bolts in orbit.

They also hope to develop a system in which satellites are programmed to automatically enter the atmosphere after they finish their missions.

Space development authorities in several countries, including Japan, the United States, Russia and France, have compiled manuals to prevent the production of space junk. However, none of them are legally binding.

The reality is that such preventive measures are not popular because they increase the cost of developing satellites.

In February last year, at a U.N. committee meeting on science and technology, Japan proposed the setting up of a three-year-long project to compile international rules for preventing space debris.

At the meeting, Hiroyuki Konno of the National Space Development Agency of Japan (NASDA) said, "Unless (the proposal) is approved by all countries acting together in the same way they handle environmental issues of the Earth itself, the proposal will be meaningless."

Nevertheless, a large number of countries were reluctant to approve the proposal out of fears it would lead to an increase in the costs and manpower required for space development.

NASDA, however, has yet to give up on its proposal to compile international rules on space debris and is working to obtain approval from as many countries as possible.

Meanwhile, Japan Space Forum, an organization conducting research on space, has been building an observatory in Biseicho, Okayama Prefecture, that will become the world's first observatory designed for monitoring space junk.

In the autumn, the observatory will be equipped with three types of optical telescopes, varying in aperture size from 25 centimeters to one meter. The telescopes are to be used for monitoring space debris in stationary orbit at altitudes of 36,000 kilometers.

In 2004, a radar system will be installed in Kamisaibarason in the prefecture to monitor space debris occupying orbits up to an altitude of 1,000 kilometers.

The facilities are not only expected to monitor space debris produced by Japanese satellites but also to provide more information on such space junk for other countries