China's first successful manned space flight received front page coverage worldwide, but little was reported about the group of scientists who kept the launch site at Jiuquan in Northwest China informed of debris in the atmosphere to ensure the safety of the expedition.
The Space Environment Prediction Centre under the Chinese Academy of Sciences (CAS) began offering this "special service" in early July, several months before Shenzhou V was launched on October 15. According to Liu Jing, a researcher with the center, the amount of debris left behind by man's previous endeavors in space is severe in the Low Earth Orbit (LEO). "Debris of less than 10 centimeters has a much greater density 300-400 kilometers above the Earth," Liu says. "Orbital debris, large or small, poses a risk not only to human activities in space but also to people on Earth should the detritus eventually fall from the sky. And collisions with floating rubbish can be disastrous to spacecraft and satellites."
The three-module Shenzhou V was launched by a Long March-2F rocket into an elliptic orbit of 200 km perigee and 350 km apogee. After circling the earth nine times, it shifted into a round orbit of 343 km. As planned, after a 21-hour flight in which the spacecraft passed round the Earth 14 times, the capsule carrying China's first astronaut returned to terra firma in a perfect soft landing in Inner Mongolia. The orbital module was left in space, where it will continue to operate for six months as a scientific satellite.
'We tried our best'
The CAS Space Environment Prediction Center was established in 1992. Scientists there say that orbital debris is an important environmental factor affecting the timing of the launch of any spacecraft, as well as its orbital path, ability to maneuver, orbital transfer and recovery. "We tried our best to accomplish this special task of providing data on the orbital debris," says Du Heng, 68, a CAS researcher and China's chief space debris research scientist. "We scrutinized and screened the relevant data in minute detail, and we ensured the supply of real-time information to the command center’s launch and elliptic orbit sections."
According to the scientist, the group's mission is far from over. "As the orbital module (of Shenzhou V) is still in the orbit, we are working to ensure that it accomplishes its tasks without a hitch."
Du Heng admits that Chinese scientists are still unable to measure debris smaller than 10 centimeters. "Despite that," he says, "we do have the ability to forecast changes in the orbital atmosphere based on what we have gathered about the orbital elements of larger debris."
Since the first man-made earth satellite "Sputnik" was launched by the former Soviet Union in 1957, Earth's atmosphere has become essential to both scientific research and commercial interests.
In the LEO region 200 to 2,000 km above the earth, traffic is heavy and debris pollution severe. "In the Geostationary Orbit (GEO) region 34,000-38,000 km above the earth, the very popular 36,000-km altitude orbit is particularly overcrowded," Du says. "Objects here remain relatively fixed above a certain region of the earth, which makes GEO ideal for communications satellites."
But, the scientist notes, the Earth's atmosphere has steadily deteriorated over time. By May 30, 2001, more than 4,150 space missions had been carried out. "Space-faring nations have dumped huge quantities of debris in space - bolts, discarded rockets, satellite appendages, retired satellites, defunct spacecraft, even flakes of paint," Du Heng says.
A total of 27,873 catalogued objects drifting through space have been tracked by ground-based radar. Of the 9,114 objects that still remain in orbit, only 6 percent are operational satellites, spacecraft or space stations. "The rest are orbital debris greater than 10 centimeters," the scientist adds. "Orbiting debris, both large and small, is moving fast enough to destroy or seriously damage spacecraft."
Pollution is the greatest threat to human activities in space. "Smaller debris is actually the most dangerous, because ground-based radar cannot detect it," Du Heng says. According to scientists, each year the international space station in the LEO region shifts its orbit at least four times to avoid hitting such detritus. Although the chances of a collision are low, the risks remain. A 1-centimeter piece of rubbish weighing 10 grams and moving at a velocity of eight km per second can be as fatal as a being hit by a one-ton motor vehicle traveling down a road at 100 km per hour.
Since April 24, 1970, Long March rockets have been used in 74 launches, including seven failures. One more launch is scheduled for December 2003. China currently has 17 active and 32 abandoned satellites orbiting the LEO and GEO regions.
In the past 13 years, there have been two cases of Long March-4 third-stage rockets fragmenting, as recorded in the 12th edition of the History of Satellite On-Orbit Fragmentation compiled by NASA. These two instances are among 170 such cases noted in the NASA documentation.
The first propulsion-related or debris-producing event occurred on October 4, 1990 and the second on March 11, 2000, altogether creating 376 detectable pieces of debris at heights of 895 km and 741 km above the earth. These two break-ups involved the third stages of the second and fourth Long March-4 rockets. One broke up one month after launch, the other, five months later.
The second event resulted in 293 pieces of debris identified by the US Space Surveillance Network (SSN). The incident aroused the concerns of the international space community.
On March 14, Nicholas L. Johnson, acting chairman of the Inter-Agency Space Debris Co-ordination Committee (IADC) and NASA's Orbital Debris Program Manager, informed the Foreign Affairs Bureau of the China National Space Administration (SNSA) that the third stage of the LM-4 rocket, launched on October 14, 1999, had disintegrated in orbit, producing some 150 chunks of debris larger than 10 centimeters.
In fact, subsequent analysis of the 1990 break-up of the third stage of the second LM-4 rocket led experts to conclude that the most likely cause of the explosion was the mixing of residual hypergolic propellants. Therefore, counter-measures were taken and tested in the third and fourth LM-4 missions on May 10 and October 14 in 1999, which were believed to be effective until the IADC addressed inquiries to the SNSA.
Experts attributed the incident to an accidental malfunction of the aestivation device and concluded there was a need for improvement through more testing.
"LM-4 rockets have done an excellent job in all of the eight launches since 1988," says Zhang Wenxiang, a senior researcher with the Shanghai Academy of Spaceflight Technology (SAST), which manufactures LM-4 launch vehicles. "It's a mature rocket with a no-failure record. It is extremely reliable and accurate. Obviously, all the credit should go to its overall design and control system. We're reluctant to make any changes except in the very necessary case of the surplus propellants. In any event, we won't tamper with its third-stage engine for fear of affecting its performance."
China's debris research
The propellants are liquids, known to experts as the fuel UDMH and the oxidant N2O4, which are contained in two separate but attached tanks that share the same bottom. The bottom structure can be destroyed if it is over-pressurized, as the two liquids would easily mix and explode.
"Our method is to deplete the liquids of both tanks after the third-stage module has moved at least one kilometer away from the satellite following separation," Zhang explains. "Each of the two tanks has a vent. As designed, the UDMH and N2O4 tanks would release their remaining fluid a distance of several kilometers, still allowing us to record the readings for 50 seconds after the venting begins."
In a July 2002 issue of the NASA newsletter "Orbital Debris Quarterly News", a different case related to the fifth LM-4 third stage is mentioned as the source of at least two new debris-causing events that occurred 30 km below the International Space Station in mid-June. If true, it would be the second infraction following the counter-measures or mitigation taken.
Since the 2000 break-up, improvements have been made to solve the problem of the venting process during the third stage. The improved device proved effective in the sixth and seventh LM-4 launches of May 15 and October 27, 2002.
The maturation of the passivation technology depends to a large extent on various ground tests made over the years by persistent researchers. This project cost 10 million yuan (US$1.2 million), funded in part by the SNSA.
At the invitation of the IADC Steering Group in October 1994 and with the approval of the State Council, the SNSA joined the IADC in June 1995. The IADC was initiated by the United States, Russia, Japan and the European Space Agency in 1993 as an international governmental forum for the worldwide co-ordination of activities related to issues of man-made and natural debris in space. The IADC now includes 11 member agencies, comprising one steering group and four specific working groups. Its primary objective is the exchange of information on space debris research between member space agencies, to facilitate opportunities for cooperation in space debris research, to review the progress of ongoing cooperative activities and to identify debris mitigation options.
Chinese researchers initiated their space debris research in the mid-1980s. In early 2000, a group of experts was organized to discuss and shape the major directions of space debris research work. By the end of the year, the group had worked out a comprehensive five-year plan (2001-2005).
As a developing nation with an interest in space, China is seriously addressing the issue of reducing space debris, says Liu Jianqiao, an SNSA official. "In this plan, the mitigation of space debris generation receives top priority. Also heading the agenda is the protection of our space objects, especially our manned spacecraft, which will become our main project after 2002."
The First National Symposium on Space Debris Research, held in Harbin from January 14-16, 2001, was attended by more than 130 experts. In addition, the SNSA sent its own experts to take part in the preparation and technical co-ordination for the IADC's compilation of the Space Debris Mitigation Guidance and Space Debris Protection Manual.
At the second symposium held in Shanghai in July 2003, China's progress in space debris research was assessed, with experts contributing 40 theses on measurement, environment, protection and mitigation. The quarterly publication Space Debris Newsletter has been circulating throughout China's space community for academic exchange.
From 2000 to 2002, a total of 18.6 million yuan (US$2.25 million) has been invested in the research of 60 subjects under different space debris research projects. Some 30 institutions have been involved in space debris research, including CAS, SAST, the Chinese Academy of Space Technology (CAST), the China Academy of Launch Vehicle Technology (CALT), the Harbin Institute of Technology (HIT) and Northwestern Polytechnical University (NPU).
Taking responsibility
CAST and HIT are now undertaking the basic research work in hopes of making a breakthrough by 2005.
As the amount of debris in orbit grows, the probability of damaging collisions increases. Orbital debris poses great risks to both manned and unmanned spacecraft. According to Han Zengyao, a CAST researcher, protective shields are required not only for these spacecraft but also for satellites with a long life span. "Unfortunately," he says, "none of our space objects are designed to deflect debris, so we have a top-priority challenge on our hands.
"In the initial stages," he continues, "we aim to design shields for space objects that are strong enough to resist collisions of centimeter-sized debris. However, we have to start from scratch because we need to understand the spear before we can create an effective shield." Han cites HIT's expertise in experimental technologies and its "hypervelocity impact laboratory" as key to solving the problems.
During the next two years, Chinese researchers will go in-depth in all aspects of space debris research, according to Guo Baozhu, vice-administrator of SNSA.
"China is a responsible country," Guo says. "We intend to fulfill our international obligation in the protection of space. We hope to exchange results on space debris research with other IADC member agencies. We are confident that we will open up new prospects in space debris research by 2005. Space-exploring nations should join together to protect outer space for future generations."
(China Daily November 26, 2003)
