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ORBITAL DEBRIS: RISK FOR THE SHUTTLE AND THE SPACE STATION

The orbital break-up of a Pegasus rocket upper stage in June of 1996 generates a record number of debris. Ground based radar tracked more than 300000 debris with a size bigger than 4mm. Until 18 months after the incident, the US Space Surveillance Network SSN tracked and catalogued 678 debris which are bigger than 10cm just from this one event.

The catalogues of the American and Russian surveillance networks currently list approximately 9000 objects. Only the biggest debris in orbit can be regularly tracked and listed. The reason is that the resolution of the sensors is limited. Objects in low earth orbit up to 2000 km must have a radar cross-section of at least 10 to 30 cm to be trackable. In GEO, where mainly optical sensors are used, the minimum size is 1 m.

The real number of orbital debris is gigantic. 40 years of spaceflight have cluttered the space around the Earth with debris. Scientist assume that there are approximately 100000 objects in the size above 1 cm in orbit. The number of objects in the micrometer size probably goes in the billions.

Every launch leaves its evidence in space. Depending on the mission and launch system used, several other objects are generated with the launch and the release of the payload, ranging from bolts, covers, cables to entire rocket stages. The survival time of the debris can be very long. Objects in 1000 km orbits can exist for hundreds of years. At 1500 km, the lifetime can go up to thousands of years already. Objects in GEO presumably can survive for a million years.

Nevertheless, every once in a while large debris fall back to Earth. According to NASA, one object with a radar cross section larger than one square meter returns to Earth every week (331 from 1992 to 1996) in average. These re-entering objects are not considered to be a risk. "Orbital debris are no problem for the Earth itself", says Prof. Dr. Walter Flury, head of the mission analysis section of the European Space Operations Center in Darmstadt, Germany. However the debris in orbit are a major concern for the spacefaring community.

Objects in LEO are flying at a hypervelocity of 7 to 8 km/s. Since the flight vectors of the objects may be opposing, the relative velocity can go up to 15 km/s (54000 km/h) and is calculated to be 10 km/s in average in low earth orbits. In GEO, the relative speed is low (approximately 2 km/s). At these hypervelocities, the debris have a tremendous kinetic energy. A 1 kg object at a speed of 10 km/s has the same amount of kinetic energy that a fully loaded truck, weighing 35000 kg has that is driving with 190 km/h.

This is reason for concern. At the end of 1998, the first module of the International Space Station (ISS) is scheduled for launch into LEO. With a mass of more than 400 t and a total surface area of approximately 11000 square meters, ISS will be the largest spacecraft ever built.

In 1997, the US National Research Council, an independent research establishment which advises the US government and NASA, has published detailed reports and recommendations concerning the protection of the Space Station and the Space Shuttle which has a vital part in the ISS assembly.

The plan is to protect ISS with shields against impacts of typical aluminium alloy objects up to 1 cm in size. In case a collision with a tracked object larger than 10 cm is predicted, based on the SSN observations, the Station will change its orbital position.

The main concern are the debris between 1 cm and 10 cm. They are probably to big for the shielding and too small to track. This risk will be counteracted by implementing procedures to mitigate the damaging effects of impacts with objects between 1 cm and 10 cm.

The ISS orbital debris analysis team has set a minimum safety level for the Station. According to this requirement, there should be a probability of 0,81 that no critical ISS component will be penetrated by an object larger than 1 cm over a 10-year period.

The American Station modules will comply with this shielding requirement. Also the European partners with the COF module (Columbus Orbital Facility) and Japan with the Japanese Experiment Module (JEM) have adopted the American shielding approach. A problem is the shielding of the Russian modules, says Nicholas Johnson, chief scientist and program manager in NASA's orbital debris program office. He says the design FGB (Functional Cargo Block) does comply with the requirements but the Service module, which is scheduled for launch in 1999, does not. According to the current plans, the Service module will be updated later in orbit.

All together, there will be 100 different shields protecting the International Space Station. The majority of the designs is based on the Whipple concept. The scientist Fred L. Whipple had discovered the possibility of shielding against hypervelocity impacts by using a multi-wall shield structure. The outer wall shatters the impactor and the inner wall is then only hit by extremely small, melted and vaporised particles.

Unfortunately, this kind of shielding is not available to the Space Shuttle orbiters which will be used in logistics missions during the ISS assembly and operational phases. The Shuttles already have a long damage history from orbital debris. Up until today, almost 60 windows had to be replaced due to damage from impacting meteoroids and orbital debris. In the past missions, the impact risk could be reduced somewhat by selecting a specific flight attitude, giving the best protection to the Shuttle crews and vulnerable parts of the orbiter structure. Also, the Shuttle relocated its orbital position a few times due to a predicted collision with a catalogued object.

However, once a Shuttle is docked to the station, the orbiter does not have above options anymore. Due to the predicted structural stress on the ISS-Shuttle unit, it is not planned to move the station in this phase.

There has been a new evaluation on the impact risk and its possible effect on Shuttle operations. The study showed that the orbiters are actually more rigid than originally thought. Still, the Shuttles are currently undergoing a modification in two areas. Along with a better protection for the radiator freon lines, the insulation of the wing mainspar is being improved. This is for the case that the reinforced carbon-carbon wing leading edge in penetrated, generating an extra heat load during atmospheric re-entry.

The task of protecting the spacecraft and the astronauts is not an easy one. Especially when assessing the population of small debris scientists depend on models. The situation, especially in LEO, changes constantly. NASA scientist Nicholas Johnson says to look at it as some kind of a weather map. In order to improve the models, NASA is conducting its own selective radar measurements. The Haystack radar, for example, is NASA's main source of radar tracking debris in the size from 1 to 30 cm. In Germany, a measuring campaign has been conducted, involving the FGAN radar close by Bonn and the radar telescope located on the Effelsberg. With both equipments combined, it was possible to accomplish measurements down to 9mm.

In the medium term, NASA is looking for a constant tracking of debris in the 5 cm size, going to 1 cm in the long term. However, this would require an expansion of the Space Surveillance Network which is operated by the US Space Command. The SSN is NASA's main source of information about trackable objects. According to a recent report by the US General Accounting Office, the Space Command would like to involve NASA in the financing of these network updates.

That a better coverage is needed is highly desired by the scientists. The number of orbital debris is constantly increasing. The situation is furthermore heated by the planned launch of satellite constellations into LEO over the next few years. Experts are already convinced that certain altitude bands in low earth orbit have already reached a critical saturation. This could possibly lead to a self-generating snowball effect: More debris are generated by collisions between objects than debris are destroyed during re-entry into the atmosphere.

"Something must be done today", says Prof. Flury from the European Space Agency. The options to minimise the generation of new debris must be used to the full extent. As far as rocket upper stages are concerned they should be vented to avoid the formation of explosive gases which could lead to a break-up. In the case of the European Ariane rockets this is being done since flight 59.

Another option is to park satellites in a so called graveyard orbit a few hundred kilometres above GEO when reaching the end of their lifetime. The ultimate approach would be to destroy the spacecraft with a controlled atmospheric re-entry after usage.

Finally, the subject of orbital debris also has a legal aspect. What, if the commercial satellite of country A collides with an expanded rocket body from country B? Who will pay for the damage and losses? Has country B done everything to avoid the generation of debris during launch? A similar case happened on 24 July, 1996, when the third stage of Ariane flight 16 (launch in 19986) hit the French satellite CERISE and cut its stabilisation mast off. What, if the satellite were not French?

From page 36 of FLUG REVUE 10/98