Space Debris Threat From Envisat Explored By University Of Leicester Students
February 20, 2014

Could Envisat Cause A Gravity-Style Space Debris Chain Reaction?

redOrbit Staff & Wire Reports - Your Universe Online

A recently-published paper from physics students at the University of Leicester explores the possibility that a massive observational satellite that lost contact with Earth in 2012 could be the catalyst that sets off a chain of events much like those depicted in the Academy Award-nominated film Gravity.

Writing in the Journal of Physics Special Topics, a peer-reviewed student journal run by the University’s Department of Physics and Astronomy, the students claim that Envisat could pose a serious collision risk due to its size and current orbit altitude. Furthermore, they note that it might be too costly and complicated to bring the probe back to Earth.

Envisat, a gigantic nine-meter-long satellite worth more than $3 billion, monitored the planet’s land, atmosphere, oceans and ice caps using a set of 10 sophisticated sensors. The ESA launched the probe in 2002 but lost contact with in April 2012, declaring an end to the satellite’s mission roughly 10 years after it started.

Envisat currently orbits at an altitude of roughly 790 km, which happens to be the region where the amount of space debris surrounding the planet is the greatest, according to the study authors. It is expected to remain in space for about 150 years, meaning that there is a chance that it will collide with other satellites or space junk.

“Each year, two objects are expected to pass Envisat to within about 200m and other spacecraft have had to move out of Envisat’s path,” the university said. While it is unlikely, they note that a possible collision with the satellite would “lead to a chain reaction effect, known as the Kessler Syndrome, where a cloud of fast-moving debris causes other collisions with orbiting bodies around the Earth.

Not only would this be bad news for the several key satellites and spacecraft that also orbit the planet, it could hamper the ability of future space missions to make it through this region of altitude, as more and more space debris joins Envisat in orbit at that height.

The satellite could be moved to an altitude where it would naturally return to Earth within the next 25 years, but the researchers said that it would require 140 kilograms of fuel to do so. Based on Envisat’s cross-sectional area and mass, they calculate that it would need to be moved to an altitude of 700 km to accomplish that task.

The students also calculated that 2.7 billion joules of energy would be needed to move the satellite 90 km closer to Earth, which would require an extra 143.1 kg of hydrazine fuel. This could be accomplished by replacing two of Envisat’s 80kg fuel tanks. However, such a maneuver has never been attempted and would be costly.

In Gravity, “the cloud of space debris is caused by a missile which was supposed to destroy a non-operational satellite and sparks the chain reaction which eventually collides with Clooney and Bullock’s spacecraft. In real life this is very unlikely to happen,” explained 22-year-old physics student Katie Raymer.

“It is even more unlikely that ESA’s Envisat could cause one of these chain reactions. However, each year two objects are expected to pass Envisat to within about 200m and other spacecraft have had to maneuver themselves out of Envisat’s path,” she added. “Also Envisat orbits at an altitude where the amount of debris is greatest. So although it is unlikely to happen, de-orbiting Envisat is certainly worth considering.”

Since Envisat was not designed to be refueled, the study authors propose alternative methods of de-orbiting Envisat. They suggest that it could be possible to use NASA’s Robotic Refueling Mission (RRM), which was specifically designed to refuel and repair non-operational satellites. However, it is still in its earliest phases of testing.

According to Professor George Fraser, Director of the University’s Space Research Centre, the paper “highlights the huge area and mass of Envisat as the major risk factors for space debris. The fact that Envisat is in a near-polar orbit doesn’t help either, since its path intersects most satellites’ orbits nearly at right angles. Imagine driving down the motorway and every so often a large truck cuts right across all four lanes right in front of you!”