ESA ENVISAT: De-orbiting and space debris threat from giant satellite

Artist's impression of Envisat orbiting the Earth. Credits: ESA

Physics students at the University of Leicester have pointed out that ESA's huge observational satellite Envisat, which lost contact with Earth in 2012, could potentially pose a space debris threat to the Earth.

What's more, the task of bringing the satellite back to Earth may be too costly and complex to be feasible, according to their final year paper for the Journal of Physics Special Topics, a peer-reviewed student journal run by Leicester University's Department of Physics and Astronomy.

Envisat, an £1.8 billion, 9 metre-long behemoth, was launched by the European Space Agency (ESA) in 2002, and used ten sophisticated sensors to observe and monitor Earth's land, atmosphere, oceans and ice caps.

However, ESA lost contact with the satellite in April, 2012 and declared the end of the mission soon after.

The satellite now orbits the Earth free from human control at an altitude of 790km, where the amount of space debris around the planet is greatest.

This means there is a greater chance of collision with other satellites and debris during the 150 years it is expected to remain in space.

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.

It is possible – though unlikely - that a collision with Envisat could 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.

This would not be good news for the many essential satellites and spacecraft in orbit.

It could also make it difficult for future space missions to pass through the region of Envisat's altitude, if the region becomes congested with space debris.

The fourth-year MPhys students' paper, 'De-orbiting Envisat', suggests that around 140kg of fuel would be required to move the satellite to a point where it would naturally return to Earth within 25 years.

Based on the object's cross-sectional area and its mass, the students calculated that the satellite would need to be moved to an altitude of 700km from its current position in order to return to the planet in 25 years.

The students calculated that the energy required to move the satellite 90km closer to the Earth was 2.7 billion joules, equivalent to an extra 143.1kg of hydrazine fuel.

This could be quite feasible, according to the students, if two of the craft's 80kg fuel tanks were replaced.

But actually getting this fuel to the satellite in orbit would be a pretty tall order due to the costs involved of such a mission – which has never been attempted for a satellite which wasn't designed to be refuelled.

More information: "De-orbiting Envisat." K. Raymer, T. Morris, O. Youle, B. Jordan, Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH. physics.le.ac.uk/journals