After 12 years in orbit, the European Space Agency (ESA) is in the process of enacting its retirement plan for the International Gamma-Ray Astrophysics Laboratory (Integral). However, while the agency is adjusting the orbit of the satellite now, the spacecraft will continue to make observations for some years, and won’t re-enter Earth’s atmosphere until the late 2020s.
The Integral satellite is one of ESA’s longest-running missions, having been placed in orbit in 2002. It’s achieved a great deal over its 12 years in space, making important observations of violent events including gamma ray bursts and black holes. Most recently, in August 2014, it made gamma-ray observations of a supernova, confirming that white dwarfs can indeed reignite and explode.
Looking towards its eventual demise, the team is in the process of adjusting the orbit of the satellite via four thruster burns, using around half of the estimated 96 kg (211 lb) of remaining fuel, and setting it up for safe re-entry in 2029 – a full 15 years down the line.
It might seem strange to plan the re-entry so far in advance, but making the adjustments now will ensure that Integral’s eventual entry into the Earth’s atmosphere will meet with the Agency’s guidelines concerning orbital debris, while allowing it to continue making observations for the next seven to eight years.
Because Integral was launched in 2002, it pre-dates the current guidelines for minimizing space debris. Despite this, the agency has decided to do things by the book anyway, carefully planning the satellite’s eventual descent over several years. If ESA opted to continue observations without making the adjustments, its fuel would last for an estimated 12-16 years, but the probe’s eventual re-entry – some 200 years in the future – would present a hazard to other missions.
Upon first inspection, the goals of maximizing the satellite’s scientific output and mitigating space debris seemed incompatible. But the longer the team worked on the problem, the clearer it became that a solution was possible.
"After detailed analysis, a sequence was developed that meets both goals," said Gerald Ziegler, a flight dynamics specialist working on the Integral mission. "Moreover, additional considerations of attitude constraints and ground station coverage has to be taken into account, making it a highly interesting and challenging undertaking."
The first of the four burns was performed on January 12-13, lasting for 16 minutes and adjusting the satellite’s orbit to allow the ESA’s Perth ground tracking station to execute future maneuvers. This allows the agency to make further adjustments at the point where the satellite is closest to Earth, allowing for the most efficient use of fuel.
The second burn, which provided around half of the overall adjustment, took place on January 24, while the final adjustments will take place on February 4 and February 12. The final two burns will better align the satellite with the agency’s Kiruna ground station in Sweden.
Integral’s fuel will run out some time in the early 2020s, but the team will also have to contend with other issues, such as the degradation of solar panels by radiation. They plan to continue making scientific observations until the satellite eventually fails.
Orbital debris is an increasingly hot topic, with the ESA recently announcing its intention to host an international workshop on the subject. With an emphasis on making the industry more sustainable, the workshop will take place March 2015, focusing on cleaning up low-Earth orbit – an area frequented by key assets such as communication satellites.