ESA's Integral gamma-ray gazer gasps its last

After almost 23 years on the job, observations end for 2029 re-entry

All good things must come to an end. So it is that the European Space Agency's (ESA) International Gamma-Ray Astrophysics Laboratory (Integral) is set to make its final observations.

Integral has been on borrowed time for the last few months, with restricted use of its instruments approved during the spacecraft's post-operations phase. However, on February 28, the end will come, and observations will cease.

The spacecraft, launched on October 17, 2002, from the Baikonur Cosmodrome in Kazakhstan, will be monitored by ESA engineers for another four years before it re-enters the atmosphere in 2029.

Integral's fate was sealed in 2015 when the vehicle's orbit was adjusted to ensure a safe re-entry and disposal some fourteen years later.

The mission has lasted far longer than any of its designers could have predicted. Initially, it was 24 months, then extended to five years, before clocking up decades in orbit, studying gamma rays, and surviving the occasional near-mission-ending incident.

The science return from the spacecraft is impressive. Integral has made many discoveries during its lengthy operational life, from a dead star seemingly revived by a nearby red giant to the spacecraft's involvement in the observation of gravitational waves.

Jan-Uwe Ness, ESA Integral Project Scientist, said: "What I find impressive about Integral are its unexpected discoveries. It turned out that Integral was ideal for tasks not at all foreseen when the mission was conceived. An example is its ability to track down the sources in the sky that generated some of the gravitational waves and ultrahigh-energy neutrinos caught by specialized instruments on the ground."

It has, however, not been all smooth sailing for the veteran spacecraft. As the mission lengthened, the issues multiplied. In 2020, Richard Southworth, Integral Spacecraft Operations Manager, told El Reg about occasional failures that needed to be worked around, and the slowly degrading power generated by the spacecraft's solar arrays.

Later that year, Integral suffered a problem that could have ended the mission. Its propulsion system, used to dump momentum from the reaction wheels, failed. Losing the thrusters could result in the spacecraft going into a spin. The solution was to spin two reaction wheels in opposing directions, causing the spacecraft to flip.

The procedure had never been tried before, and after more experimentation, Integral's scientific efficacy was restored.

Then there was the time in 2021 when the spacecraft did go into a spin after a reaction wheel unexpectedly shut off. The team had mere hours to regain control of the vehicle before its batteries were depleted, and the mission likely ended. Again, Integral was kept running.

As recently as 2023, a new software-based safe mode that worked using the reaction wheels rather than thrusters was uploaded to the spacecraft and tested.

Professor Carole Mundell, ESA Director of Science, said: "That Integral's spacecraft and instrumentation have performed so exquisitely well for so many years is testament to the quality of the technology developed by the European scientific community and space industry at the turn of the millennium, and the science and engineering teams at ESA who have operated this mission ever since. Congratulations to all our communities for their dedication and achievements."

From a technical perspective, does Integral need to end? Judging by the extra months of observations, the motivation is financial. In 2023, ESA announced that the mission was "in jeopardy financially" but that the science case supported an extension. This ran until December 31, 2024.

An ESA spokesperson told The Register that "the last extension was coupled to the operations of the Gravitational Wave Observatories."

Some limited operations of Integral's instruments were approved at the start of the year. However, time is now up.

So, raise a glass to the achievements of Integral and its team as instrument operations conclude. NASA's Fermi Gamma-ray Space Telescope might be more sensitive, but with only a mission duration of 16 years under its belt, it has a while to go before it matches Integral's longevity. ®