Inara Pey: Living in a Modemworld

They are the grande dames, so to speak, of space-based astronomy, observatories launched into orbit around Earth and the Sun to provide us with unparalleled insight into the cosmos around us, born of ideas dating back to the early decades of the space age. They form three of the four elements of NASA’s Great Observatories programme, and all operated, or continue to operate well beyond their planned life spans; they are, of course, the Hubble Space Telescope (HST – launched in 1990), the Chandra X-Ray Observatory (CXO and formerly the Advanced X-ray Astrophysics Facility or AXAF – launched in 1999), and the Spitzer Space Telescope (SST, formerly the Space Infrared Telescope Facility or SIRTF – launched in 2003).

Today, only Hubble and Chandra remain operational. The fourth of the observatories (and 2^nd to enter space after Hubble), the 16.3-tonne Compton Gamma Ray Observatory (CGRO), had its mission curtailed in June 2000, after just over 9 years, when it suffered an unrecoverable gyroscope failure. With fears raised that the failure of a second unit could leave the observatory unable to control its orientation, the decision was made to shut it down and de-orbit it in a controlled manner so it would break-up on entering the atmosphere and any surviving parts fall into the Pacific Ocean, rather than risk an uncontrolled re-entry which could shower major pieces of the observatory over populated areas.

Whilst the “youngest” of the surviving three observatories, Spitzer was placed into a “safe” mode in January 2020, ending 16.5 years of service. By then, the nature of the observatory’s orbit – it occupies a heliocentric orbit, effectively following Earth around the Sun  – were such that it was having to perform extreme rolls back and forth in order to carry out observations and then communicate with Earth, and these were affecting the ability of the solar arrays to gather enough energy to charge the on-board batteries. The “safe” mode meant that Spitzer could continue to recharge its batteries and maintain electrical current to its working instruments, potentially allowing it to be recovered in the future. However, while it is true that Hubble and Chandra continue to work, neither is without problems.

Hubble orbits close enough to Earth that even at over 500km, it is affected by atmospheric drag, causing it to very slowly but inexorably lose altitude. This used to be countered through the semi-regular servicing missions, when a space shuttle would rendezvous with HST to allow astronauts to carry out work, and then gently boost the telescope altitude using its thrusters. But the shuttle is no more, and the last such boost was in 2009 to 540 km; currently Hubble is at around 527 km, and at the present rate of decent, it will start to burn-up in another 10-15 years. However, a boost now could see Hubble – barring instrument / system failures – continue to operate through the 2050s.

Chandra, meanwhile, faces a different challenge. It lies in a highly elliptical orbit around the Earth, varying between 14,508 km at its closest and 134,527 km at its most distant. It has therefore been operating untended for its entire operational life, and is starting to show signs of wear and tear. In 2018, it suffered a glitch with one of the gyroscopes designed to keep it steady during observations (and orient it to look at stellar objects). Whilst the gyroscope was recovered, it was put into a reserve mode lest it fail again. This led to fears that should a second gyro fail, either orientation control might be lost if the 2018 gyro fail to come back on-line correctly to take over the work. Also, and while the main science instructions are in good order, they are aging and presenting concerns as to how well they are actually doing.

Ideas for both boosting Hubble’s orbit and carrying out a robotic servicing of Chandra have been floated for the last few years – but there are now signs both might actually get potentially life-extending missions.

In December 2022, NASA issued an RFI on how Hubble’s orbit could be boosted, and have received eight responses, one of which has also been publicly announced and would seem to offer potential. It involves two companies: Astroscale Holdings and Momentus Space, a US-based company. The former is in the business of clearing space junk from Earth’s orbit, and has already flown prototype vehicles capable of doing this in orbit. This includes the ability to carry tools to mate or grapple junk and then move them. Momentus, meanwhile, are in the satellite servicing business and recently demonstrated a small “space tug” in orbit that was largely successful in meeting its mission goals (7 out of nine small satellites deployed into individual orbits).

In their proposal, the two companies indicate Momentus would provide a variant of their tug, and Astroscale a dedicated capture tool designed to use the grapple holds on Hubble. Following launch, the Momentus craft would self-guide itself to Hubble’s orbit and rendezvous with it using the Astroscale mating tool. Once attached, the Momentus vehicle would use its thrusters to gently raise Hubble’s orbit by 50 km, then detach. The vehicle could then be used to remove orbital debris in orbits approaching Hubble, thus protecting it from the risk of collision.

NASA has yet to comment on any of the proposals received under the RFI, but the Momentus / Astroscale option, using equipment already being flight-tested and refined and which is of relatively low-cost, would appear to be a real option.

A similar, but more expensive and complex idea has been proposed by Northrop Grumman – the company effectively responsible for building Chandra – to help keep the X-Ray observatory going. This would involve the construction and deployment of a “mission extension vehicle”, a “space tug” capable of departing Earth and gradually extending and modifying its orbit to rendezvous with Chandra and link-up with it, taking over the operations related to orienting and steadying the platform using gyros, potentially extending the mission by decades.

This is important because Chandra has already proven invaluable in supporting the James Webb Space Telescope (JWST) which operates in the infra-red. The ability to observe targets in both X-ray and infra-red can reveal a lot more about them.

NASA has given no word on whether it would finance such a mission, although Northrop Grumman has apparently forwarded the results of its own study on the idea to the US agency. However, given the most recent U.S. decadal survey in astrophysics, released in 2021, included mention of a new X-ray telescope to replace Chandra, a servicing mission – even one this complex – capable of extending Chandra’s operations for decades at a fraction of the cost of a new telescope, which itself would take years if not decades to develop, could be highly attractive.

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