Space Sunday: aiding three space telescopes

The Hubble Space Telescope, the Chandra X-Ray Observatory and the Spitzer Space Telescope. Credits: NASA

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 2nd 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.

The Hubble Space Telescope sitting on its holding platform in the cargo bay of the space shuttle Atlantis in 2009, seen through the orbiter’s rear deck windows during the 5th and final (and most extensive) servicing mission. Credit: NASA

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.

A set of X-ray images of regions of space – including supernova remnants and merging galaxies – captured by the Chandra X-ray Observatory, released by NASA in 2009 to celebrate the telescope’s 20th anniversary. Credit: CXC/NASA, SAO

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.

However, perhaps the most intriguing idea for the Great Observatories is that of bringing the Spitzer Space Telescope back up the full operational capacity. Were it to go ahead, it would be potentially the most complex robotic space missions undertaken. To start with, and as noted above, Spitzer follows Earth around the Sun – but at a slightly greater distance, taking 373.2 days to complete an orbit. This means that over time, the distance between Earth and the observatory has been steadily increasing, and is now some 300 million km. This alone makes reaching  spitzer a challenge. And once the vehicle reaches the telescope, it then has to assist in getting the telescope back up and running.

Despite these complexities, the relatively new start-up Rhea Space Activities has been awarded $250,000 NASA to formalise a proposal they call the Spitzer Resurrector Mission (SRM). This will involve launching an automated vehicle into an orbit around the Sun which, over a period of time – months -, it would essentially “come up behind” Spitzer. By altering its velocity and orbital track when en route, potentially by using the gravity of a near-Earth asteroid (or two), SRM would be able to bring itself alongside Spitzer where it can be used to complete the telescope’s recovery.

An illustration of the Spitzer Resurrector Mission (objects not to scale) Credit: Rhea Space Activity

Exactly what tasks the vehicle will perform hasn’t been fully spelt out, but the overall aim – according to Rhea – is to leave Spitzer pretty much in the same condition as when it was initially commissioned following its launch, again extending its operational life by decades. This is itself an intriguing proposition; some of Spitzer’s instruments require extremely low temperatures in which to operate, but the liquid helium used to achieve this was expended before the telescope was put to sleep. So does this mean SRM will try to re-stock Spitzer’s liquid hydrogen reserves (assuming the telescope has a means to accept any such replenishment)? Well, NASA, Rhea and Rhea’s partners – the Smithsonian Astrophysical Observatory, Johns Hopkins University Applied Physics Laboratory, Blue Sun Enterprises, and Lockheed Martin – have indicated the mission, which could fly as soon as 2026, would use ISAM – In-space Service Assembly and Manufacturing.

ISAM is a multi-faceted approach to the on-orbit servicing, replenishment and support of satellites, including restocking them with propellants and coolants. Again, it’s by not means certain this will be involved in SRM – but the mission has gained the support of the Department of the Air Force (DAF) and US Space Force, both of whom may contribute to the cost of the mission, seeing it as a technology demonstrator for the ISAM technologies they wish to leverage.

In Brief

Ax-2 Due for Launch

At the time of writing, the Axiom Ax-2 mission to the International Space Station (ISS) was readying for launch at the Kennedy Space Centre). The second fully private mission to the ISS undertaken by the company will spend eight days a docked with the station as Axiom continues to work towards establishing its own modules at the ISS and, eventually, its own space station.

Axiom-2 crew from left to right: Mission Specialist Ali Alqarni, Mission Specialist Rayyanah Barnawi, Commander Dr. Peggy Whitson, and Pilot John Shoffner. Credit: Axiom Space/Vytal/Chris Zuponcic

The crew of four comprises mission commander and former NASA astronaut Peggy Whitson, who now serves as an Axiom special consultant; American racing driver, investor, and pilot John Shoffner, who is both a fare-paying passenger and serving at the mission’s pilot, and Saudi Mission Specialists Ali Alqarni, a fighter pilot in the Saudi Royal Air Force and Rayyanah Barnawi, a biomedical researcher and first Saudi woman to fly in space. I’ll have more on this mission is an upcoming Space Sunday.

Virgin Orbit: Offers and Refusals

Troubled Virgin Orbit has received an offer from rival air-launch company Stratolaunch Systems, operators of the massive Roc carrier aircraft. However, it is not to buy-out the company, but a stalking horse bid, with Stratolaunch specifically wishing to acquire Cosmic Girl, and all equipment related to operating the aircraft.

The offer was revealed on May 16th, and is interesting as Virgin Investment Group poured some US $15 million into Virgin Orbit in April in order to help it stay afloat through the Chapter 11 process – in return for first refusal on the transfer of rights of Cosmic Girl (which would allow VIG to effectively return the 747 to Virgin Atlantic). In offering $17 million for the aircraft + associated assets, Stratolaunch appear to be merely allowing VIG to recover their money. Virgin has yet to declare if this offer, or any others received for the company and its assets, will be accepted – that news is expected to come later in the coming week.

In the meantime, the UK government has indicated it will not step in to save Virgin Orbit (much as it did with OneWeb), with George Freeman, minister for science, innovation and technology somewhat dismissively referring to the company as a “platform that hasn’t worked”, despite it having four consecutive successful launches out of the six it has managed to fly.

Earth Safe from Major Asteroid Clobbering for 1,000 Years (Maybe)

Astronomers at the Smithsonian Astrophysical Observatory (SAO) have completed a comprehensive examination of the major asteroids (e.g. those 1 km across or larger) known to periodically zoom close to our planet, extrapolating the data available to determine that – outside of a couple of exceptions – none of them are likely to strike Earth any time in the next 1,000 years.

So does that mean we’re safe? Well, no. Sadly, there are many thousands of these near-Earth objects (NEO) ranging in size from just a couple of metres across right up to the 1 km mark , and many of many of those are capable of causing local or even regional damage. A 20m chunk of rock entering Earth’s atmosphere can cause an airburst explosion measuring 0.5 megatons about 20km above the planet’s surface, for example, while an asteroid 100m across could flatten a city the size of Paris. Many of these smaller objects are in chaotic orbits, making it impossible to track their orbits beyond a decade or three into the future.

A computer generate image of the orbits of 2,200 potentially hazardous to Earth objects as they travel around the Sun, as calculated by JPL’s Centre for Near Earth Object Studies (CNEOS). Highlighted are the orbits of Earth and Mars and the double asteroid Didymos, the target of NASA’s Double Asteroid Redirect Test (DART) mission. Credit: NASA/JPL

Plus, there are also the “known unknowns” and “unknown unknowns”. That is, NEOs we’ve spotted as they’ve zipped by Earth but then vanished before we’ve been able to gather enough data to track their probable orbit; and other we know are statistically likely to be passing Earth on a regular basis, but we have yet to spot them. What this study does give us, is a measure of assurance that – barring something extraordinary, such as an exceptionally large “unknown unknown” – we’re unlikely to go the way of the dinosaurs, courtesy of an asteroid.

Although – there are those two exceptions to the SAO study: (7482) 1994 PC1 and (143651) 2003 QO104. The former (roughly 1.3 km across) is going to continue to pass close to Earth throughout the next thousand years, and if its orbit were to be influenced by another large body, it could potentially be deflected onto a collision course. The latter, at almost 2 km across, has a very chaotic orbit, making it impossible to predict its orbit with certainty beyond a few decades. Both of these have the potential to cause an explosive release of energy equivalent to 10 million megatons.


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