Space Sunday: water on the the Moon; asteroids & comets

The Stratospheric Observatory for Infrared Astronomy (SOFIA), a joint US / German programme, Credit: NASA

Earlier in October, NASA teased the world with news of a special announcement concerning the Moon, using social media to announce the fact … they would be making an announcement on Monday, October 26th.

The announcement of the announcement led to a lot of speculation (and a lot of ribbing at NASA’s expense) with some correctly identifying the fact that the news would have something to do with the Stratospheric Observatory for Infrared Astronomy (SOFIA), the world’s largest flying telescope. This is a joint NASA / DLR (German space agency) venture that flies a German-built 2.5m diameter reflecting telescope aboard a short-bodied 747 SP operated by NASA.

Flying at 12 km above the ground, and so well above the worst of the distorting effect of the Earth’s atmosphere and capable of 10-hour observation sorties, SOFIA is almost as capable as space-based telescopes of a similar nature (having around 85% of the infra-red capability of a similarly-sized space telescope), whilst offering fair easier and lower-priced maintenance, upgrade and general operational costs. In addition, the range of the 747 aircraft means that SOFIA can operate over almost any location on Earth and so be available for almost any observational requirements than fall without the telescope’s capabilities.

The German-built telescope on the Sofia aircraft during flight tests. The “spots” around and on the open telescope bay door are airflow indicators to help with monitoring air passing over the open door. Credit: NASA

When finally made public, the announcement – which was billed as being related to NASA’s current plans to return humans to the Moon, Project Artemis -, proved to be that SOFIA has detected water molecules on the sunlit surfaces of the Moon.

Whilst an important discovery, marking a further increase in the presence of water on the Moon (which we’ve known about since 2009), it is important to offer a measure of context to the discovery: this is about water molecules bound within the regolith (surface material) of the Moon, not actual water ice, as was confirmed in 2018 for many of the permanently shadowed and very cold craters of the Moon’s south polar regions.

In particular, SOFIA detected the infra-red signature for water molecules within the crater Clavius (perhaps most famous for being the location of the lunar administrative base in 2001: A Space Odyssey).  Located in the southern highlands at 58.4°S 14.4°W, Clavius is one of the oldest formations on the lunar surface, believed to have formed some 4 billion years ago; it is some 230 km across and some 3.5 km deep.

Clavius crater as seen via the Johannes Kepler Observatory, Linz, Austria, 2004. North is towards the top of this image. Credit: H. Raab

That water molecules may be widely present in lunar regolith had been long suspected. However, previous estimates as to how much might be present had been hampered by the fact that previous studies could not clear differentiate between the presence of water molecules (H2O) and hydroxyl (OH). During extended observations of Clavius, utilising  a special instrument, the Faint Object infraRed CAmera for the SOFIA Telescope (FORCAST), the airborne observatory was able to detect water molecules at around 100 to 400 parts per million in a cubic metre of regolith.

To put this in proportion, this means that SOFIA detected the equivalent of one third of a litre of water trapped in a cubic metre of lunar surface material – which is actually a lot.  If the SoFIA findings hold true for all of the surface material within the sunlit parts of the Moon, it means there a potentially a lot of water to be had;  but whether or not it is actually accessible or have any significant bearing on human activities on the Moon is open to debate. Certainly, it is unlikely to have any significant impact on America’s Project Artemis, despite claims otherwise.

Simply put, the water molecules detected within Clavius are most likely bound in glass beads that resulted from micrometeoroid impacts. As such, it is nowhere near as potentially accessible as the water ice in the south polar region craters, and it is going to need relatively intensive processing in order to be properly extracted and turned into usable water – and the kind of heavy engineering required to achieve this at scale isn’t going to be available for use on the Moon any time soon, and may not even been cost-effective even when it is.

Clavius crater as seen by NASA’s Lunar Reconnaissance Orbiter (launched in 2009 and still operational). Note that is this image, north is at the bottom. Credit: NASA

Nevertheless, the discovery is important for our understanding of the Moon and our longer-term exploration of the lunar surface. It might also mean a new lease of life for SOFIA. whilst not mentioned in the release, NASA had sought to quietly terminate the 10-year-old telescope in 2021, citing it’s “lack of scientific output”.

OSIRIS-REx Stows Sample Head

Over the last couple of Space Sunday reports, I’ve covered NASA’s  Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx) mission, which is designed to gather samples samples from the asteroid 101955 Bennu and return them to Earth for analysis (see: Space Sunday: OSIRSIS-REx: sampling an asteroid and Space Sunday: asteroid sampling & starship building)

The original aim of the sampling operation had been to try to gather about 60 grams of surface material from the asteroid in a delicate “touch and go” mission that was completed on October 20th.However, as I reported in the second of the above articles, the sample gathering operation was rather more successful than anticipated, over-filling the sample gathering head on the vehicle’s robot arm to the point where a Mylar cover designed to cover the opening in the sample head and trap the gathered material have failed to seat itself properly, ad as a result material was “leaking” out of the sample head and back into space.

A gif showing material “leaked” by the sample head floating around the TAGSAM mechanism. Credit: NASA

Because of this, it was decided to suspend a further operations related to the sample gathering and move directly to moving the sample head to the Sample Return Capsule (SRC) that will parachute the sample back to Earth in 2023 as OSIRIS-REx makes its return. However, due to the complexities involved in moving this operation forward – it had originally been planned for late November 2020, to allow time for other experiments related to the gathered samples to be carried out (most notably just how much material had been gathered), this operation could not commence until October 26th.

In all, the operation took 36 hours to complete, much of the time simply being taken up waiting for signals to / from the space craft taking an hour to complete the two-way trip, but on October 27th, OSIRIS-REx returned images from its on-board camera systems showing the sample head being placed within its retaining cradle in the SRC by the TAGSAM robot arm, and of it seated successfully in the SRC with the robot arm fully detached, ready of the “lid” of the capsule to be closed and sealed over it.

The sample head from TAGSAM is eased into the sample return capsule (l), and is shown in its stowed position, the rest of the TAGSAM arm having detached (r), ready for the capsule lid (seen on the left of both images) to hinge shut and lock, securing the gathered sample. Credit: NASA

Other images returned by OSIRIS-REx during the operation have led mission managers to believe as much as 1 kilogramme of material may have been rammed into the sample head,  thanks to the father than Bennu’s surface is so brittle that despite the gentleness of the sample heads contact, it actually broke through the asteroid surface and penetrated to the depth of around 48cm

In addition, these images, couple with others taken as material was “leaking” from the sample head have led scientists to conclude that Bennu’s surface is made of of very thin layers of dust that have built up over time, but within which there is little or nothing to actually bond the material together, leaving it exceptionally fragile. Not that this is a bad thing for OSIRIS-REx; the fact that it did break through the surface deposits so easily not only means the sample head gathered far more material than every hoped for – but it also obtained  some of that material from within the asteroid, making it pristine samples of the solar system’s early history, untouched by solar radiation.

Of  Philae’s Bounces and a “Skull”

In 2014, the European Rosetta mission reached its study target, 67P/Churyumov–Gerasimenko (67P/CG) after a 10-year flight to catch it. The arrival of the spacecraft, also called Rosetta, marked the start of 2-year study of the comet in an attempt to further our understanding of these artefacts from the creation of the solar system.

A  part of this study included dispatching a lander – called Philae – to the surface of the comet so it could be examined in detail. This was always going to be a difficult goal, because given the tiny amount of gravity exerted by the comet meant there was a real risk the lander – roughly the size of a washing machine –  would bounce back into space.

Philae, its science instruments and the SESAME harpoons that failed to secure the lander as it touched comet 67P/C-G. Credit: ESA

To combat this, the three legs on the lander were equipped with harpoons designed to fire at the moment of contact, hopefully firmly anchoring it to the comet. Unfortunately, and as I extensively reported at the time, the harpoon mechanism failed, and Philae ended up bouncing across 67P/C-G for a fair distance before finally coming to rest in the shadow of a rocky overhang that presented it from receiving sufficient sunlight to keep its batteries properly changed. And while the core of Philae’s mission was successfully completed before it went into hibernation, there was the question of exactly were and how it bounced across the comet before finally coming to rest.

Thanks to the work of a team reviewing data from Philae’s magnetometer, which recorded every bounce and impact, and comparing it with data and images obtained by Rosetta, a team has finally determine the lander’s path across the comet – and in doing so have confirmed some of the Rosetta mission findings and given us a quirky little gift from Philae in time for Halloween.

After initially touching-down at a point known to the mission team, it had been thought the lander had bounced backwards as the comet turned beneath it, slowing drifting back down under gravity to impact and bounce again. However, the data put together by the ESA team shows that after that initial contact, Philae actually launched itself sideways, relative to the comet,  possibly because on of the landing leg harpoons had initially snagged on something.

This sideways move cause the lander to punch its way through a “wall” of loose rock and dust to one side of the landing zone, and then catch itself on a more solid rocky overhang, dragging away surface material to expose underlying water ice, before it was flipped over and lifted up so that it collided with a top of a ridge line, and then finally colliding with the “cliff” where it came to rest the right way up.

The impacts created by Philae’s passage were actually imaged by Rosetta before the end of its mission in 2016, but while the exposed water ice was of interest to scientists, that they might be linked to the lander bouncing across the comet had never previously been realised.

In particular, by re-examining the impact points in light of the data gathered by the magnetometer, together with known factors such as the lander’s mass, and the depth of some of the impacts (obtained by 3D analysis of the Rosetta images, the team studying Philae’s path along the comet were able to confirm one of Rosetta’s finding about 67P/C-G: that around 75% of it is in fact the void space between the dust and ice from which it has formed, and that as such, the material comprising it is is about as solidly bounded together as the material used in the average snowball.

Gif showing how the inverted Philae lander gouged the “eye sockets” of “skull top ridge”, after exposing the ice of the “mouth” (near the bottom of the image in an earlier impact). Credit: ESA

This, coupled with the ease with which the OSIRIS-REx sample head smashed through the surface layers of Bennu, suggests that a lot of the asteroids and comets in the solar system aren’t particularly sturdy, thanks to their lack of mass and gravity.

This is not to say a sufficiently large enough one colliding with our atmosphere wouldn’t do Earth a mischief, but it does raise hopes that were we to spot one that was going to do so early enough, the chances are good that we could break it up into relatively harmless debris well before any impact of substance occurred.

And the Halloween link?

The back of the ridge Philae struck after being flipped over was, before the impact, marked by an oval, head-like outline. In striking it, the lander gouged out two “eye sockets”, which together with the water ice exposed it exited exited the “dust wall” at the base of the ridge, resulted in the area having something of a grinning skull-like appearance in the Rosetta images, which has led to it being informally dubbed “skull-top ridge”.