Space Sunday: asteroid sampling & starship building

Kissing an asteroid: the sample head of TAGSAM, the sample gathering mechanism on NASA’s OSIRIS-REx makes contact with asteroid Bennu on October 20th, 2020. Credit: NASA

In my previous Space Sunday update, I covered the (then) upcoming attempt by NASA’s Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx) to snag samples of material from the surface of asteroid 101955 Bennu, a carbonaceous near-Earth asteroid.

The attempt was successfully made on Tuesday, October 20th – although just how successful it was did not become apparent until a few days later, when mission engineers realised they now had a slight problem.

The mission required OSIRIS-REx slowly descended from its close-in altitude of 770m, a sample gathering called TAGSAM (Touch-And-Go Sample Acquisition Mechanism) extended beneath it. This was intended to make very brief contact with the surface of Bennu, absorbing the spacecraft’s momentum in springs, and allowing it to fire a nitrogen jet to blast material up from the asteroid some of which would hopefully be caught in the arm’s sampler head, prior to the arm “pushing off” from Bennu once more, allowing OSIRIS-REx to gently back away to a point where it could examine what it has gathered.

OSIRIS-REx: the sample-gathering operation. Credit: NASA

The entire operation was scheduled to take some 4.5 hours from start to back-away and parking. The event was live-streamed, but due to the current distance between Earth and Bennu, those on Earth were witnessing events 18.5 minutes after they had actually occurred. This also meant the entire operation was carried out autonomously, the software controlling it having been previously uploaded to the satellite.

OSIRIS-REx, following Bennu’s rotation about its axis, struck the asteroid a metre one metre away from its intended contact point, which lay within a shallow crater on Bennu that has been christened “Nightingale”. It remained in contact with the surface for 6 seconds – very slightly longer than had been anticipated.

The moment of contact for TAGSAM – and of actually smashing through the surface material on Bennu (film speeded up). Credit: NASA

Whilst there was a camera on the robot arm recording the operation, the footage could not immediately be sent back to Earth. Instead, mission controllers relied on the telemetry OSIRIS-REx did immediately transmit back to Earth. This revealed that everything had apparently gone as planned: TAGSAM made contact, the gas was fired and regolith (surface material blasted upwards. The telemetry then confirmed OSIRSIS-REx was backing away from the asteroid towards the point where analysis of the amount of captured material could be carried out.

This was transcendental. I can’t believe we actually pulled this off. The spacecraft did everything it was supposed to do. Even though we have some work ahead of us to determine the outcome of the event, this was a major accomplishment for the team. I look forward to analysing the data to determine the mass of sample collected.

– OSIRIS-REx Principal Investigator Dante Lauretta

Then came the first of the surprises.  When the video footage captured by the TAGSAM arm camera was received and processed (above right) on October 21st, it revealed that the sample head hadn’t so much touched the surface of Bennu as smashed straight through it to an estimated depth of almost 50 centimetres – and in doing so, had pulverized a rock roughly 20 cm across which, when first viewed in the footage, caused the mission team to worry it might prevent sample gathering and damage the sample head.

The next step in the operation was to analyse the state of the sample head once TAGSAM had been returned to its stowed position against the spacecraft. To do this, one of the star tracker cameras used for navigation was tasked to capture an image of the sample head. When this was returned to Earth, mission staff had a second surprise: the sample head was “leaking” material.

Following the sample gathering operation, a Mylar diaphragm should have rotated over the opening of the sample head to seal any material gathered inside it – but the star tracker camera revealed this had failed to sit correctly, and a small cloud of material was forming around the sample head as it persistently “leaked” out. Given the force of the contact with Bennu, the mission team realised that, rather than just collecting 60 grams of material, the sample head had likely been filled to capacity, preventing the Mylar cover from correctly sealing it.

A small but growing cloud of material gathers around the TAGSAM sample head on OSIRIS-REx following a sample gathering operation that likely overfilled the sample containment area, preventing a Mylar diaphragm (the white semi-circle, visible in the middle of the circular sample head) from sealing the cavity. Credit: NASA

With material slowly but steadily escaping, the decision was been taken to cancel the attempt to estimate the amount of material gathered, and instead move to transferring the sample head to the Sample-Return Capsule (SAC). This is the unit that will return the sample to Earth when OSIRIS-REx return here in 2023. As the SAC is sealable, moving the sample head there as soon as possible – in this case, October 27th – will ensure the remaining material from Bennu is preserved.

In the meantime, and while OSIRIS-REx cannot start on its return to Earth until March 2021, the decision has been made not to return the vehicle to a low-level “hover” orbiting Bennu, but to instead allowing it to continue away from the asteroid at around 44 metres per hour until it reaches a more extended orbital position.

Starship SN8: Static Fire Complete and Vehicle Stacked

SpaceX has cleared two major milestones on the route towards  starship prototype SN8 completing a flight to 15 km altitude:  a static fire test of the vehicle’s engines, and the stack of the upper sections of its hull.

The static fire test – a standard part of pre-flight operations for SpaceX vehicles, allowing them to “clear their throats”, so to speak, in preparation for flight – came on Tuesday, October 20th, with an initial pre-burner ignition test.

Each Raptor engine actually comprises three motors: the main engine itself, and two smaller motors that power the main engine’s turbo pumps. In the pre-ignition test, these small motors (six for starship) are first briefly fired to ensure they are operating correctly and in unison, then after an inspection to make sure they have worked correctly, the main motors  are test fired. During a static fire test on Monday, October 19th, things got as far as the pre-burner ignition test, but it appears it wasn’t satisfactory, as no follow-up full engine firing occurred.

Lighting up the sky: the SN8 static fire test of its three atmospheric Raptor engines in the early hours (local time) of Tuesday, October 20th, Boca Chica, Texas.

On October 20th, however, things went as planned, with  successful pre-burner ignition test followed 2 hours later by the full static firing of the three engines for a few seconds, allowing them to deliver 90% of their total thrust. This marked the first time three Raptor engines have ever been fired in unison.

At the time of the test, SpaceX were also preparing the upper sections of SN8 for stacking – mating them with the rest of the ship. First, the nose cone, complete with forward aerodynamic canards, was mated to the upper hull section. Then, following the static fire test, the combined unit was rolled to the launch stand and lifted aloft to be wielded to the lower hull.

In an operational vehicle, these upper sections will enclose living spaces and / or cargo, and so will be naturally strengthened. However, for SN8, they are hollow structures, other than the liquid oxygen header tank right up in the nose. Because of this, the sections have added support stringers inside to give them (hopefully) the necessary strength to withstand the dynamics of the upcoming test flight, resulting in their somewhat rough-and-ready finished look.

Standing tall and with aerodynamic surfaces folded into their launch configuration, the upper section of Starship SN8 is held aloft by a massive crane as teams weld it to the vehicle’s core section. Credit: via Twitter

No date for the flight attempt has been given, but there is anticipation within the space community that it might by as soon as the coming week.

In the test, the three Raptor engines will propel SN8 to 15km, where it will then pitch over into a “belly flop”, using the fore and aft aerodynamic surfaces like a skydiver uses their arms and legs to maintain stability, as well as to control pitch (nose elevation up or down) and roll. As it reaches a point a few hundred metres from the ground, the vehicle will stand upright and use one of the Raptor engines as a brake to touch-down on a landing pad. Such is the complexity of the flight, SpaceX gives this first attempt perhaps a 50% chance of success.

Venus: No Significant Phosphine

In September, I reported on a new study that suggested the upper reaches of Venus’ atmosphere contain phosphine (PH₃), a colourless, flammable, toxic gas compound most commonly produced by organic life forms (see: Space Sunday: phosphine on Venus, test flights and Jupiter). At the time, study triggered a lot of excitement about the potential for microbial life floating  in the relatively calm layers of the planet’s atmosphere, although the authors of the study cautioned that more research was required to confirm their findings.

Now a group of astronomers have subjected the original findings of that first study to a rigorous re-examination – and have come to the conclusion there is actually little or no phosphine in the Venusian atmosphere.

What appears to have happened is that the original study team thought they had detected a phosphine signature using the James Clark Maxwell Telescope (JMCT) in Hawaii. To confirm their findings, they turned to the more capable (for this kind of work) Atacama Large Millimetre/sub millimetre Array (ALMA) in Chile, writing a series of scripts to assist in the analysis of the ALMA data.

Two of the ALMA telescope dishes used in the study of Venus’ atmosphere, here pictured on October 19th, 2020 against the backdrop of the constellation of Orion as seen from Chile canted on its side. Orange Betelgeuse can clearly be seen towards the upper right, with the massive and brilliant Rigel diagonally opposite it, sitting just above the foremost telescope. Credit: Y. Beletsky (LCO)/ESO)

However, an unintended bias in the scripts seems to have drawn undue attention on a single 267 GHz spectroscopic line in the ALMA observations, appearing to reveal a 20 parts per billion concentration of phosphine in Venus’ upper atmosphere. In identifying this bias and removing it, the Dutch-led review team found the resultant report of Phosphine drops to a point where it is no longer statistically meaningful, leading them to their conclusion that is fact their is little or no phosphine present above Venus, and thus there are unlikely to be any little microbes making it.

Studying Tea Leaves Finds ISS Pressure Leak

For much of the past year, the International Space Station (ISS) has suffered a series of atmospheric pressure losses. While none have threatened the crews on the station, by the summer of 2020, things had reached a point where the pressure loss was so persistent, mission controllers ordered crew on the station to commence a “hard target” search for the cause of the loss, and they eventually traced it to the Russian Zvezda module. However, the exact source couldn’t be identified within the module, so the decision was taken to close it off from the rest of the station and minimise its use.

The problem with this idea is that Zvezda, actually about the oldest of the ISS modules, having been originally built in the mid-1980s for Russia’s scrapped Mir-2 space station, is the station’s main life support centre, as well as containing crew facilities. As such, the leak needed to be found.

To do so, the ISS crew turned to a novel technique – albeit one those who like sci-fi films may well have seen employed in similar situations: the release a suitable material into the atmosphere, and then following the trail as it is inevitably drawn to the source of the leak. In this case, the most innocuous of items was selected – the humble tea leaf.

The smaller transfer compartment of the Zvezda module, indicating how hard finding a leak can be given all the equipment. Credit: NASA / Roscosmos

Released from a tea bag, the tea leaves were left in the sealed module while the crew observed them via video feed, noticing the leaves eventually drifted to a scratch on one of the wall panels – and stayed there. Examination of the scratch, close to some communications equipment, proved it was in fact a tiny crack in the module’s inner surface, allowing the crew to make repairs using another humble item: duct tape.

2 thoughts on “Space Sunday: asteroid sampling & starship building

    1. It’s been a low-key story for the last several months – bubbling up and dying away again. Most “space” publications reported it responsibly (like, when the leak started to cause a noticeable loss / the ISS crew had reached a point in their work cycle more time could be devoted to leak hunting. The news of the fix itself came via Roscosmos, the Russian space agency rather than NASA, as both agencies effectively manage the space station.

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