How do you keep a space probe clean?

Last Friday, NASA’s OSIRIS-REx probe zipped past Earth, just 17,000 kilometres up, on its way to collect what might very well be the most expensive handful of dirt in history.

The probe, which was launched on September 8, 2016, is on its way to an asteroid called Bennu. Once it arrives, in 2018, the probe will spend 12 months imaging and collecting data before, finally, swooping down to the surface in a manoeuvre characterised as “touch and go”, scooping up 60 grams of gunk, and then ferrying it back to Earth for analysis.

The craft is scheduled to touch down on September 24, 2023, and the findings that may eventually arise from patient analysis of the sample could provide new evidence regarding not only the early history of the solar system, but also, perhaps, about how life arose on our own planet.

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Aside from the considerable construction and navigational challenges involved in getting a 6.2 metre long, 2000 kilogram delicate piece of instrument-laden machinery all the way from Cape Canaveral to a tiny lump of rock over two million kilometres away (at the closest point of its 1.2 year orbit) and back again, there is another, less acknowledged but vital matter that scientists at NASA have had to confront.

How do you keep a space probe clean? To be more precise, how do you ensure that the material scraped from the top of Bennu by the probe’s expendable Touch-and-Go Sample Arm Mechanism (TAGSAM) isn’t contaminated by microscopic debris that has been along for the ride since Day One?

Ensuring that the OSIRIS-REx sample will be kept in a pristine condition, is the responsibility of a large team of researchers headed by Jason Dworkin of NASA’s Sciences and Exploration Directorate.

Along with 61 colleagues, Dworkin compiled a 100-page protocol detailing how the need to ensure the integrity of the collected material had to inform the design and construction of the probe from the very start.

Failure to do so would be catastrophic. One of the mission’s primary aims is to see if Bennu’s surface contains organic materials, particularly amino acids. 

Some theories suggest that amino acids arriving on the ancient Earth during asteroid or meteorite impacts might have been a key ingredient for the emergence of life. Establishing that such organic compounds are present on Bennu would represent a key piece of evidence.

It is evidence, however, that would be immediately invalidated if there was even the slightest chance that an amino acid present on or in OSIRIS-REx had contaminated the sample.

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Thus, it fell to Dworkin and his colleagues to ensure that couldn’t happen. 

The first task they faced was to accept the brute reality that their task was impossible. A strict definition of pristine, he and his team note in the protocol – which is available on the physics pre-print server Arxiv – requires no “alteration of the physical, chemical, textural, or other state that compromises sample integrity.”

In a spacecraft a long way from home, the team conceded, that outcome “is beyond the scope of the science requirements” of the mission.

Some level of contamination, the team wrote, was “probable”: “Decisions and actions which impact sample cleanliness can occur at any time in the lifecycle of spacecraft fabrication, operations, and sample curation.”

The key, therefore, was to ensure a level of mitigation and hygeine that ensured any contamination that did occur will be of sufficiently low level – or sufficiently obvious – that it can be excluded and not interfere with analysis.

To do that, Dworkin and his team focussed on the TAGSAM, reasoning that the danger of contamination increases as the distance between any given component and the collected sample itself decreases.

Thus, special attention was reserved for the 192-square-centimetre surface area of the TAGSAM interior, and the collection and storage containers that will hold the Bennu dirt once it is scooped up.

The collection mechanism at the end of the TAGSAM uses a high pressure, high purity mixture of helium and nitrogen gas to fluidise loose particles from Bennu’s surface and then propel them into a collection chamber.

This tiny cylindrical chamber is enclosed in polyethylene terephthalate – known commercially as Mylar. Before the samples are deposited the chamber is flooded with a 5% helium gas, which will reveal any microscopic leaks that may have developed during the journey, which is then vented out again.

The collected particles in the helium-nitrogen mixture are then introduced. The gas mixture escapes through a metal mesh enclosing the outside of the cylinder, leaving the bits of dirt – up to 2.5 centimetres in diameter – trapped inside and hopefully as pristine as Dworkin’s team intended.

In their document, the NASA team say the cleanliness protocols for OSIRIS-REx match those for Mars missions. They also concede that they could – technically – have been even better, but raising the bar higher risked causing some very severe problems back on Earth.

Introducing more aggressive hygeine standards, the team writes, would have jeopardised the timing of the project and possibly made it miss its 39-day launch window. Doing so, the authors note, would have meant a year-long delay, which would have consumed “all available cost reserves.”

It will be six years before Dworkin – and the world – learns whether his compromise was worth it.

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