On September 8th, 2016 at 23:05 UTC, an Atlas V 411 rocket lifted-off from Space Launch Complex (SLC) 41, Cape Canaveral Air Force Station (now Space Force Station). Launched by United Launch Alliance (ULA), the rocket carried aloft NASA’s Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx), an ambitious mission to study a carbonaceous near-Earth asteroid and obtain as large a sample of material as possible for a return to Earth.
More recently, on September 24th, 2023, the mission achieved its goal, returning an estimated 250 grams of material – four times the minimum amount scientists hoped to obtain at the start of the mission – from the 500m diameter asteroid 101955 Bennu. It is not the first mission to return a sample of material from an asteroid; Japan holds that record with its Hayabusa and Hayabusa-2 missions. The first rendezvoused with asteroid 25143 Itokawa in 2005, the second with asteroid 162173 Ryugu in 2018; however, given both these missions returns a total sample cache of under 6 grams, OSIRIS-REx is the most successful to date.
Over the intervening seven years since its launch and return, OSIRIS-REx completed a round-trip journey of some 6.4 billion kilometres. Along the way it performed a fly-by of Earth some 12 months after launch, allowing it to enter an orbit around the Sun from which it could intercept Bennu. This passage around the Sun allowed OSIRIS-REx to past through the Earth-Sun Lagrange L4 position, where it performed a search for a class of near-Earth objects known as Earth-Trojan asteroids. Whilst no previously unknown asteroids were located during the 11-day survey in February 2018, the exercise yielded valuable data on vehicle manoeuvring for the kind of precise imaging required on reaching Bennu.
As it approached OSIRIS-REx Bennu in late 2018, OSIRIS-REx was able to observe Jupiter, adding to his science mission, prior to entering an initial orbit at the start of December 2018. It then spent most of the month generally characterising the asteroid, detecting hydrated minerals in the form of clay across the asteroid’s surface, suggesting it was once a part of a larger object rich with frozen water, offering a further pointer to how life-forming minerals and water may have been carried to Earth and the inner planets.
On December 31st, 2018 OSIRIS REx closed to just 1.75 km above Bennu’s surface, allowing it commence an extensive remote mapping and sensing mission which allowed the science team to identify potential areas which might be suitable for gathering one or more samples. In reaching that altitude, OSIRIS-REx set a new record for the closest distance any spacecraft has orbited a celestial object, beating ESA’s Rosetta mission’s orbit of 7 km around the comet 67P/Churyumov–Gerasimenko.
In all, 14 months were spent carefully surveying Bennu, allowing for potential sample-gathering sites to be identified, with the spacecraft closing to just 1 km above the asteroid, breaking its own record and allowing a final survey of the four preliminary landing sites so a final selection could be made. In the end, a site dubbed “Nightingale”, a fairly open shallow depression on the asteroid’s surface, was selected, and the mission moved to the rehearsal phase.
In order to collect samples, OSIRIS-REx had to make physical contact with the asteroid in a “touch and go” (TAG) manoeuvre. This would see the spacecraft deploy a robot arm underneath itself. Called the Touch-And-Go Sample Arm Mechanism or TAGSAM, this spring-loaded arm carried a camera system and, on its end, a sample gathering system. The craft would then use its thrusters to gently push itself down towards Bennu, bringing the sample head into contact with the asteroid’s surface.
At this point, several things would happen in rapid succession: the springs in the arm would absorb the spacecraft’s motion, allowing it to maintain contact for a second or two as a jet of inert nitrogen would be directed at the surface under the sample head in order to blast material up into it while Velcro-like rings on the end of the head would snag dust particles and the like. Then, as the springs in the arm recoiled under the mass of the spacecraft and very gently push it back away from the asteroid, allowing a Mylar cap to close over the sample head, trapping whatever had been captured inside the head. Finally, once the spacecraft was sufficiently clear of the asteroid – 40m or so -, OSIRIS-REx would fire its thrusters an position itself back in orbit a few hundred metres above the asteroid, where the sample gathering operation could be assessed for success and from which, if required, a further attempt made to grab material.
All of this was obviously quite complex – and due to the the delay in communications between vehicle and Earth, had to be carried out entirely autonomously. Hence the rehearsal phase of the mission. These were carried out in April and August 2020, with the first bringing the craft to within 65 metres of the sample site and the second stopping just 40 metres above it. Both saw the craft go through all phases of the TAG operation, sans actually touching the asteroid, with a small burst from the thrusters substituting from the recoil of the TAGSAM springs to push it away from the asteroid once more. Both rehearsals were flawless and paved the way for the first – and only, as it turned out – sample gathering attempt.
Inara Pey: Living in a Modemworld 