By Will Sager
In late 2020, the NASA OSIRIS-REx (Origins Spectral Interpretation Resource Identification Security – Regolith EXplorer) spacecraft tagged asteroid 101955 Bennu hoping to collect a sample to be returned to Earth for study. Scientists hope that these samples, which are thought to be primordial fragments from the early solar system, will provide clues into about planetary formation. Although Bennu threatened to eat the intrepid spacecraft, OSIRIS-REx had a blast.
Asteroid sampling has been attempted twice before by Japan’s JAXA (Japan Aerospace Exploration Agency), in 2005 from asteroid 25143 Itokawa by the Hyabusa 1 spacecraft and in 2019 from asteroid 162173 Ryugu by the Hyabusa 2 spacecraft. The first Hyabusa mission had technical difficulties that resulted in a malfunction of the sampling probe and recovery of < 1 milligram of material. Hybabusa 2 was more successful, but the recovered sample was still quite small at 5.4 grams ( about 0.2 oz). OSIRIS-REx was more ambitious, with a larger sampler called TAGSAM (Touch-and-go Sample Acquisition Mechanism), which was supposed to gather about 60 grams of material from the asteroid surface. To visualize TAGSAM, think of a large, high-tech toilet plunger about 30 cm (11.8 inches) across. The sampler, of course, is made of aluminum (not rubber). It has a ring of jets around the periphery that fire nitrogen gas to stir up small particles and force them into the sample chamber. The TAGSAM is attached to the spacecraft by a thin, articulated arm that allows the sampler to be unfolded and extended about 3.35 meters (11 feet). To collect a sample, the plan was to extend the TAGSAM and move the spacecraft slowly towards the surface of Bennu, make contact, and fire the nitrogen jets to stir up loose debris, which would be trapped inside TAGSAM by a cover flap.
After launch in September 2016 and a trip by Earth for a gravity boost in 2017, OSIRIS-REx survived another year in transit, arriving at Bennu in December 2018. It spent nearly two years surveying the surface of the little 500 meter diameter asteroid to find a good spot for sampling. Details about the sample site and sampling are contained in an article in the 15 July 2022 issue of the journal, Science (Lauretta et al., 2022). Mission scientists found a small depression, which they named Nightingale, within a small crater named Hokioi, which has a diameter of about 20 meters (66 feet). At the Nightingale site, the surface was easy for the spacecraft to reach and dominated by grains of small size, which are ideal for the TAGSAM. In contrast, much of the rest of the Bennu surface is covered by boulders. Furthermore, photographic and spectral images suggested that the site contained a mixture of lithologies typical of the Bennu surface.
Sampling occurred on October 20, 2020. After approaching the asteroid surface at a velocity of 10 centimeters per second (0.2 mph), TAGSAM contacted the surface – and kept on going because the asteroid afforded little resistance. One second after contact, the nitrogen gas jets fired as planned. Acting as retro-rockets, the jets slowed the TAGSAM advance to 4 centimeters per second. Six seconds after contact, OSIRIS-REx jets fired to back the spacecraft away from the surface. By that time, the TAGSAM had penetrated about 49 centimeters (19 inches) beneath the surface. Project scientists described the surface as a “compliant viscous fluid” that provided minimal resistance to the sampler owing to near-zero interparticle cohesion. Bulk density was calculated to be only 500 – 700 kilograms per cubic meter, which is significantly less than that of water (1,000 kilograms per cubic meter).
Cameras trained on the TAGSAM showed that the nitrogen sampling jets blasted fine particles and small rocks in all directions. One boulder near the contact point was pulverized, but another displayed more coherence and was tilted. Before and after pictures of the Nightingale surface defined a blast crater about 9 meters (29.5 feet) in diameter and half a meter deep. The blast blew away about 12 cubic meters of material.
Lauretta and colleagues describe efforts to secure the sample and estimate its mass. After backing away from the asteroid, cameras showed TAGSAM full of sample material, but not properly closed because larger fragments held open the flap meant to cover and secure the sample. Project scientists engaged in a series of spin maneuvers with the TAGSAM boom wrist meant to dislodge these particles and allow the flap to close. Although the wrist-flicks caused some material to be lost, they were successful in closing the sampler. To judge the mass of the sample, scientists rotated the arm and noted the force required. They estimate that the TAGSAM contains 250 ±100 grams (about 9 ounces) of sample. The OSIRIS-REx spacecraft is currently on its way home and will arrive next year, hopefully with the sample intact.
The interaction of OSIRIS-REx with Bennu raises interesting questions about spacecraft missions to deflect asteroids. NASA has been studying the use of spacecraft to redirect asteroids for planetary defense. The DART (Double Asteroid Redirection Test) mission is currently underway. It will crash a small probe with a mass of 610 kg (1,340 pounds) into a moonlet (Dimorphos) of the asteroid 65803 Didymos in late 2022. The objective is to see if this crash will deflect the moonlet, supporting this type of maneuver as a way of deflecting dangerous asteroids from a collision path with Earth. Bennu is a “rubble pile” asteroid, which is a very loose conglomeration of material held together weakly by mutual gravity. Some scientists think that many asteroids are of this type (Walsh, 2018). Although it may seem that a rubble pile asteroid could easily be blasted apart by explosives, the internal structure and particle distribution is unknown. The interiors may harbor larger fragments, but their size and location cannot be easily determined. This uncertainty will undoubtedly will make asteroid deflection missions more complex.
References and Sources
Hecht, J., 2022. Asteroid Bennu almost swallowed spacecraft whole. Sky and Telescope Magazine (online), 15 July 2022, https://skyandtelescope.org/astronomy-news/asteroid-bennu-almost-swallowed-spacecraft-whole/
Hecht, J., 2021. The first analysis is in of samples taken from asteroid Ryugu. Sky & Telescope Magazine (online), 12 December 2021, https://skyandtelescope.org/astronomy-news/the-first-analysis-is-in-of-s...
Lauretta, D. S., et al., 2022. Spacecraft sample collection and subsurface excavation of asteroid 101955 Bennu. Science, v. 377, p. 285-291, doi: 10.1126/science.abm1018.
Walsh, K. J., 2018. Rubble pile asteroids. Annual Review of Astronomy and Astrophysics, v. 56, p. 593-624, doi: 10.1146/annurev-astro-081817-052013.