NASA’s Osiris-REX successfully stowed its sample of Bennu’s regolith, collected earlier this week. Images show a good haul.
Space history was made last week, as NASA's Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (Osiris-REX) mission descended over the Nightingale site on asteroid 101955 Bennu, briefly contacted the surface, puffed nitrogen gas onto the regolith to jostle material into its sample collector, and backed away.
On contact, the TAGSAM head dug up to 48 cm (19 inches) deep into Bennu's surface. While the retreat thrusters fired 6.1 seconds after contact, it took 9 seconds for the spacecraft to change velocity, so the head was in contact with regolith for at least that long. In part due to Bennu's make-up and in part due to the long, deep contact, Bennu came away with more than enough material.
The day after the collection took place, researchers got a good look at the touch-and-go sample sample acquisition mechanism (TAGSAM) head using the spacecraft's SamCam imager. The view showed that the head was overflowing. Its mylar flap, which should have sealed in the sample, was wedged open, allowing tiny grains of material to escape.
The team acted quickly to store the precious cargo on the spacecraft. This meant forgoing the moment of inertia spin test, which would have "weighed" the sample. Nevertheless, images and calculations show that the sample collection likely exceeded expectations. The team estimates that the head is filled with 258 to 575 grams of material — well above the minimum target of 60 grams set for the mission.
The spacecraft's retreat seems to have blown a fresh new depression into the surface, apparent in the image sequence taken during sample collection. Unfortunately, further close imaging of the Nightingale site is unlikely. "Sadly...we are never going back," says Kevin Walsh (Southwest Research Institute). "We are trying to get something out of the backaway images, but the dust/debris cloud is making it very hard."
Stowage, Sampling Complete
The mission to explore Bennu has been a long and complex one, from the launch on September 8, 2016, from Cape Canaveral to the arrival at Bennu in December 2018. Moreover, celestial mechanics dictates a long stay at Bennu, with no return possible until March 2021. Therefore, safeguarding the sample was paramount, and the team waited until they had a full span of coverage via NASA’s Deep Space Network to stow the sample.
StowCAM caught the action on Wednesday, October 29th: The TAGSAM arm placed the sample in the capsule, tugged on it to ensure the latches were in place, then backed out of the container. Stowing the sample also meant severing the nitrogen gas tube and the TAGSAM arm itself: Osiris-REX had enough nitrogen for two sample attempts if needed, but given the hugely successful sample collection, the team decided one attempt was enough. The stowage maneuver means that the mission's approach and sample phase is officially over.
In addition to the sampling event, researchers presented interesting perspectives on Bennu at the virtual meeting of the Division for Planetary Sciences (DPS). One focus was how the asteroid got its distinctive shape.
"One of the first traits that we all notice about Bennu is its shape," says Kevin Walsh (Southwest Research Institute) during the recent 52nd DPS meeting. "This is the familiar 'top shape' or equatorial bulge. We can see it very clearly in all of the imaging, and it was actually resolved by radar ahead of time."
Sampling the crater distribution over the surface of the asteroid, researchers saw that the very largest craters (those larger than 100 meters across) are overprinted on the equatorial ridge. That means that the ridge — which researchers had initially thought was a recent development — must be quite old.
Researchers had thought the YORP (Yarkovsky-O’Keefe-Radzievskii-Paddack) effect, in which asymmetric heating and re-radiation of sunlight spins up asteroids and other small bodies near the Sun, could explain the ridge's formation in recent times. Indeed, Bennu is considered to be a fast rotator, spinning on its axis every 4.3 hours. But if its equatorial ridge is old, then another explanation is more likely. Ideas abound, but perhaps one of the most interesting is the recent proposal that Bennu was "born that way" — that is, perhaps the ridge is a remnant of the rubble-pile asteroid's formation.
However, while the ridge may be ancient, it's still changing. The YORP effect acts on the surface of the fast-rotating asteroid, moving material from its mid-latitudes toward the ridge on the equator. "The surface is actually really dynamic," says Erica Jawin (Smithsonian Museum of Natural History). "There is drastic resurfacing over much shorter timescales than I would have thought." Jawin presented evidence of the large flow of material — which even partially buries boulders — at the DPS meeting.
Next Up: Sample Returns . . . and Secondary Targets
Now, Osiris-REX will prepare for its long journey home. Departure from Bennu will occur on March 3, 2021, at the earliest, and the Sample Return Capsule will jettison prior to Earth flyby on September 24, 2023 for re-entry and collection over the Utah Test and Training Range. This maneuver is risky: NASA's solar wind sampler, Genesis, smashed into the Earth early on the morning of September 8, 2004, after its parachute failed to deploy.
Osiris-REX will stay in solar orbit after Earth flyby in 2023 and could potentially visit a secondary target. Meanwhile, the Japanese Aerospace Exploration Agency’s Hayabusa-2 mission, which visited the strikingly similar 162173 Ryugu, will deliver its own sample to Earth on December 6, 2020. JAXA has now extended this mission: Hayabusa 2 will visit asteroid 2001 CC21 for a high-speed flyby in 2026, then rendezvous with asteroid 1998 KY26 in 2031.
Perhaps we will encounter some of the particles that escaped from the TAGSAM collector one day, as Bennu may also contribute to a very faint meteor shower every late September. But the real finds will come in 2023, when Osiris-REX finally delivers its sample to Earth.