The date is finally set for OSIRIS-REx’s sampling maneuver. The spacecraft has been at asteroid Bennu since the end of December 2018. During that time, it’s found a few surprises, and mapped the surface in great detail.
Now, we can circle October 20th on our calendars, as the date OSIRIS-Rex will collect its sample.
OSIRIS-REx stands for Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer. Its target, asteroid Bennu, is a carbonaceous asteroid, and it’s ancient. Scientists think that the basic chemical and mineral makeup of the asteroid was determined in about the first 10 million years of the Solar System’s history. This makes it nearly ideal as a scientific target.
OSIRIS-REx launched from Cape Canaveral on an Atlas V rocket, on September 8th 2016.
The mission has a number of objectives, including mapping the surface, understanding the Yarkovsky Effect, and generally trying to understand all the global properties of such an asteroid. But it’s the sample return that is the meat of the mission.
Astronomers are keen to collect a sample from this primordial rock, so that they can understand its minerals and organic compounds. Asteroids like Bennu may be one of the keys to understanding how the Solar System formed, and even how life began.
OSIRIS-REx uncovered a few surprises at Bennu. Shortly after arriving at the asteroid, the spacecraft discovered particles being ejected. That makes Bennu an “active asteroid,” or one that is losing mass. In Bennu’s case, its mass loss is episodic rather than continuous. Some of the ejected particles orbited the asteroid for a few days before falling back to the surface, while some were ejected into space.
According to one study, there were three causes for these ejected particles: meteoroid impacts, thermal stress fracturing, or release of water vapor.
Bennu’s surface was also a surprise. Rather than a more uniform surface with lots of suitable sampling locations, the surface is a mayhem of jumbled, hazardous boulders. That’s why finding a sampling site, and a backup site, has takes as long as it has.
Eventually, mission operators came up with four candidate sampling sites, all named after birds. Now they’ve settled on a primary and a backup site. The primary site is called Nightingale, and the backup site is called Osprey.
On October 20th, OSIRIS-REx will approach Nightingale using its Natural Feature Tracking (NFT) system to guide itself to the surface. The NFT uses a database of onboard images of Bennu’s surface, and compares them to what its cameras are seeing in real time as it descends. By comparing the two, the spacecraft can guide itself around hazardous boulders.
If it gets into trouble, it can autonomously abort and put itself back into a safe orbital position. OSIRIS-REx has enough resources for three samplle attempts.
The sampling procedure itself will be over fairly quickly. The spacecraft will touch the surface of Bennu with its sampling mechanism, called TAGSAM, or Touch-And-Go Sample Acquisition Mechanism. TAGSAM will emit a puff of nitrogen gas and then collect airborne particles from Bennu’s regolith. That puff-then-collect action will take just five seconds.
This is NASA’s first asteroid sampling mission, and there’ve been some challenges.
“The OSIRIS-REx mission has been demonstrating the very essence of exploration by persevering through unexpected challenges,” said Thomas Zurbuchen, NASA’s associate administrator for science, in a press release. “That spirit has led them to the cusp of the prize we all are waiting for – securing a sample of an asteroid to bring home to Earth, and I’m very excited to follow them through the home stretch.”
But the challenges OSIRIS-REx faces aren’t all out there in space, and on an asteroid. The COVID-19 pandemic is creating challenges nobody foresaw. And it’s the pandemic that’s partly responsible for this delay in sampling. Because of the inherent complexity of spacecraft operations, and because of the difficulty mission personnal face in gathering in groups, NASA and Goddard have given the mission an additional two months. So the original August sampling date has been moved to October.
Nobody could have foreseen the pandemic when the mission was intially planned. But missions like this always contain contingencies in the scheduling, because there are always unexpecteds.
“In planning the mission, we included robust schedule margin while at Bennu to provide the flexibility to address unexpected challenges,” said Rich Burns, OSIRIS-REx project manager at NASA’s Goddard Space Flight Center. “This flexibility has allowed us to adapt to the surprises that Bennu has thrown at us. It’s now time to prioritize the health and safety of both team members and the spacecraft.”
The team is still confident that OSIRIS-REx will successfully collect—and return—a sample of Bennu to Earth.
“This mission’s incredible performance so far is a testament to the extraordinary skill and dedication of the OSIRIS-REx team,” said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, Tucson. “I am confident that even in the face of the current challenge, this team will be successful in collecting our sample from Bennu.”
If successful, the sample will be back on Earth by September 24th, 2023.
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