Asteroid Bennu is about to play ‘tag’ with a NASA spacecraft

The spacecraft will perform the entire sequence of approaching the asteroid and collecting the sample autonomously since live commands from Earth won’t be possible.

Based on the data returned by the spacecraft, the mission team should be able to confirm if the touchdown was successful on Tuesday.

The spacecraft has conducted two rehearsals without actually touching the asteroid, so its team is prepared for this highly anticipated event.

However, Bennu isn’t what scientists expected it to be. The Nightingale landing site is surrounded by hazards, so the mission will need to be executed with the utmost precision.

The OSIRIS-REx spacecraft will have to detect hazards and delay its own mission if any obstacles get in the way of the sample collection. But if everything runs smoothly, the spacecraft and its prized sample will begin the long journey back to Earth next year and land the sample on Earth in 2023.

Dodging building-size boulders

Imagine a van that can seat 15 passengers flying through space and approaching a rock that has the height of the Empire State Building and is rapidly rotating. That’s the OSIRIS-REx mission to Bennu.

Bennu has an orbit that brings it close to Earth, which is why it’s considered to be a near-Earth asteroid. One of its future approaches could bring it perilously close to Earth next century; it has a one in 2,700 chance of impacting our planet.

The samples from Bennu could help scientists understand not only more about asteroids that could impact Earth but also about how planets formed and life began. In total, the spacecraft will collect about 2 ounces, or 30 sugar packets, worth of material from the asteroid.

Although this sounds small, it’s the largest sample return from space since the Apollo program, according to NASA.

But first, OSIRIS-REx has to navigate a small, narrow path to the Nightingale landing site inside a crater on the asteroid’s Northern Hemisphere.

The touchdown area has been reduced to about one-tenth of the original plan, so accuracy is key. On the crater’s eastern rim sits a building-size boulder that could threaten the spacecraft as it backs away from the asteroid once the sample has been collected.

Bennu “is not nearly the sandy beach we hoped and were expecting,” said Thomas Zurbuchen, associate administrator for the Science Mission Directorate at NASA, during a September press conference.

Instead, the scientists discovered the asteroid was covered with house-size boulders. The team used the spacecraft to map the entire asteroid and determine four potential landing sites. Nightingale was ultimately chosen as the best one.

“We selected Nightingale because, by far, it has the most fine-grained material of all the four sample site candidates,” said Dante Lauretta, the mission’s principal investigator and a professor at the University of Arizona Lunar and Planetary Laboratory, in September.

“We spent early 2020 doing low-altitude reconnaissance passes over this site, ultimately imaging at about an eighth of an inch per pixel. We basically have incredibly detailed images covering the entire crater, and we counted all of those rocks.”

Going in for a touchdown

The Nightingale landing site is 52 feet in diameter, about the size of a few parking spaces where the van-size spacecraft will briefly touchdown for just seconds. There is no room for error; building-size boulders are just steps away.

The event will take about 4.5 hours to unfold and the spacecraft will execute three maneuvers to collect the sample.

The spacecraft will first fire thrusters to leave its safe orbit around the asteroid, which is about 2,500 feet away from the surface, and travel for four hours before reaching just 410 feet away. Then, the spacecraft will adjust for position and speed to continue descending.

Next, OSIRIS-REx will slow its descent to target a path so it matches the asteroid’s rotation during contact. Its solar panels will fold into a Y-wing configuration above the spacecraft to protect them.

At last, OSIRIS-REx will touchdown for less than 16 seconds. The spacecraft will fire a pressurized nitrogen bottle into the asteroid, using the gas as a way to lift material off Bennu’s surface.

The spacecraft’s collector head will capture the stirred up material. This head, located on the 11-foot-long robotic sampling arm, is the only part of the spacecraft that will touch Bennu. The team compares it to an air filter in an older model car, perfect for collecting fine material.

Small discs, which can collect dust like sticky pads, are also located on the head in case part of the sampling maneuver doesn’t go according to plan.

A camera on the spacecraft will take footage of the collection event.

“We’ll be able to tell if we were tilted, if gas blew out to the side, if material was sufficiently stirred up,” Lauretta said. “We also will have a very good indication of the exact location in Nightingale where we made contact and we can compare that to our samplability map, to assess if we touched down in an area where there is abundant sampleable material or one of the rockier locations.”

Then, the spacecraft’s thrusters will fire and help it back away from Bennu’s surface and reach a safe distance once again.

The OSIRIS-REx spacecraft will go through these maneuvers on its own and ultimately has to decide if it’s safe or not. The spacecraft will rely on its Natural Feature Tracking navigation system, which actively takes images during the descent and compares them with images stored in a catalog onboard to ensure it’s on the right path. This will influence each step of the descent and approach.

If it gets off course, the spacecraft will abort the attempt.

The navigation system also contains a hazard map of the landing site, and if it detects that the spacecraft is approaching a hazard, it will halt at 16 feet above the asteroid and not continue with the attempt.

This means that OSIRIS-REx can stay safe and attempt another sample collection in the future.

Checking in with Earth

Throughout each step, the spacecraft will send back data to NASA researchers on Earth, which will allow them to determine if touchdown was successful or not. While only a limited number of the team will be on site at Lockheed Martin Space’s Mission Support Area due to the pandemic, others will be on-site at different locations to monitor the event.

To make sure the spacecraft actually collected a sample, one of the OSIRIS-REx cameras will take images of the collection head on October 22. And on October 24, the spacecraft will conduct a spin to determine the mass of the material it collected.

Although 2 ounces is the goal, OSIRIS-REx can hold up to 4 pounds.

If the sample collection goes as planned, the head and sample will be placed and sealed in the sample return capsule to be returned to Earth.

But if the collection failed or there isn’t enough material, there are two more nitrogen charges for future attempts at a second site called Osprey in January 2021.

Altogether, the team members will spend about a week assessing what they collected and ensuring that the spacecraft and its instruments managed to survive the encounter unscathed.

“By far, the most likely outcome we will have on October 20 is we will contact the surface and come away with a large sample that exceeds our requirements. But Bennu has thrown us a number of curve balls already, which is why we are fully prepared to tag at Osprey (the backup site) if that becomes necessary,” said Mike Moreau, deputy project manager at NASA’s Goddard Space Flight Center in Maryland, in a statement.

Recently, scientists discovered that the fine material at the Nightingale site was only recently exposed to the space environment. This means that the material gathered by OSIRIS-REx will be some of the most pristine material on the asteroid.

And the sample of Bennu that will be returned to Earth won’t be anything like the meteorites we have on Earth, NASA scientists say, because so much of what they’ve learned about Bennu is unexpected.