After five years in space, a NASA spacecraft is on its way back to Earth after collecting dust and rocks from the asteroid Bennu.

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Osiris-Rex reached asteroid Bennu in 2018 and spent two years flying near and around it, before collecting rubble from the surface last fall.

The University of Arizona’s Dante Lauretta, the principal scientist, estimates the spacecraft holds between a half pound and 1 pound (200 grams and 400 grams) of mostly bite-size chunks. Either way, it easily exceeds the target of at least 2 ounces (60 grams).

It will be the biggest cosmic haul for the U.S. since the Apollo moon rocks. While NASA has returned comet dust and solar wind samples, this is the first time it’s gone after pieces of an asteroid. Japan has accomplished it twice but in tiny amounts.

On Monday, the OSIRIS-REx spacecraft fired its main engines for seven minutes thrusting the craft away from the asteroid at 600 miles per hour (nearly 1,000 kilometers per hour), setting it on a 2.5-year cruise towards Earth.

After releasing the sample capsule, OSIRIS-REx will have completed its primary mission. It will fire its engines to fly by Earth safely, putting it on a trajectory to circle the sun inside of Venus’ orbit.

After orbiting the Sun twice, the OSIRIS-REx spacecraft is due to reach Earth on Sept. 24, 2023. Upon return, the capsule containing pieces of Bennu will separate from the rest of the spacecraft and enter Earth’s atmosphere. The capsule will parachute to the Utah Test and Training Range in Utah’s West Desert, where scientists will be waiting to retrieve it.

“OSIRIS-REx’s many accomplishments demonstrated the daring and innovate way in which exploration unfolds in real-time,” said Thomas Zurbuchen, associate administrator for science at NASA Headquarters. “The team rose to the challenge, and now we have a primordial piece of our solar system headed back to Earth where many generations of researchers can unlock its secrets.”

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To realize the mission’s multi-year plan, a dozen navigation engineers made calculations and wrote computer code to instruct the spacecraft when and how to push itself away from Bennu. After departing from Bennu, getting the sample to Earth safely is the team’s next critical goal. This includes planning future maneuvers to keep the spacecraft on course throughout its journey.

“Our whole mindset has been, ‘Where are we in space relative to Bennu?'” said Mike Moreau, OSIRIS-REx deputy project manager at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Now our mindset has shifted to ‘Where is the spacecraft in relation to Earth?'”

The navigation cameras that helped orient the spacecraft in relation to Bennu were turned off April 9, after snapping their last images of the asteroid. With Bennu in the rearview mirror, engineers are using NASA’s Deep Space Network of global spacecraft communications facilities to steer the OSIRIS-REx by sending it radio signals. By measuring the frequency of the waves returned from the spacecraft transponder, engineers can tell how fast OSIRIS-REx is moving. Engineers measure how long it takes for radio signals to get from the spacecraft back to Earth in order to determine its location.

The spacecraft’s course will be determined mainly by the Sun’s gravity, but engineers will need to occasionally make small course adjustments via engine burns.

“We need to do regular corrections to bring the trajectory increasingly closer to Earth’s atmosphere for the sample release, and to account for small errors that might have accumulated since the last burn,” said Peter Antreasian, OSIRIS-REx navigation lead at KinetX Aerospace, which is based in Simi Valley, California.

The team will perform course adjustments a few weeks prior to Earth re-entry in order to precisely target the location and angle for the sample capsule’s release into Earth’s atmosphere. Coming in too low could cause the capsule to bounce out of the atmosphere like a pebble skipping off a lake; too high and the capsule could burn up due to friction and heat from the atmosphere. If OSIRIS-REx fails to release the capsule, the team has a backup plan to divert it away from Earth and try again in 2025.

“There’s a lot of emotion within the team about departure,” Moreau said. “I think everyone has a great sense of accomplishment because we faced all these daunting tasks and were able to accomplish all the objectives thrown at us. But there’s also some nostalgia and disappointment that this part of the mission is coming to an end.”

NASA has lots more asteroid projects planned.

Set to launch in October, a spacecraft named Lucy will fly past swarms of asteroids out near Jupiter, while a spacecraft known as Dart will blast off in November in an attempt to redirect an asteroid as part of a planetary protection test. Then in 2022, the Psyche spacecraft will take off for an odd, metallic asteroid bearing the same name. None of these missions, however, involve sample return.

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