[Reading Science] Why Was Asteroid Bennu Soft Like a Swamp?

Sample Capsule to Be Dropped into Atmosphere of Probe on 24th
Scientists Shocked by Surface Landing in October 2020
Unexpected Soft Surface Submerging up to 50cm
Possible Malfunction Due to Backdraft and Debris from Thruster Ignition

The US asteroid Bennu exploration spacecraft OSIRIS-REx is finally returning to Earth after a long seven-year journey. Not only is it remarkable that it safely traveled billions of kilometers round trip, but the process of landing on the asteroid, collecting samples, and retrieving them was also fraught with difficulties. Attention is focused on the possibility that the samples may contain materials that reveal the secrets of the birth of the universe and the origins of life on Earth.

NASA-launched OSIRIS-REx spacecraft and asteroid Bennu.

The US National Aeronautics and Space Administration (NASA) announced on the 22nd that on Sunday the 24th (local time), the OSIRIS-REx spacecraft will arrive in Earth's orbit and drop a capsule containing samples from Bennu into the atmosphere. This capsule is scheduled to land at a US Department of Defense training ground in Utah, and NASA's OSIRIS-REx operations team will immediately begin recovery. NASA plans to broadcast this process live starting at 10 a.m. Eastern Time on its official website, internet streaming sites such as YouTube, and social media platforms including Facebook and X. Additionally, an event will be held allowing the public to virtually experience the entire sample recovery process.

The OSIRIS-REx spacecraft was launched on September 8, 2016, from Cape Canaveral Space Launch Complex in Florida. After flying approximately 1.9 billion kilometers over two years, it arrived near the asteroid Bennu in August 2018. It then spent two years photographing, mapping, and analyzing Bennu's surface. Finally, on October 20, 2020, it landed on Bennu's surface, collected samples, and departed for Earth in May 2021, now approaching its return.

However, it is not yet time to relax. If OSIRIS-REx sustains unexpected damage and problems arise during the capsule's atmospheric reentry and landing, the efforts of the past seven years could be wasted. NASA researchers particularly recall a harrowing incident caused by incorrect predictions when the spacecraft landed on Bennu on October 20, 2020. NASA had expected Bennu's surface to be hard, composed of visible solid rocks, stones, gravel, and dust. However, the surface at the landing site was soft like a swamp, and the spacecraft suddenly sank about 50 cm. Moreover, when the spacecraft ignited its thrusters to return after sucking in samples through its collector, a massive backblast occurred. NASA scientists acknowledge the possibility that debris ejected at that time may have damaged the spacecraft's body.

NASA scientists still do not know why Bennu's surface was so soft. They only speculate that uneven particles on the surface left many voids in the surface layer. After dropping the sample capsule into Earth's atmosphere, OSIRIS-REx plans to begin a second mission to explore another asteroid, Apophis.

Asteroid Bennu. Photo by NASA

Scientists expect that samples from the asteroid Bennu will provide insights into the secrets of the universe and solar system's birth and the origins of life. If the existing hypothesis is correct?that water, organic matter, and carbon, the basic elements of life, were delivered to Earth via asteroids?then similar components should be found in Bennu. In fact, Bennu is known to contain carbon as well as other organic materials.

Meanwhile, this is humanity's third asteroid sample collection and return mission. Japan succeeded in collecting and retrieving samples from the asteroids Itokawa and Ryugu in 2010 and 2020, respectively, using the Hayabusa 1 and 2 spacecraft. Notably, the sample collected by Hayabusa 2 from Ryugu contained uracil, one of the bases that make up RNA, a fundamental element of life, attracting significant interest.

South Korea currently has no asteroid exploration plans. The Korea Astronomy and Space Science Institute originally planned to launch a probe using the Nuri rocket in 2029 to explore the asteroid Apophis, which would approach within about 31,400 km of Earth. Although the atmosphere was favorable when the plan was made, it was canceled after failing to be selected for a preliminary feasibility study in April 2022.