On September 24, the United States National Aeronautics and Space Administration (NASA) made history by returning to Earth a sample collected from an asteroid located far from our planet for the first time.
This asteroid is called Bennu, measuring 262 meters in diameter and situated approximately 6.2 billion kilometers from Earth. The sample collection from this asteroid was conducted by a NASA probe in 2020, before the spacecraft embarked on a lengthy three-year journey back to Earth.
Reportedly, when the spacecraft was about 108,000 kilometers from Earth, the OSIRIS-REx spacecraft released the capsule containing the samples into the atmosphere. The capsule was then closely approached and examined by NASA personnel. Initial inspections indicated that the capsule was intact and undamaged during the landing. Estimates suggest that the collected sample weighs around 250 grams, exceeding the initial target of 60 grams. This is the largest sample mass returned from space to Earth since the Apollo program.
The “stormy” landing process
Previously, since Bennu is one of the asteroids with the potential to collide with Earth, NASA decided to launch the OSIRIS-REx spacecraft in 2016 with the mission to explore this asteroid and return samples for research. It was not until two years later, in 2018, that the spacecraft arrived at Bennu.
Before OSIRIS-REx, there were several missions aimed at exploring asteroids conducted by Hayabusa 1 (Japan) and NEAR-Shoemaker (USA). These missions confirmed a long-held hypothesis by meteorite researchers: many asteroids are not solid rock, but rather a combination of boulders, sand, and gravel formed from collisions between larger objects throughout the history of the Solar System.
Stereo image of a rock outcrop on the surface of asteroid Bennu.
For OSIRIS-REx’s mission, observations from telescopes suggested that the geological structure of Bennu is quite similar to Itokawa, based on information brought back by Japan’s Hayabusa spacecraft in 2010. Bennu is believed to be composed of interspersed pebbles and smooth surfaces filled with sand, where the spacecraft could land comfortably.
However, NASA researchers were truly “offside.” When OSIRIS-REx first observed Bennu up close, it discovered a “hellish landscape” with sharp rocks protruding high, defying Bennu’s weak gravity. This made the initial landing plan, which relied on a laser altimeter, completely ineffective.
To find a suitable landing spot on Bennu, the scientists operating OSIRIS-REx had to meticulously map its surface features down to the centimeter using the spacecraft’s camera.
Later, with special assistance from astrophysicist and renowned musician Brian May, a 3D stereo image (created by merging two photographs to recreate depth perception) was produced. This allowed the OSIRIS-REx operational team to assess the safety of potential landing sites.
The landing of OSIRIS-REx shows that the surface of Bennu has a much lower material density than the rest of the asteroid.
Nearly “swallowed” by an asteroid far from Earth
After 22 months of careful consideration, researchers decided on a site they named Nightingale, where OSIRIS-REx touched down on October 20, 2020. Instead of landing in a 50-meter-wide area as initially planned, the truck-sized spacecraft had to attempt to land in a crater only 10 meters wide. Moreover, the operational team had to remotely reprogram the spacecraft to complete the mission.
However, even after selecting the “calmest” landing site, the probe still encountered unexpected issues. First, as soon as it landed on the surface of Bennu, OSIRIS-REx sank about 50 centimeters deep, as if the spacecraft was being “swallowed” in a swamp. Only when the spacecraft rapidly expelled a burst of nitrogen gas from the TAGSAM propulsion system did OSIRIS-REx’s fate escape a tragic end on Bennu.
However, the powerful thrusts caused a large amount of gravel and debris to be ejected from the crater and into space, enveloping the ascending spacecraft and threatening its safety. Fortunately, no incidents occurred with OSIRIS-REx.
At this moment, the team at NASA breathed a sigh of relief, but they were also perplexed. According to projections, the surface of Bennu should have been able to support the weight of the OSIRIS-REx quite well, similar to a pile of gravel.
However, the landing process indicated that the surface of Bennu must have a much lower density than the rest of the asteroid. Scientists are still searching for an explanation.
The low-density surface could be due to the existence of voids between the rocks on the surface. In the past, these voids may have been filled with fine material, but over billions of years of Bennu traveling through the Solar System, these fine particles may have settled into the asteroid, which is much denser than the surface.
To know for sure, scientists will need to gather some information about the interiors of asteroids from the upcoming HERA mission by the European Space Agency, which will study the aftermath of NASA’s DART spacecraft’s impact test on the asteroid Dimorphos that occurred in September 2022.
