In 1999, astronomers discovered 101955 Bennu, a billion-year-old asteroid half a kilometer wide, some 330 million kilometers from us. Asteroids are relics from when planets were forming. Bennu has intrigued scientists for many reasons: its bizarre shape, spinning like a top, and the many craters that dot its surface. Additionally, the asteroid has a carbon-rich composition resembling some terrestrial meteorites. Scientists believe Bennu holds answers to our origins and may have brought the building blocks of life to Earth in a collision.
So in 2016, Nasa sent a mission to Bennu: a van-sized spacecraft, OSIRIS-REx, that would not only study Bennu, but also take soil from its surface and bring it back to Earth for analysis. . OSIRIS-REx traveled for two years (2016-2018) to arrive at Bennu, to encounter many surprises that have upended our understanding of the asteroid. For the next two and a half years, the spacecraft circled the tiny space object releasing millions of images and treasure troves of data from its observations.
Earlier ground and space telescopes and spectral data showed that Bennu had a smooth, sandy beach-like surface with some rocks and boulders. However, what OSIRIS-REx discovered is that Bennu is far from smooth. Instead, it’s littered with rocks and boulders. “The rugged terrain went against all of our predictions,” NASA reports, citing Dante Lauretta, lead investigator for the OSIRIS-REx mission. In early 2019, as the spacecraft circled the asteroid at a distance of 1.6 km, the probe noticed that Bennu was spewing a plume of particles from its surface! “This is one of the biggest surprises of my scientific career,” Lauretta remarks.
Following this observation, the researchers intensified the orbits of the probe around Bennu. Over the following months, OSIRIS-REx observed further plumes of such particles. Some of the many ejected particles circled the asteroid, like satellites, and then returned to its surface. The team continues to study the data for plausible reasons for this. Fortunately, these particles did not pose a threat to the spacecraft. Other adjustments had to be integrated into the OSIRIS-REx agenda. Previously, the mission planned to land the spacecraft on a 25-foot safe area on the surface. However, the rock-dense surface now posed obstacles for the probe to collect soil samples as it was no longer a feasible location. Additionally, the mission required high-precision spacecraft performance, so sample collection was postponed.
After zeroing in on a safe place called Nightingale, in late October 2020, OSIRIS-REx achieved the magnificent feat. The space gymnastics, called the Bullseye Touch-and-Go (TAG) sample collection event, lasted 5-6 seconds. First, the spacecraft hovered above the surface, then gently touched down within three meters of the target location. He then deployed a robotic arm with a foot-wide sampling head that picked up from the ground. Then the container was immediately sealed and the spacecraft took off.
Data analysis revealed that lift-off happened in no time as Bennu’s surface particles looked like a pit of plastic bullets. They offered no landing resistance and could have sunk the probe. Quoting Rich Burns, the OSIRIS-REx project manager at NASA’s Goddard Space Flight Center, NASA says: “Throughout OSIRIS-REx operations near Bennu, our spacecraft and operations team demonstrated that we can achieve system performance that exceeds design requirements.
Analyzes of the probe data show that Bennu’s rotational speed changes due to its uneven heating and cooling as it rotates in sunlight. As a result, the asteroid’s rotation period decreases by about one second per 100 years. Additionally, the data reveals that there are not many craters on its surface as previously thought. The reason could be that Bennu is made up of a pile of rubble of boulders and rocks with only a little gravity holding them together. Because of this, rocks absorb impacts, preventing them from penetrating.
Scientists describe this as “the crumple zone in a car”, which absorbs the least energetic impacts without leaving a trace. The probe is the first to orbit a small space body. OSIRIS-REx, short for Origins, Spectral Interpretation, Resource Identification, Security, Regolith-Explorer, is now on its way back to Earth. Time will reveal what other surprises these pristine sands hold.
(The author is a science communicator)