HOUSTON — The asteroid Bennu, with the shape of a spinning top, turns out to be extremely rugged. That is going to make it difficult for a NASA spacecraft, Osiris-Rex, to vacuum up a sample to take back to Earth. It was designed to collect sand and gravel, not boulders.
In addition, Bennu is shooting back.
“We are seeing Bennu regularly eject material into outer space,” said Dante Lauretta, Osiris-Rex’s principal investigator, during a telephone news conference on Tuesday. He and other mission scientists have been presenting their findings at the Lunar and Planetary Science Conference in Texas. “We saw the first evidence of this in January of this year and have since observed 11 such events.”
The NASA spacecraft, which launched in 2016, entered orbit around Bennu on Dec. 31. It is not the only spacecraft from Earth exploring an asteroid. Hayabusa2, launched by Japan’s space agency in 2014, began orbiting the asteroid Ryugu last year. Its mission is also to collect samples for return to our planet for study, and members of its team were presenting findings at the Texas conference on Tuesday as well.
Both missions have found that the objects they are studying have terrain much more jagged than anticipated. But while Hayabusa2 already collected its first sample from Ryugu’s surface last month, the particles erupting from Bennu posed an additional challenge for the Osiris-Rex mission.
Varying in size from inches to perhaps a few feet in diameter, some of the ejected debris escaped Bennu’s tenuous gravity, and launched in the right direction and speed to enter orbit, becoming tiny moons for at least a short while.
“We certainly did not expect to see this activity,” Dr. Lauretta said.
When the first burst was detected on Jan. 6, the mission planners made quick calculations to determine whether their spacecraft was in any danger. “Were we safe in orbit?” said Rich Burns, the project manager for the mission.
With the same tools used to assess the danger to satellites around Earth from orbital debris and meteorites, the team concluded that the chances of Osiris-Rex suffering a hit was very low.
The spacecraft may be out of peril for now. But the discovery adds Bennu to a rare category of space rocks called active asteroids. Of the 8,000 asteroids discovered, only 12 have ever been observed releasing their material into space. The Osiris-Rex mission team reported these and other findings in seven papers published in the Nature family of journals on Tuesday.
The mission of Osiris-Rex — the name is a shortening of Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer — is to study a carbon-rich asteroid in hopes of understanding bits of the early solar system and perhaps how the building blocks of life came to Earth. (Japan’s Hayabusa2 team has a similar mission.)
Bennu, some 1,600 feet wide, is covered, almost top to bottom, with boulders. Some are only a few feet wide. The largest, some 250 feet across, protrudes like a giant pimple.
The spacecraft’s primary mission is to collect samples, and it was designed to aim at a target circle 82 feet in diameter. It is to briefly place its collection apparatus, which resembles an automobile air filter, on the surface, blow a burst of nitrogen and gather material that is knocked upward.
However, there is no 82-foot-wide hazard-free location on Bennu. Dr. Lauretta and Mr. Burns both expressed confidence that they will be able to guide the spacecraft to a smooth enough site. They have time to prepare. Osiris-Rex is not to make its collection attempt until July next year.
The Hayabusa2 mission overcame a similar challenge earlier this year at Ryugu, another carbon-rich asteroid.
In outward appearances, Ryugu is similar to Bennu — a faceted spinning top, covered with boulders. Hayabusa2 had been designed to set down on Ryugu within an accuracy of 164 feet to collect its sample, but there was no clear swath big enough on the asteroid.
Refining their techniques, the Hayabusa2 navigators were able to touch down within 9 feet of their target, firing a projectile into its surface to free material for its sample collection tool. In April, Hayabusa2 is to shoot a larger bullet into Ryugu to create a crater and allow its instruments to peer at what lies below the asteroid’s surface. Later, the spacecraft may collect another sample from the artificial crater.
While they look similar, measurements show distinct differences between the two asteroids. Bennu contains a bounty of waterlogged minerals while Ryugu — according to papers published Tuesday in the journal Science — appears to hold only wisps of water, as if the material had been heated to hundreds of degrees and dried out.
Ryugu’s mineralogical signal could just be in the surface layer, and underneath it is more like Bennu, or the object the Japanese researchers are studying could be dry all the way through. That’s one of the things excavation by the bullet might reveal.
Hayabusa2 is to leave Ryugu at the end of a year and arrive at Earth a year later while Osiris-Rex will remain at Bennu a bit longer. It is to depart sometime after March 2021 and drop off its sample of Bennu at Earth in September 2023.
The papers presented by the Osiris-Rex team also reported additional results about Bennu’s shape, rotation and surface. Among the findings are the possibility that Bennu’s rate of rotation is triggering landslides. Those may shift large amounts of material from the poles down to the equatorial region, causing clumps of rocky debris to build up.
Bennu’s twirling also may have sped up over time, which has contributed to its condensed diamond-like shape. And it doesn’t seem to be slowing down, which could cause its shape to be altered even more.