JAXA/EPA, via Shutterstoc, left; NASA/Goddard/University of Arizona

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.

[Sign up to get reminders for space and astronomy events on your calendar.]

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…