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.