The Osiris-Rex team said Tuesday that the surface of the asteroid, which measures 490 meters (1,600 feet) in diameter, was covered in stones and boulders. They had expected it to be smoother and easier for the probe to touch.

"We go back to the drawing board and start thinking again," Dante Lauretta, the head of the mission, told a press conference. The team's observations also appeared in the Nature journal on Tuesday.

The probe was designed to head for a flat area with a radius of 25 meters, but the images beamed back since December showed that there is no area that big which is free of boulders.

As a result, the team will have to aim more tightly.

"Now we're going to try to hit the center of the bullseye," said project manager Richard Burns.

Since December, the probe has been using its instruments to map Bennu from a close distance, currently three miles.

The asteroid, which orbits the sun, is 85 million kilometers (52 million miles) from the Earth.

The goal is touch the surface with a robotic arm for just five seconds in July 2020, retrieving a sample of between 60 grams and two kilograms (two ounces to 4.4 pounds) of regolith, which means relatively small particles such as gravel or sand, since the machine can only suck up particles measuring less than two centimeters.

The samples will be stowed in the probe, which will return to Earth in 2023.

Bennu is technically known as "rubble-pile asteroid", that is, it is made up of pieces of debris that had broken off larger celestial bodies and come together under the effect of gravity.

It has more than 200 boulders larger than 10 meters in diameter, and some stretching up to 30 meters, according to researchers writing in Nature Astronomy. It has a number of craters between 10 and over 150 meters in length.

"It is not trivial to deliver a spacecraft with meter scale resolution to the surface of an asteroid in the microgravity environment," said Lauretta, who nevertheless said he was "confident" that the team would rise to the challenge.

Another surprise Bennu had been withholding was that it emits particles which fall back to the surface like rain. That should not however endanger the probe, the team said.