NASA’s plan to scoop up dirt and gravel from an asteroid has hit a snag.

Scientists thought the asteroid Bennu had wide, open areas suitable for the task. But a spacecraft now circling the asteroid revealed the surface is covered with boulders and there aren’t any big, smooth spots for sampling.

In a paper released Tuesday by the journal Nature, scientists say they plan to take a closer look at a few smaller areas that might work. But they said sampling from those spots poses “a substantial challenge.”

The spacecraft, called Osiris-Rex, is supposed to bring the samples to Earth for further study.

The small asteroid is 70 million miles (110 million kilometers) from Earth.