The U.S. space agency said Friday that it expects to have captured some hundreds of grams in rock samples during a recent visit by a spacecraft to the asteroid Bennu, but added the collector is leaking some of the material.

The OSIRIS-REx spacecraft is on the first U.S. mission to carry samples from an asteroid back to Earth. On Tuesday, it unfurled its robotic arm, attached with a disk-shaped sample collector at its end, to touch the surface of Bennu for six seconds.

The OSIRIS-REx spacecraft's collector. (Courtesy of NASA)(Kyodo)

According to the National Aeronautics and Space Administration, images have shown that the sampler, about 30 centimeters in diameter, is full of rocks and dust. But the team also discovered that material is escaping through a flap that is wedged slightly open by what is thought to be some larger rocks picked up in the collection process.

There is "definitely evidence of hundreds of grams of material, and possibly more," Dante Lauretta, a University of Arizona scientist who leads the mission, told a teleconference, adding, "My big concern now is that the particles are escaping."

As any movement of the spacecraft and sampler may lead to further asteroid material loss, the team decided to forgo its planned sample mass measurement scheduled for Saturday to focus on swiftly stowing it in a capsule in which any loose material will be contained during the spacecraft's journey back to Earth.

The mission's goal is to obtain samples of between 60 grams and 2 kilograms and return the material to Earth in 2023.

NASA plans to exchange material with Japan's space agency, which is expecting samples from the asteroid Ryugu collected by its Hayabusa2 explorer, currently en route back to Earth.

Asteroids are remnants from the early formation of the solar system about 4.6 billion years ago, and studying them is expected to provide fresh insights into the formation of the solar system and the origins of life.

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