NASA/Goddard/University of Arizona

Before NASA's Osiris-Rex spacecraft can reach out and grab a piece of asteroid Bennu, it needs to find a safe spot on the space rock's surface. And for that, NASA wants your help.

Osiris-Rex, which arrived at Bennu on Dec. 3, 2018, aims to become the first US spacecraft to return a sample from an asteroid to Earth. Japan's Hayabusa mission brought back asteroid particles in 2010, with another asteroid-wrangling mission out of Japan under way this year.

Since the NASA craft arrived at Bennu, the team has discovered an extremely rocky terrain that threatens the vehicle's safety. So NASA is asking volunteers to develop a hazard map by measuring Bennu's boulders and mapping its rocks and craters via a web interface.

"Bennu has surprised us with an abundance of boulders," Rich Burns, Osiris-Rex project manager at NASA Goddard Space Flight Center, said in a statement. "We ask for citizen scientists' help to evaluate this rugged terrain so that we can keep our spacecraft safe during sample collection operations."

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All that's needed to hang out on the asteroid is a mapping app, a screen large enough to clearly see images of the asteroid's surface and a mouse or trackpad that can make precise marks.

An interactive tutorial explains how to get around the CosmoQuest app circling craters, measuring boulders and erasing mistakes. CosmoQuest, a project run out of the Planetary Science Institute that supports citizen science initiatives, offers additional user assistance through an online community where mappers can share tips and high-fives and ask questions. CosmoQuest also shares guidance through livestreaming sessions on Twitch.

The original design for capturing a piece of the space rock was based on locating a hazard-free zone with a 160-foot (25-meter) radius on Bennu's surface. However, because of the unexpectedly rocky terrain, the team is yet to identify such a site.

The volunteer asteroid mapping is straightforward work that involves dragging and dropping, an eye for detail and a bit of perseverance. When I signed up Wednesday night at about 9 p.m. PT, more than 70 "Bennu Mappers" were online scouring the asteroid's surface. The Bennu mapping campaign continues through July 10.

Screenshot by Leslie Katz/CNET

Bennu measures about 1,600 feet (500 meters), only slightly wider than the height of the Empire State Building, and is considered potentially hazardous. "There's a very small chance that it will impact Earth in the next century," Burns has said. Which means you probably don't need to get out your helmet just yet.

In its first five months communing with Bennu, Osiris-Rex has already discovered trapped water and determined that Bennu is between 100 million and a billion years old, making it significantly more mature than predicted. But the mission's coup de theatre, the sample grab, won't take place until 2020. That's when the spacecraft's fancy robotic arm will reach out and tag the asteroid's surface with its Touch-And-Go Sample Arm Mechanism, or Tagsam.

While touching the rock, the arm will blow a burst of nitrogen gas to loosen up bits of debris that will be then be brought back to Earth when the spacecraft returns in September 2023.

The asteroid may contain unaltered material from the beginning of our solar system. The hope is that Bennu's cargo will yield insights into astronomical processes and resources in near-Earth space and improve our understanding of asteroids that could impact Earth.

Originally published May 23.