One of the most exciting science projects for the University of Arizona is taking place some 170 million miles away.

Last week, the OSIRIS-REx spacecraft conducted its final sample retrieval rehearsal on the asteroid Bennu. This four-hour rehearsal was in preparation for the spacecraft's attempt at collecting dust and rocks from the surface of the asteroid in October. If successful, OSIRIS-REx will be the first American spacecraft to bring samples of an asteroid back to Earth.

The mission has been a center of attention here in Tucson, as its principal investigator is Dante Lauretta, a professor of planetary science and cosmochemistry at the University of Arizona's Lunar and Planetary Laboratory. Part of NASA's New Frontiers program, the OSIRIS-REx mission also features UA's Heather Enos as deputy principal investigator and Michael Nolan as science team chief.

This final rehearsal, conducted on Tuesday, Aug. 11, involved the spacecraft testing its sampling acquisition system, collecting data from the asteroid's surface, moving solar panels and using its thrusters as it orbits the rock.

During the rehearsal, the OSIRIS-REx spacecraft passed 144 feet above the asteroid's surface—the closest it's ever come. From that close proximity, the spacecraft's camera suite captured its highest quality images yet.

According to NASA, because the spacecraft is millions of miles from Earth, it takes approximately 16 minutes for the spacecraft to receive the radio signals used to command it. This time lag prevented live commanding of flight activities from the ground during the rehearsal. As a result, the spacecraft performed the entire rehearsal sequence autonomously. Prior to the rehearsal's start, the OSIRIS-REx team uplinked all of the event's commands to the spacecraft and then provided the "Go" command to begin the event. The actual sample collection event in October will be conducted the same way.

OSIRIS-REx will not land on Bennu's surface to capture its sample. Instead, it will use the Touch-And-Go (TAG) Sample Acquisition Mechanism to shoot a jet of nitrogen, dislodging particles from the asteroid. The spacecraft is expected to be able to capture upwards of 60 grams worth of carbonaceous dust and rock ejected from Bennu's surface.

The OSIRIS-REx spacecraft launched from the Earth in September 2016 and is planned to return in 2023. The spacecraft will collect rocks and dust from the surface of Bennu in order to better understand "the initial stages of planet formation and the source of organic compounds available for the origin of life." Since arriving at Bennu, OSIRIS-Rex has mapped the asteroid's rocky and carbon-rich surface.

Its extremely rugged terrain will make the sample acquisition more difficult than previously anticipated. However, the OSIRIS-REx team has selected a potential site, named "Nightingale," that is comparatively less hazardous.

"Many important systems were exercised during this rehearsal—from communications, spacecraft thrusters, and most importantly, the onboard Natural Feature Tracking guidance system and hazard map," Lauretta said. "Now that we've completed this milestone, we are confident in finalizing the procedures for the TAG event. This rehearsal confirmed that the team and all of the spacecraft's systems are ready to collect a sample in October."

Since arriving in December 2018, researchers have learned much about the asteroid. Possibly the most important announced discovery is the roughness of Bennu's surface. Scans reveal Bennu is "packed with more than 200 boulders larger than 33 feet (10 m) in diameter and many more that are 3 feet (1 m) or larger. The largest boulder measures 63 yards (58 m) across." This is a far cry from the originally expected surface of gravel and pebbles. This complicates the original plan for the spacecraft to collect its sample; instead of having roughly half a football field to navigate in, OSIRIS-REx will have to collect a sample in a site half the size of a basketball court or smaller.

Bennu is also one of the darkest objects in the solar system. High resolution images shot with OSIRIS-REx's multiple cameras reveal Bennu reflects only 4 percent of sunlight. This is due to Bennu being a carbonaceous asteroid, meaning its composition includes a large amount of naturally dark carbon. However, researchers also found Bennu's surface to possess a higher-than-expected variety of albedo, or solar reflection. Meaning the brightness of Bennu's surface varies greatly from one area to the other, due to the rough surface. This presents an unexpected challenge for the spacecraft's laser scanning system, or lidar.

With a "spinning-top shape" already known before arrival, Bennu's surface is older than expected, but shows evidence of recent activity. Scientists describe this as a "dynamic surface," formed from multiple collisions with smaller asteroids. Bennu also contains a lot of empty space, up to 60 percent, but its shape indicates interior stiffness, with enough internal strength to allow the surface to crack.

Some of the first scientific data returned from the OSIRIS-REx mission was visible and infrared imaging that revealed hydrated minerals on Bennu's surface. For hydrated minerals to be formed, they must come in contact with water at some point in the past. Hydrated minerals are "ubiquitous across the surface of the asteroid," and while Bennu is too small to have ever hosted liquid water, the finding indicates liquid water was present at some time on Bennu's parent body, a much larger asteroid.

Reseachers also found that Bennu's rotation is steadily accelerating at about one second per century. This increase is due to the YORP effect, where differences on the surface of a small astronomical body unequally scatter solar radiation. Due to Bennu's aforementioned roughness and variety of albedo, it is catching and reflecting sunlight at different angles, causing its rotational rate to drift. Researching this phenomenon is one of the original goals of the OSIRIS-REx mission.