Why is OSIRIS-REx collecting a sample from asteroid Bennu?

Our solar system is filled with countless asteroids, comets, and other small worlds left over from the disk of dust and gas that formed the planets 4.5 billion years ago. Some of these worlds collided with early Earth, and in doing so may have brought water and carbon-containing materials called organics here that formed the basis of life as we know it.

On Earth, weather and geological processes constantly alter the surface, but that’s not the case on asteroids, which have remained largely unchanged. By studying them, we learn what our infant solar system was like.

Although modern spacecraft instruments can tell us a lot about other worlds, certain kinds of experiments can only be done on Earth. What we really need is a sample, so NASA’s OSIRIS-REx spacecraft will bring us one from Bennu, a near-Earth asteroid containing carbon and water. The spacecraft launched in 2016 and arrived at asteroid Bennu in 2018. OSIRIS-REx will depart Bennu in 2021 and return the sample to Earth in 2023, providing material for decades of scientific study.

💡 Why sample return matters

Precision. Some space-bound experiments can’t be done very precisely. One example is determining the origin and age of a rock, which is important for piecing together timelines of what happened in the early solar system before life arose on Earth.

Reproducibility. Science is all about being able to reproduce your results. NASA and Japan’s space agency JAXA are sharing asteroid samples collected by the OSIRIS-REx and Hayabusa2 missions, allowing researchers to analyze them with more than one science instrument in more than one laboratory.

Duration. When NASA returned samples from the Moon during the Apollo program, it knew technology would improve over time, so it stored some samples aside and even kept some sealed for future research with more advanced tools. The samples OSIRIS-REx brings to Earth will be available for future generations to study.

How OSIRIS-REx sample collection works

Collecting a sample from Bennu is no small challenge. The asteroid, which measures 500 meters (a third of a mile) wide, ended up being much rockier than mission designers expected. The sample site is just 16 meters in diameter and surrounded by boulders bigger than OSIRIS-REx itself. The spacecraft must collect its sample without guidance from Earth, since it currently takes nearly 20 minutes for signals to travel between our planet and Bennu at the speed of light.

The entire process takes almost 5 hours. OSIRIS-REx will match Bennu’s 4-hour rotation rate and slowly descend to the surface. To give the spacecraft more room to maneuver, it adjusts itself into a Y-shape, extending its sample arm 3 meters and tilting back its two solar panels. Eventually OSIRIS-REx must turn its high-gain antenna away from Earth, restricting the volume of information ground controllers can receive. The spacecraft figures out where it is by comparing surface views from prior flyovers with real-time camera images. It will back away immediately if it thinks it’s going to crash.

Bennu barely has any gravity, so OSIRIS-REx can’t land. Instead, the spacecraft will high-five Bennu with a cylindrical dinner plate-sized device at the end of its arm called TAGSAM, the Touch-And-Go Sample Acquisition Mechanism. TAGSAM blasts nitrogen gas into the surface, kicking dust and small rocks into a collection chamber that runs around the inside of the device.

OSIRIS-REx won’t overstay its welcome, immediately backing a safe distance away from Bennu. The mission team will take pictures of TAGSAM to verify they got a sample, and later spin the spacecraft to weigh it. If for some reason things go awry, the spacecraft carries enough nitrogen for two more collection attempts. But if everything goes according to plan, OSIRIS-REx will store the sample in a capsule and depart for Earth next year. In September 2023, the capsule will parachute to a landing in Utah.

How to get involved

For space fans, moments like these are the equivalent to a championship game. Join us at planetary.org/live for NASA TV coverage and watch OSIRIS-REx complete this exciting mission milestone. We won’t see any close-up images from the sample collection until later, when OSIRIS-REx turns its high-gain antenna back toward Earth. But ground controllers will be able to tell how things are going through limited telemetry data, and we’ll likely see them react with joy upon touchdown.

NASA TV coverage starts on 20 October 2020 at 2: 00 p.m. PT / 5: 00 p.m. ET / 21: 00 UTC. Touchdown confirmation signals will reach Earth at about 3: 10 p.m. PT / 6: 10 p.m. ET / 22: 10 UTC.

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