Why Is a NASA Spacecraft Currently Orbiting an Asteroid?

The OSIRIS REx spacecraft will perform a cosmic smash-and-grab on the surface of the potentially hazardous asteroid, Bennu. What do scientists hope to learn?

Sabrina Stierwalt, PhD
5-minute read
Episode #373
The Quick And Dirty
  • NASA's OSIRIS-REx spacecraft is currently orbiting the potentially hazardous asteroid Bennu so that it can grab a sample in August to bring back to Earth
  • Studying the sample of Bennu will reveal more about what the asteroid is made of and thus how much damage it could do in a future impact with Earth
  • Scientists will also learn more about whether asteroids could be the potential source of water on Earth and how our solar system formed 

While much of planet Earth stays home to stop the spread of the coronavirus, the OSIRIS-REx spacecraft hovers 105 million miles away in the ultimate act of social distancing. The NASA spacecraft has been in orbit around the asteroid Bennu since 2018 with the goal of eventually grabbing a sample from its surface and returning that sample to Earth.

The sample grab is planned for August 2020, so in April, the team of scientists and engineers behind the OSIRIS-REx mission had a dress rehearsal of sorts. They practiced two of the four steps the spacecraft will have to perform to leave its orbit, approach the asteroid’s surface, and retrieve the sample before backing away.

First, they fired the spacecraft’s engines so that it left its safe orbit of 1 kilometer (or 0.6 miles) away. Then, while on its approach, they performed a checkpoint burn or a second engine firing. That's when the spacecraft checks its location and adjusts its position so that its trajectory heads toward the planned sample retrieval site. The spacecraft continued its approach, making it within about 75 meters (or 246 feet) of Bennu, but then backed away.

The whole smash-and-grab event will take only five seconds of actual contact between spacecraft and asteroid.

The team also practiced successfully deploying the spacecraft’s Touch-And-Go Sample Acquisition Mechanism—a fancy name for its arm, which will extend toward the asteroid’s surface to grab the sample. The whole rehearsal took about four hours and was performed mostly by a team of scientists operating from home. (Talk about working remotely!) During it's close approach, the spacecraft took pictures. You can check out the detailed images of the asteroid's surface on the mission website.

When the real sample grab happens in August, OSIRIS-REx will fire a charge of pressurized nitrogen. This blast is meant to stir up the surface so that the arm can collect a sample before the spacecraft backs off. The whole smash-and-grab event will take only five seconds of actual contact between spacecraft and asteroid. The spacecraft is planned to eventually begin its two-and-a-half-year journey back to Earth in March, 2021.

Why are we orbiting an asteroid?

Asteroid Bennu is one of a subset of elite asteroids classified as PHAs, or potentially hazardous asteroids. This means they cross Earth’s orbit and are large enough to cause significant damage should there be an impact. Bennu, specifically, is about a third of a mile across (or just shy of 500 meters.) It has a “relatively high probability of impacting the Earth” one hundred or so years from now. 

So learning about Bennu is most immediately an act of self interest—we want to know what it's made of to better understand the damage it could potentially do. Asteroids made of ice and porous rock, for example, are less dense than asteroids made of, say, iron. The ice-and-rock asteroid has less damage potential. 

The OSIRIS-REx mission will help us answer deeper questions about how our solar system formed and how water—and thus, life as we know it—came to be on Earth.

Want to learn more about the effects of an asteroid impact? This simulation, created by scientists at Purdue, allows you to experiment with different asteroid sizes, densities, and impact angles to see just how much these factors matter in terms of eventual destruction

Being able to study a sample from the asteroid up close will give us an idea of Bennu’s physical and chemical properties. While in orbit, OSIRIS-REx is also making detailed maps of the asteroid’s surface and monitoring its trajectory closely. All of this information together will help guide a plan toward mitigating any future impacts. 

The OSIRIS-REx mission will also help us answer deeper questions about how our solar system formed and how water—and thus, life as we know it—came to be on Earth. As we’ve discussed before, scientists still aren’t sure whether Earth’s water was brought here originally on a comet, an asteroid, or even during the same event that gave us the Moon. Getting a detailed look at a sample of Bennu—in particular, its isotope budgets—will shed some light on whether the makeup of the asteroid is consistent with being Earth’s cosmic water source.

The unique level of detail to which we’ll get to know Bennu will also help us in all future asteroid studies. Of course, we’ve examined meteorites—cosmic debris that has fallen to Earth—but only after their trajectories have brought them across our path and through our atmosphere.

Since we don’t usually have the opportunity to examine an asteroid sample up close in the lab, we mostly study asteroids through ground- and space-based telescopes. If all goes well, after 2023, we will have both an original sample and all of these other measurements for Bennu so we can see how they compare. We can get an idea of how the conclusions we draw from those telescope-based measurements stack up to the real thing. Then, we can apply any adjustments to future studies of asteroids.

Asteroid mining 

Another outcome of the OSIRIS-REx mission is that it serves as a potential dry run for attempts to mine asteroids. Asteroids are rich in raw materials like water, iron, and precious metals like gold and silver. These materials could be brought back to Earth for processing or processed on-site.

Currently, the costs and risks of asteroid mining outweigh the benefits. It is still hard to tell which asteroids might be the best candidates for mining. Not to mention, visiting them is expensive. Asteroids are also far less massive than a planet or even a moon, which means they don't have strong gravity. That means drilling into the surface could cause a significant disruption of the drill site and even of the asteroid’s trajectory.

But if supplies begin to run out on Earth, and if more public and private investments drive the cost of spaceflight down, extracting these resources from asteroids may be an option in the future.

Asteroid citizen science 

If you’re stuck at home and looking for a way to contribute to our understanding of asteroids like Bennu, there are a few citizen science efforts that need your help. 

NASA scientists combine amateur observations with professional ones to get a more complete picture.

If you have access to a telescope (8” or larger), you can make and submit your own observations of asteroids through NASA’s program Target Asteroids!. When trying to pin down an asteroid’s precise trajectory, the more data the better. NASA scientists combine amateur observations with professional ones to get a more complete picture. 

Don’t have access to a telescope? No problem. CosmoQuest is looking for help with mapping craters made by impacts on the Moon and the brightest asteroid, Vesta. Because no matter how advanced we build our software, it’s still not as good as the human eye.

And over at Zooniverse, astronomers are looking for help finding evidence of asteroids in images taken by the Hubble Space Telescope. Given the stay at home orders, they've released more datasets and are hoping to reach one million classifications done by people like you. Catching an asteroid in the act of streaking across an image is rare, but even if you don’t see one, you get to look at some gorgeous Hubble Space Telescope images while on your hunt.  

Please note that archive episodes of this podcast may include references to Ask Science. Rights of Albert Einstein are used with permission of The Hebrew University of Jerusalem. Represented exclusively by Greenlight.

About the Author

Sabrina Stierwalt, PhD

Dr Sabrina Stierwalt earned a Ph.D. in Astronomy & Astrophysics from Cornell University and is now a Professor of Physics at Occidental College.