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OSIRIS-REx, NASA’s first mission to sample an asteroid, is on a seven-year journey to collect remnants of the early solar system. When the minivan-sized spacecraft arrived at its destination – an ancient near-Earth asteroid called Bennu – in December 2018, it beamed back images of a tiny world that had never before been seen up close.
The sample of asteroid material OSIRIS-REx will return in 2023 will offer a wealth of new clues about the origins of water and life in the solar system that will keep scientists busy for generations. In many ways, the entire mission is breaking new ground, and the achievements of the spacecraft and the team behind it are forging a path for future missions. Here are 10 ways the mission’s operations and scientific exploration are advancing the asteroid frontier.
1. Operating in Close Quarters
On Dec. 31, 2018, as many on Earth were preparing to welcome the New Year, OSIRIS-REx etched its name into the spaceflight history books. With a short burn of its thrusters, the spacecraft entered orbit around asteroid Bennu at a distance of about one mile (1.75 kilometers) from the asteroid’s center, setting a new record for the closest a spacecraft has ever orbited a planetary body. Previously, that distinction belonged to the European Space Agency’s Rosetta spacecraft, which orbited Comet 67P/Churyumov-Gerasimenko at a distance of about four miles (seven kilometers).
2. Appreciating (and Orbiting) the Small Things
Closest orbit of a planetary body was not the only record that OSIRIS-REx set on New Year’s Eve 2018. As the spacecraft made its first tight loop around Bennu, the asteroid, in turn, became the smallest object ever orbited by a spacecraft.
Orbiting an object as small as Bennu (about 500 meters or 1,640 feet in diameter) is an incredibly challenging navigational feat. Unlike larger planetary bodies, Bennu has barely enough gravitational pull to keep a spacecraft in a stable orbit. Because of this, small forces – like heat radiating from the asteroid or the pressure of sunlight hitting the spacecraft – that do not have much effect on a spacecraft orbiting a planet, become major factors in OSIRIS-REx’s orbit around Bennu.
3. Going Where the Action Is
Earlier this year, while in orbit around Bennu at a distance of about one mile (1.75 kilometers), OSIRIS-REx captured images with its NavCam 1 camera that showed particles being ejected from the asteroid’s surface. These observations, which appear to depict several distinct particle ejection events, represent the first close-up images of material being released from an asteroid. Viewing and studying this phenomenon at close range will help scientists better understand the processes that are shaping Bennu and other “active asteroids” now and into the future.
4. Inventing the Reverse Space Vacuum
The ultimate goal of OSIRIS-REx – collecting a sample of loose surface material from asteroid Bennu – is trickier than collecting material from a larger body like a planet. In a gravity field as weak as Bennu’s, loose rocks and dirt would be readily lost into space if disturbed by a mechanical sampling tool like a scoop. To meet this challenge, the Lockheed Martin spacecraft team developed an entirely new sampling system called the TAGSAM (Touch-and-Go Sample Acquisition Mechanism), which works like a reverse vacuum. When it’s time to collect a sample, the TAGSAM arm will touch Bennu’s surface for about five seconds and shoot a blast of nitrogen gas to stir up and capture loose material. Because of this innovation, OSIRIS-REx can retrieve its precious cargo without having to land on the asteroid.
5. Bringing Home More Space
OSIRIS-REx will return a minimum of 2.1 ounces (60 grams) of material from Bennu’s surface, which is roughly the mass of a full-size candy bar. Although that may not seem like much, it will be enough for scientists to study for generations to come. In fact, it will be the largest sample humans have ever retrieved from an asteroid. It will also be the largest sample NASA has collected from anywhere in space since the Apollo era, when astronauts picked up hundreds of pounds of lunar rock and dust over the course of six missions to the moon. Bonus: in some pre-launch testing, TAGSAM, the spacecraft’s sampling mechanism, demonstrated that it was able to collect up to 4.4 pounds (two kilograms) of material.
6. Keeping it Clean
Most spacecraft, including OSIRIS-REx, are constructed in clean rooms, where their contact with contaminants like dust and grease is carefully controlled. For OSIRIS-REx, the team was particularly interested in limiting the spacecraft’s exposure to organic molecules. Scientists will be looking for organics in the sample returned from Bennu, and they need to be confident any materials they study came from the asteroid and did not hitch a ride from Earth. To accomplish this, the OSIRIS-REx team set a new clean room standard that had never been attempted before: a strict limit on amino acids, the molecules that make up proteins. The contamination control team set that limit at 180 nanograms (roughly 300 times less than the weight of a human fingerprint) per square centimeter of the spacecraft’s sampling system. Monitoring and testing as the spacecraft was assembled and prepared for launch revealed that the standard was not only met, but far surpassed. Amino acids were found in concentrations no higher than 13 nanograms per square centimeter – less than one tenth of the allowed limit.
7. Going Where No One has Gone Before
From Earth, scientists group asteroids into categories based on their spectral properties, or how much and which wavelengths of sunlight they reflect. Bennu is classified as a C-complex asteroid, a group that is dark in color and presumed to be carbon-rich. Some contain organic molecules and volatiles like water. But Bennu is also part of rare B sub-group asteroids – a category with less than 100 known examples in the solar system – which reflect more light in the blue wavelengths. OSIRIS-REx is the first mission to visit a B-type asteroid. In addition, the sample that returns to Earth in 2023 will be our first chance to put pristine material from this type of asteroid under a microscope.
8. Investigating the Danger Zone
The Center for Near-Earth Object Studies (CNEOS) at NASA's Jet Propulsion Laboratory classifies asteroids as potentially hazardous based on two factors: their size and their potential for crossing paths with Earth in the future. CNEOS maintains a running list of potential Earth impactors, which changes over time as we discover new asteroids and learn more about the orbits of known asteroids. Owing to its size (about 500 meters across), an orbit that brings it close to Earth every six years, and uncertainty in its exact position many years into the future, Bennu is currently second on that list – making it the highest-threat asteroid ever visited by a spacecraft. Still, the odds of Bennu colliding with Earth are slim: only 1‐in‐2700 over the next couple of centuries (or a 99.963% chance that Bennu will not impact Earth). The data OSIRIS-REx collects will help scientists better understand the risk and prepare to deflect any possible collision in the future.
9. Predicting the Unpredictable
One of the reasons scientists are uncertain about Bennu’s exact position in the future is a phenomenon called the Yarkovsky Effect. Like any planetary body, Bennu absorbs heat from the Sun. As the asteroid rotates, the sunlit (day) side moves into darkness (night), and the heat is radiated back into space. That process creates a small thrust that alters the asteroid’s path slightly. The exact amount and direction of thrust varies depending on factors like the asteroid’s shape, spin, intrinsic brightness, and texture, making long-term predictions very challenging. OSIRIS-REx’s visit to Bennu offers scientists the first chance to directly measure and test their understanding of the Yarkovsky Effect on an asteroid. This improved understanding will allow for more precise predictions of Bennu’s trajectory over time and better overall models of the forces that affect all asteroids’ motion.
10. Mapping with Lasers
One of the instruments in OSIRIS-REx’s scientific toolbox (which also includes cameras and spectrometers to visually map and study Bennu’s composition) is an instrument called OLA – the first scanning laser altimeter ever used to study an asteroid. OLA, contributed by the Canadian Space Agency, maps Bennu’s shape by measuring the round-trip time of laser pulses traveling from the instrument to millions of points on Bennu’s surface and back. The scanner allows OLA to take up to 10,000 measurements per second – more than 1,000 times faster than any previous asteroid altimeter. Ultimately, OLA will scan the entire surface of Bennu, taking up to a billion measurements, to create a highly-accurate 3D model of the asteroid’s topography and the distribution of boulders, craters and other features.