For nearly two years, a small spacecraft called OSIRIS-REx has been orbiting an asteroid more than 100 million miles away, patiently biding its time by studying the rock’s surface. Scientists believe that this asteroid, Bennu, is a piece of a much larger one that formed just a few million years after Earth. It’s a perfectly preserved cosmic time capsule that could reveal the secrets of the ancient history of our solar system. Tomorrow, OSIRIS-REx will make a daring plunge to Bennu’s surface and use a robotic arm to vacuum up some of its space dust, which it’ll bring back to Earth. The encounter will last for just a few seconds, but it is a technological feat that has been more than a decade in the making.
Shortly before 2 pm ET on Tuesday, OSIRIS-REx will fire its thrusters and begin a torturously slow descent from its orbit above Bennu. It will take the craft four and a half hours to travel just a kilometer to the surface, and by the time it gets there, it will be moving only about 4 inches per second. “Everything around this asteroid happens slowly,” says Richard Burns, the OSIRIS-REx project manager at NASA’s Goddard Flight Center. Although the team has done two practice approaches earlier this year, getting the spacecraft as close as 120 feet above the asteroid, this will be the first physical contact. “This is the one thing we haven't done, so we don’t know what’s going to happen,” says Burns. “We’re cautiously optimistic that this will be the only time we touch the surface.”
The planned sample collection area, dubbed Nightingale, is a rugged 66-foot-wide crater near Bennu’s north pole. It was selected primarily because the crater appears to be young, which means that the exposed rock is likely to consist of pristine remnants from when the asteroid was formed billions of years ago. As OSIRIS-REx approaches Bennu, it will extend an 11-foot-long arm capped with a sample collection device that looks like a large shower head. When the arm contacts the surface, it will blow a small burst of nitrogen gas onto it to stir up some dust and rocks. This asteroid dirt will be collected in a ring around the head, which can store a little more than 4 pounds of material.
The maneuver requires extreme precision. The surface of Bennu turned out to be a lot more rugged than researchers expected, but the sample collection arm works best on a flat, sandy surface. If the arm touches down on top of rocks more than a few inches in diameter, it could limit how much material it’s able to collect.
“We weren’t certain what the surface was going to look like,” says Mark Fisher, a space engineer at Lockheed Martin who worked on building the sample collection arm. “We thought it was going to have a lot more fine-grain material, but there’s lots of big rocks on the surface. And that makes it hard to get some smaller rocks into the sampler.”
Fisher says he’s not too concerned about the spacecraft’s ability to pick up at least some material. He says the nitrogen jet that will be used to stir up rocks on the surface has “a lot of kick,” so even if the sampler head touches down on a rocky area, it should be able to blow out some loose dirt lying below the rocks. The outside of the sampler head also acts a bit like velcro, which means that as long as the arm makes contact with Bennu, researchers are guaranteed to at least get some material stuck to the collector plate.
Asteroid researchers have been waiting for years to get their hands on dirt from Bennu. The asteroid was selected as a target because it’s about 4.5 billion years old—approximately the same age as the solar system—and is thought to contain water locked up in minerals. Earlier this month, researchers on the OSIRIS-REx team published a series of papers in Science based on data from a year of observations around Bennu that confirmed the asteroid is also rich in organic material.
“Bennu turned out to be exactly the kind of target we hoped it would be,” says Dante Lauretta, a planetary scientist at the University of Arizona and the lead investigator for OSIRIS-REx. Based on data collected from orbit, water accounts for somewhere between 5 and 10 percent of Bennu’s weight, and carbon-rich molecules appear to be smeared across its surface. Lauretta says this means the samples that will return from Bennu can help scientists understand the role asteroids may have played in bringing water to an ancient Earth, perhaps even seeding it with the prebiotic material that provided the building blocks for life.
“If this kind of chemistry is happening in the early solar system, it probably happened in other solar systems as well,” says Lauretta. “It helps us assess the likelihood of the origin of life occurring throughout the galaxy and, ultimately, throughout the universe.”
The samples may also have a lot to teach us about how our solar system formed, says Bill Bottke, a planetary scientist and asteroid expert at the Southwest Research Institute who is studying asteroid dynamics as part of the OSIRIS-REx mission. Bottke and other planetary scientists believe that Bennu was once part of a much larger asteroid that broke apart more than a billion years ago. Bennu’s progenitor was likely formed when the solar system was still a swirling cloud of gas and dust, but what happened between the time it formed and the time it got blown to bits is a mystery. Whatever happened to Bennu’s parent, it was a cataclysmic event that probably left traces of that violence in the structure and chemistry of the asteroid. Studying that process can help scientists understand the dynamic forces that shaped the solar system billions of years ago. “By understanding Bennu, we can learn more about the fundamentals of how planets and their building blocks formed,” says Bottke.
If OSIRIS-REx doesn’t scoop up enough rocks on Tuesday, it has enough propellant to make two more attempts. But each dive to the surface is risky, so if the team doesn’t have to go back, they won’t. If they do have to try again, OSIRIS-REx may have to go to its backup site, a crater near Bennu’s equator called Osprey. The switch would be an effort to avoid contamination if the craft got close enough to Nightingale, the original landing spot, to potentially pollute it with thruster exhaust. In any case, it will take a few months before OSIRIS-REx is ready for another descent, because the team has to make minute adjustments to the spacecraft’s orbit to perfectly align it with its target.
“Getting back into orbit is a tricky business, and it’s time consuming to plan the observations necessary to get our bearings again,” says Burns. “The spacecraft is taking images and matching them with known surface features to update the orbit.” He says the earliest a second attempt could happen is December. If the spacecraft doesn’t collect a sample the second time around, it will only have enough time and nitrogen left to make one last try before it has to depart for Earth.
Once Bennu has its sample, it will return to an orbit around the asteroid so it can tag along while the rock makes its annual journey around the sun. Bennu is the smallest object ever orbited by a spacecraft, and keeping a stable orbit around something so small comes with all sorts of challenges that aren’t involved in orbiting a planet. It only takes a tiny amount of energy to achieve escape velocity—a human standing on the surface of Bennu could jump off of it— which means that OSIRIS-REx has to do everything extremely slowly if it doesn’t want to fly off into space.
“The gravitational acceleration of Bennu is very small,” says Kenny Getzandanner, the OSIRIS-REx flight dynamics manager. “During the descent, we’ll come down at around 10 centimeters per second, which is about the level Bennu was tugging on us to begin with. Getting down slowly gives us plenty of time to do navigation updates, but that’s also just naturally the dynamics of the situation.”
Next March, Bennu will make its closest pass to Earth for the next six years, and OSIRIS-REx will use this window to decouple from the asteroid and start its journey back home. The spacecraft will be booking it at around 100,000 miles per hour, but even at that speed it will take about a year and a half for it to catch up with Earth.
OSIRIS-REx will orient itself on an impact trajectory with Earth. But a few hours before it would enter the atmosphere, in September 2023, it will jettison the sample return capsule and perform a deflection maneuver that will rescue the spacecraft, putting it in orbit around the sun. Meanwhile, the capsule will slam into the atmosphere at 27,000 miles per hour. Fisher says the 2-foot-wide capsule is protected with the same sort of heat shield that safely returned samples from NASA’s Stardust mission through the tail of a comet. The OSIRIS-REx team expects it to land under parachute on the Utah Test and Training Range, a military ordnance facility just outside of Salt Lake, where it can be tracked during its descent.
“We're coming in rougher and faster than astronauts coming back from the International Space Station,” says Lauretta. “So the samples will get shaken, and the one thing I do worry about is there might be some fragmentation of the material during reentry. But there’s really no way to protect it from that.”
If there’s enough propellant left, OSIRIS-REx might be able to extend its mission and do more science, otherwise it will be left to die in solar orbit. Getzandanner says the team is focused on the main mission now, but he doesn’t rule out the possibility of a mission extension. “Once OSIRIS-REx jettisons the capsule, we'll have a pretty capable spacecraft that would certainly be capable of performing another mission,” says Getzandanner. “Once we get through touch-and-go, we can start thinking about extended missions and doing some more work on that.”
Once the asteroid samples are returned, scientists at NASA’s Johnson Space Center will catalog them and keep the bulk of the material, studying some of it immediately and reserving a portion that will be sent to an undisclosed secure location in New Mexico for safekeeping. They’ll divide the rest of it up among research groups around the world, including partners at the Japanese and Canadian space agencies, both of which contributed to the mission. Japan, which has successfully launched two asteroid sample-return missions, shared data and techniques that helped shape the OSIRIS-REx mission. Researchers in Canada contributed a laser altimeter that has been used to precisely measure how high the spacecraft is orbiting the asteroid and map its surface in detail.
OSIRIS-REx will help researchers get a better understanding of everything from the origin of life to the formation of planets. But there are bound to be some lingering mysteries. For instance, last year the OSIRIS-REx team noticed that rocks appeared to be exploding off the surface of the asteroid, and no one can really explain why. Like all asteroids, Bennu is no longer geologically active, so the movement couldn’t have been caused by earthquakes or tiny volcanoes. And the rocks were popping off the surface with too much force to be explained by other obvious mechanisms, like heating from the sun. Unfortunately, OSIRIS-REx won’t be sticking around Bennu long enough to resolve the mystery, but it underscores how much there is to learn about the millions of space rocks drifting through our solar system.
While OSIRIS-REx is primarily a scientific mission, it also has implications for planetary defense. There is a 1-in-2,700 chance that Bennu will crash into Earth in about 150 years, which would cause death and destruction on an international scale. The world’s space agencies are looking into ways to stop this sort of cataclysmic event; next year, NASA will launch the world’s first planetary defense mission as a test run. The mission, called DART, involves ramming a spacecraft into a nonthreatening asteroid to study whether an oncoming space rock could potentially be rerouted. Another possible solution being considered by US government researchers: nuking a killer asteroid.
But no defense systems will be very effective unless astronomers have a pretty good idea of when an asteroid might hit the Earth. While it’s easy enough to predict the path of a massive object like a planet, it’s trickier with rotating asteroids. That’s because they are subject to the so-called Yarkovsky effect: The side of the rock facing the sun absorbs sunlight, but later emits it as heat when that side faces away, which produces a small amount of thrust. The effect is small, but over time it can cause an asteroid to significantly deviate from its predicted orbit.
“Because Bennu is a retrograde rotator, it’s radiating heat in the opposite direction that it’s orbiting,” says Lauretta. “That’s like putting the brakes on the asteroid. And when you decrease orbital velocity, Bennu actually moves into the inner solar system.”
This means that, about a century from now, Bennu will pass within one Earth-moon distance of our planet. But researchers can only predict where it will pass by Earth somewhere within a 100,000-mile window. That’s too much uncertainty for comfort, so OSIRIS-REx has been studying Bennu’s trajectory during its sojourn at the asteroid to learn how it deviates from its predicted orbit. The spacecraft has been able to detect minute changes that would be hard to observe from Earth, which will help scientists refine their models of how the Yarkovsky effect influences asteroids and how close Bennu might pass by Earth.
“We’ve been able to measure that with a very high signal-to-noise ratio,” says Lauretta. “So the Yarkovsky effect is no longer the number one contributing uncertainty factor on Bennu’s orbital trajectory.” Instead, he says, the largest uncertainty is how the gravitational pull of other asteroids in the solar system slightly alters Bennu’s path.
When OSIRIS-REx kisses Bennu on Tuesday, it will mark a major milestone in the high-stakes mission. The spacecraft won’t be out of the woods yet; some of the riskiest parts of the mission are still ahead of it. It still has to collect those samples and return them safely to Earth. But if OSIRIS-REx is successful, it could change our understanding of how life arose in the solar system—and how to protect it in the future.