Hunting asteroids: Remarkable missions to rocky remnants of the solar system

01 May 2024
By Rebecca Pool
In 2029, the asteroid Apophis will pass less than 20,000 miles from our planet’s surface—closer than the distance of geosynchronous satellites. Photo credit: NASA/JPL-Caltech

In less than five years, on Friday 13 April 2029, an asteroid the width of the Hoover Dam will streak past Earth, coming closer than geosynchronous satellites. Originally predicted to collide with our planet when the asteroid was discovered in 2004, optical and radar observations now indicate that Apophis—some 380-m across and named after an Egyptian serpent demon—will instead be visible as a moving dot of light from Europe, Africa, and Asia as it skims past at some 20,000 miles above Earth’s surface. Asteroids this size only come this close to Earth once every 7,500 years, and the space community can barely wait.

“This is not just any old asteroid—it is going to be a global scientific event,” says Dathon Golish, an imaging scientist at the Lunar and Planetary Lab of the University of Arizona, a key research institution for the study of asteroids. “A lot of people are going to hear about Apophis and it’s going to be a big deal.”

Amy Simon, senior scientist for planetary atmospheres research at NASA’s Goddard Space Flight Center, is equally excited. “Apophis was originally our most hazardous asteroid and now we’re going to have this very, very close flyby in 2029,” she says. “I mean, it will be really close—so we’ve got this amazing opportunity to study an object that is getting this close to Earth.”

Both Simon and Golish are part of a team that will study Apophis and are among hundreds of planetary scientists, geologists, physicists, aerospace engineers, and instrumentation developers worldwide studying asteroids. Multiple missions are underway to reach these rocky, airless remnants of our solar system, to better understand how they formed and maybe even discover water on them to glean clues to the beginnings of life. Researchers are also keen to learn about the near-Earth asteroids that may pose an impact threat to our planet so they can assess the risk and, ultimately, deflect potential hazards.

For example, the Hayabusa2 spacecraft from Japan Aerospace Exploration Agency (JAXA) returned samples from the 900-m-wide near-Earth asteroid Ryugu in late 2020, and is now enroute to a small water-rich asteroid, 1998 KY26. Meanwhile NASA’s Lucy probe set off on a 12-year mission in late 2021 to fly by three asteroids in our solar system’s main asteroid belt and then onto eight of the thousands of Trojan asteroids—all named after participants in the legendary Trojan war—close to Jupiter.

NASA, in 2021, also sent its Double Asteroid Redirection Test (DART) spacecraft to Dimorphos, an asteroid moon orbiting a larger body called Didymos. The spacecraft’s deflection technology altered the small moon’s orbit, and now the European Space Agency is set to launch its spacecraft, Hera, later this year to Dimorphos to perform a post-impact survey. This desire to hone planetary defense strategy also makes Apophis, with its near-Earth flyby, such a compelling object to study.

“There’s going to be a lot of assets on the ground looking up at Apophis to check its trajectory,” says Simon. “Because Apophis comes so close to Earth, it’s going to be tidally torqued [by our gravity] and understanding its internal strength and how it is held together is going to be really important, [especially] for planetary defense reasons.”

Enter NASA’s Origins, Spectral Interpretation, Resource Identification, and Security-Apophis Explorer (OSIRIS-APEX or APEX for short). As Apophis skirts Earth, this spacecraft will be in hot pursuit.

Apex was originally launched in 2016 to reach an entirely different asteroid, Bennu, which is 55-m in diameter and has a chance, albeit tiny, of hitting Earth. The spacecraft was then known as OSIRIS-Regolith Explorer (O-REx), as it was tasked with mapping the surface and retrieving a sample of regolith—loose rock and dust fragments that cover many planets (and asteroids)—from Bennu. Headed up by University of Arizona planetary scientist Dante Lauretta, the mission was a success, and said sample was dropped in the Utah desert on 4 September 2023. And so, O-REx, renamed APEX to reflect its new mission—and now being led by planetary scientist Dani Mendoza DellaGiustina, also from the University of Arizona—is on its way to Apophis.

The spacecraft is home to an arsenal of cameras and spectrometers designed to image Bennu at meter-to-millimeter scales and map its surface, identifying minerals and organic compounds while providing data for navigation and sample collection. For example, an eight-inch telescope, PolyCam, acquired Bennu data from some 2 million km away, while MapCam more closely mapped the asteroid in color, and SamCam surveyed regolith sampling. At the same time, thermal, X-ray, and visible and infrared (IR) spectrometers measured the asteroid’s surface temperature and identified minerals and organic compounds, while lidar measured the distance between the spacecraft and Bennu’s surface, also creating high-resolution topographic maps.

“We match up, say, camera images with spectrometry to verify what we’re seeing. We don’t want complexity in instruments we’re sending millions of miles away and can’t fix once they’ve gone,” explains Golish. “We have few moving parts and even the detectors seem quaint nowadays—they’re one megapixel, 1024 by 1024 detectors. However, they are also robust and reliable.”

And this was certainly needed for Bennu. According to Ryan Olds, engineer at Lockheed Martin, (as well as guidance navigation and control lead for O-REx and APEX) O-REx performed a “pretty complex campaign of flybys and orbits” of  the asteroid. Surface imagery was captured at various angles with the resolution reaching 5-cm-per-pixel—one of the highest resolutions to which such a planetary body has been mapped.

The sample return capsule containing Bennu regolith from NASA’s O-REx mission shortly after touching down at the Pentagon’s Utah Test and Training Range. Photo credit: NASA/Keegan Barber

However, analyses also revealed the asteroid’s surface to be littered with many boulders rather than covered in smoother regolith as indicated by pre-mission observations. Olds and his colleagues quickly realized that although the lidar could measure the spacecraft’s distance to Bennu, it couldn’t accurately guide the spacecraft between the boulders to a safe landing. So, they turned to a back-up optical navigation system that used natural feature tracking (NFT) to detect the craft’s position and velocity in 3D.

During descent, SamCam captured images of the surface that were processed in real-time on the spacecraft’s onboard computer using NFT algorithms to identify and track recognizable features. By comparing this real-time footage with the wealth of data already collected from Bennu, O-REx could autonomously adjust its descent to ensure safe touchdown.

“NFT was a new technology and hadn’t been used in this way before,” says Olds. “But the team had a blast combing over the surface of this alien world to pick and choose strategic [landing] places to sample safely. We learned a lot here and I think NFT will offer us a lot of [options] for visiting other places.”

Indeed, during regolith collection, O-REx performed a sophisticated touch-and-go or TAG maneuver so it could match its velocity with Bennu’s surface velocity and navigate towards the surface. At some 10 m from the asteroid, the spacecraft’s sample acquisition arm extended, and a short burst of nitrogen gas was released to stir up the regolith for collection.

“Due to light-time [the time taken for light to travel between celestial objects], the telemetry coming from the spacecraft to Earth had happened 40 minutes previously,” recalls Olds. “We really were just passive spectators, which made watching this nerve-wracking but then extremely exciting once we knew it had all worked.”

So, on 20 October 2020, the regolith was successfully collected, stowed in a safety return capsule, and after six more months of analyses, O-REx waved goodbye to Bennu and embarked on its return journey to Earth. By this time, JAXA’s Hayabusa2 had returned from Ryugu. As Seiji Sugita, from the Earth and Planetary Science department at the University of Tokyo, and science principal investigator of Hayabusa2’s optical camera, highlights, he and colleagues met with surprise when they encountered their asteroid.

“We thought that Ryugu and Bennu would be very different in shapes, but when we arrived at Ryugu, we saw it had a spinning-top shape [like Bennu],” he recalls.

“We thought, ‘Oh my goodness, we’ve reached the wrong asteroid!’—and that’s what the O-REx team thought, too,” he jokes. “But seriously, this similarity was so surprising and has made our missions so much more interesting.”

The payload aboard Hayabusa2 has its similarities to O-REx and APEX, including wide-angle, telescopic, near-infrared (NIR) and thermal IR cameras, an NIR spectrometer, and lidar. However, unlike the NASA spacecraft, JAXA’s craft also had an ingenious fleet of deployable instruments, used to remarkable effect.

On reaching Ryugu, Hayabusa2 created a crater on the asteroid with a small, bullet-like carry-on impactor (SCI) and released its deployable load. Three rovers, equipped with cameras, thermometers, and accelerometers, bounced around Ryugu alongside a microwave radar scout, developed by the Germany Aerospace Center and French Space Agency, CNES, and a camera designed to capture SCI deployment and sampling.

Sugita reckons these instruments were critical to analyses and minimized damage to Hayabusa2 during its Ryugu visit. “One of the rovers actually travelled very long distances for its size [less than 20 cm wide], travelling [across hundreds of meters] from the northern to southern hemisphere on the asteroid,” he says.

But resounding successes aside, Sugita also highlights the value of international cooperation in the world’s concurrent asteroid missions. “NASA has been one of our biggest advocates—we’ve really enjoyed flying at the same time… for example, the O-REx camera team has been so generous in sharing data so we can cross-calibrate our cameras,” he says.

Hayabusa2 traveled roughly 180 million miles back towards Earth, to release its asteroid sample, which landed in Woomera, South Australia, on 5 December 2020. Then, like APEX, the JAXA spacecraft was re-routed towards a further asteroid as part of an extended mission. This time around, the craft is heading for 1998 KY26 which it’s scheduled to reach by 2031. The 30-m-wide micro-asteroid’s high-water content is of huge interest to researchers, and Sugita is excited. “You know, you don’t need a gigantic spacecraft for these small-body missions,” he says. “What we have shown is that a relatively small nation such as Japan can have a compact spacecraft and really open up new frontiers in space.”

The 1998 KY26 asteroid will be Hayabusa2’s final destination. This extended mission will have used the craft’s remaining ion engine fuel, and as Sugita puts it, the spacecraft must be very exhausted at this point. Still, the team will make the most of their remarkable spacecraft and are mulling over its final operations. These could include firing the last SCI at 1998 KY26 to disturb its surface and study the aftermath, or completing a short touchdown to study the surface mobility of asteroid particles. “Project engineers tell me that what Hayabusa2 [finally] does remains an open question... so the young scientists and engineers who recently joined our team can make a choice,” adds Sugita.


Illustration of the two rovers from Hyabusa2 as they explored the Ryugu asteroid. These rovers were designed to take advantage of low gravity to “hop” over the surface of a small celestial body, and could move up to 15 m per hop. Photo credit: JAXA

As Hayabusa2 speeds towards its latest asteroid, APEX has set off on its one-way trip to Apophis. With three Earth-gravity assists, six closer-than-comfort passages by the Sun—the first of which took place on 2 January—the route isn’t easy.

As Olds highlights, the craft wasn’t designed to pass by the Sun, so he and colleagues have configured one of the craft’s solar arrays to be oriented backwards so as to shield components from extreme temperatures. “We can put the craft in this strategic attitude when we get close to the Sun, and so far, so good,” he says. “In the coming months, we’ll take images of the stars to make sure the cameras and instrumentation are still operating correctly—we’re not worried yet.”

Assuming safe passage, come 2 April 2029, APEX will start taking imagery of Apophis. By 13 April when, at 19,794 miles, the asteroid will be at its closest to Earth, the spacecraft will make more detailed analyses. From there, all involved will be intently watching how Apophis reacts and responds to Earth’s gravitational field.

The Earth flyby is expected to cause quakes and landslides on the asteroid’s surface. Calculations indicate our planet’s forces will not disaggregate the asteroid, but millimeter- to centimeter-sized particles will become dislodged, uncovering what lies beneath—a literal celestial scoop for researchers keen to examine Apophis’ freshly uncovered subsurface.

Countless calculations from researchers worldwide also indicate the close encounter will alter the asteroid’s orbit and change the way it spins. Although radar and orbital analyses clearly indicate there is no risk of Apophis impacting our planet for at least a century, studying these characteristics is set to shape our all-important future planetary defense strategies.

Victoria Hamilton, a planetary scientist from the Southwest Research Institute in Boulder, Colorado, and co-investigator on several missions—including to Bennu, Apophis, and the Trojan asteroids—highlights how visiting these bodies provides a critical opportunity to study the so-called Yarkovsky effect. Here, the uneven heating and cooling of an asteroid rotating in sunlight can accelerate its rotation and ultimately, over geologic time, alter its orbit. “We’re trying to understand if…we can take measurements to predict the Yarkovsky effect on an asteroid and see if that asteroid will become more or less hazardous based on what we understand about its changing orbit,” she says.

Chronicling these effects is only a small part of APEX’s journey with Apophis. As with Bennu, the spacecraft will map the asteroid and characterize its mass and structure. Then, come July 2030, towards the end of the mission, the spacecraft will dip within 16 feet of Apophis’s surface, fire its thrusters downwards, stir up dust and rocks, and back away. This spacecraft thruster investigation of regolith (STIR) maneuver was first performed at Bennu, but as Golish points out: “We can’t sample the surface as we’ve already used our sampling device… but we will get a look at the subsurface.”

The STIR maneuver also draws an end to the APEX mission—at least for now. Researchers are already toying with ideas for further actions on Apophis, including more STIR maneuvers, additional orbits, and even landing the spacecraft on the asteroid so it can end its days operating as a radio beacon. What’s clear is that APEX’s one-way trip from Earth makes current investigations into Bennu regolith all-the-more meaningful.

Hamilton reflects on when she watched the Bennu sample descend to the desert. Like most of her fellow O-REx colleagues, she was at home following the NASA coverage. “You’re watching this capsule come down and all the time you’re thinking, when’s the chute going to deploy, when’s it going to deploy,” she says. “Then it does, you see the capsule land in the dirt and with a big sigh of relief you say, okay it didn’t crash, it’s intact—we’re really going to get the samples and do this whole other part of the mission.”

And for Hamilton and fellow researchers, this is what asteroid missions are also about. O-REx brought home some 121 g of Bennu regolith—more than twice the mission’s goal—with particles ranging in size from microns to centimeters. So far, Hamilton has studied 10-µ-sized particles but she’s hoping to look at larger stones soon.

“If I showed you a sample and didn’t tell you what it was, you’d say, that’s just a little black rock,” she says. “But I look at my sample and think, wow, that came from up there. We went, we got it, and we brought it back—you know, it’s amazing to hold something like that.”

Rebecca Pool is a science and technology journalist based in the UK.


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