If you have an ACS member number, please enter it here so we can link this account to your membership. (optional)

ACS values your privacy. By submitting your information, you are gaining access to C&EN and subscribing to our weekly newsletter. We use the information you provide to make your reading experience better, and we will never sell your data to third party members.



The tale of 2 asteroid sample-return missions

OSIRIS-REx and Hayabusa2 are set to collect data that could reveal clues to the origins of life, as well as aid in planetary defense and resource mining

by Cici Zhang
September 30, 2018 | A version of this story appeared in Volume 96, Issue 39
An illustration shows OSIRIS-REx contacting the asteroid Bennu with the Touch-And-Go Sample Acquisition Mechanism, or TAGSAM.

Credit: NASA | OSIRIS-REx, shown here in an artist's rendering, will arrive at the carbon-rich asteroid Bennu in December 2018 and return to Earth in 2023.


@OSIRISREx: After traveling for nearly 2 years, last week I caught my first glimpse of the new world I’m destined to explore: asteroid Bennu, blinking across the sky 1.4 million miles in the distance. The secrets of the early Solar System are calling, and I must go …

@haya2e_jaxa: Congratulations, @OSIRISREx! We cannot wait to see what you will discover as you approach Bennu.

@OSIRISREx: Thank you, fellow #asteroid explorer! It has been fascinating and inspirational to see your images of Ryugu. Can’t wait to see what @MASCOT2018 and MINERVA-II find on the surface this fall ...

This Twitter conversation that began Aug. 24 between the National Aeronautics & Space Administration’s OSIRIS-REx spacecraft and the Japan Aerospace Exploration Agency’s Hayabusa2 spacecraft is just one example of the friendly banter passing between the explorers in recent months. These spacecraft, each approaching a different asteroid to collect samples and return them to Earth, have been cheering each other on as they venture deeper into space, aiming to accomplish technical and scientific feats that have never before been attempted. The week C&EN went to press with this story, Hayabusa2 deployed two MINERVA-II rovers to the surface of Ryugu and was poised to send another rover, MASCOT, some weeks later. OSIRIS-REx is set to rendezvous with Bennu in December.

Ever since the first one was discovered in 1801, asteroids, which orbit the sun, have fascinated generations of humans, from the author of the French novella “The Little Prince,” whose title character originated on the asteroid B-612, to scriptwriters for doomsday movies such as “Armageddon.” Scientists are no different.

They have been studying asteroids for decades, mostly from afar, to help answer questions about the beginning of time. Thanks to meteorites—chunks of asteroids and other interstellar objects that have fallen to Earth—and to telescope data, researchers know that these small celestial bodies originated during the beginning days of the solar system, from the same cloud of dust and gas that condensed to form our sun and planets 4.6 billion years ago. These minor planets, most of which reside in a belt between the orbits of Mars and Jupiter, hold minerals and organic compounds that could help answer questions about the formation of our solar system.

So far, only one mission from the Japan Aerospace Exploration Agency (JAXA)—the original Hayabusa—has returned pristine samples from an asteroid to Earth. And that mission, which almost failed, collected only about 1,500 tiny grains, less than 1 mg, of material. Hayabusa2 and OSIRIS-REx aim to collect more substantial amounts of asteroid—at least 0.1 g for Hayabusa2 and 60 g for OSIRIS-REx—to give scientists more definitive answers about asteroids’ chemical and physical properties. Not only will the data enable the space sleuths to test hypotheses about planetary formation and even life’s origins, but they may also help officials make decisions about protecting Earth from collisions and help firms better focus their plans for mining asteroids for precious resources in the future.

From Hayabusa to Hayabusa2

An illustration of Hayabusa2 flying over an asteroid is shown here.
Credit: JAXA
Hayabusa2, shown here in an artist’s rendering, is set to return asteroid samples from Ryugu to Earth in 2020. Before the launch of Hayabusa2 (2014) and OSIRIS-REx (2016), only one spacecraft had accomplished such a feat.

As you are reading this line, Hayabusa2 is dancing with Ryugu in a quiet place 3.2 billion km away, waiting for the right moment to make close contact. The diamond-shaped asteroid measures 900 meters in diameter, dwarfing its dancing partner, which spans only a few meters. Hayabusa2 is small but mighty: It’s on its way to collect and return samples through three touchdowns, the first of which is scheduled to happen in late October. At the same time, it will deploy four rovers, including the two MINERVA-IIs that landed the week C&EN went to press, the German-French MASCOT, and a third MINERVA-II, to provide supplemental information about the regions on the asteroid from which the samples are collected.

Having learned from the original Hayabusa mission, Japanese space scientists equipped Hayabusa2 with many updated technologies, including its sample collection mechanism, before it launched in 2014. The main design is still the same, however: The spacecraft has a cylindrical horn attached to it. During its first two descents, the spacecraft will fire a small projectile into the asteroid’s surface as the tip of the horn touches down. Materials kicked up by these bullets will be collected by the horn.

Many people think that 0.1 g are very small, but if we have 0.1 g of sample, we can do all the analyses that we planned.
Makoto Yoshikawa, mission manager of Hayabusa2, Japan Aerospace Exploration Agency

In addition to the first two sampling runs, Hayabusa2 will aim to create a crater on Ryugu—the first time such a feat has been attempted. The goal is to wait for the dust to clear, then collect samples from the hole left behind, hunting for samples untouched by radiation, weathering, or collision that has happened in space.

“This time we want to make a hole on the surface to get to the subsurface material,” says Makoto Yoshikawa of JAXA, veteran astronomer and mission manager of Hayabusa2. “We discussed many ways to make a hole.” But the easiest is a small explosive device, or Small Carry-on Impactor, he says.

A complicated mission worth at least ¥30 billion ($300 million), Hayabusa2 may surprise some with how little sample it will return.

“Many people think that 0.1 g are very small, but if we have 0.1 g of sample, we can do all the analyses that we planned,” Yoshikawa says. Those analyses include examining the mineralogy of coarse- and fine-grain samples and measuring volatile materials, condensed organic matter, and organic molecules.

Assuming the mission is successful, when Hayabusa2 returns to Earth with its samples in late 2020, Yoshikawa and coworkers will also learn about the materials’ grain patterns. Those patterns could yield clues about the physical environments that Ryugu has experienced over time.

For instance, when researchers examined microscopic grains in samples returned from the asteroid Itokawa during the original Hayabusa mission, they found four types of surface patterns instead of the one pattern that had been expected (Geochim. Cosmochim. Acta 2016, DOI: 10.1016/j.gca.2016.05.011). Each of the patterns revealed a piece of the history of Itokawa, including crystallization under intense heat about 4.5 billion years ago and a collision that happened almost 3 billion years later. In a similar way, Hayabusa2’s samples will reveal Ryugu’s story to the world.

A pie chart showing that asteroids are mainly C-type.
Credit: C&EN/Shutterstock
What we know about asteroid composition
The majority of asteroids in our solar system are believed to be C-type.
Source: NASA, Simon Green

A glimpse of the origin of life

Meteorites have long been used as a proxy to study asteroids. Every day, these rocks—debris from comets and asteroids—fall to Earth for scientists to collect. It’s easier to identify the dark chunks after they’ve come to rest on vast sheets of white ice and snow, which makes Antarctica an ideal place to spot them.

Using spectroscopic data collected from telescopes, researchers have learned to link these earthbound samples to asteroids orbiting above. For example, scientists predict that samples from Bennu will contain millions of organic compounds. Spectral data they’ve collected suggest that Bennu, a C-type asteroid (carbon rich), is similar in composition to a well-studied carbonaceous chondrite meteorite recovered in Australia, the Murchison meteorite.

In the lab of OSIRIS-REx project scientist Jason Dworkin, researchers have used liquid chromatography/mass spectrometry and gas chromatography/mass spectrometry to study several meteorite samples. His and other teams have found that meteorites contain amino acids, the building blocks of life, and that the amino acids show a bias toward left-handed chirality over right-handed chirality (ACS Cent. Sci. 2016, DOI: 10.1021/acscentsci.6b00074). This finding is significant because amino acids on Earth have shown exclusively left-handed chirality, but it’s still unclear how that preference came to be. One hypothesis is that chiral molecules may have hitched a ride on asteroids that fell to Earth long ago, seeding life’s left-handed chirality when the rocks landed.


Meteorite samples are far from perfect surrogates for asteroids, though. They experience extreme pressures and lose a great deal of their bulk when falling through Earth’s atmosphere. Once they land, they pick up contaminants from the terrestrial environment that are hard to distinguish from native substances. So although the missions to Ryugu and Bennu are expensive and challenging, the potential for scientific advancement is significant.

Pristine samples from an asteroid might not only answer questions about the origins of life’s chirality but also offer clues about how water ended up on Earth. One hypothesis suggests that asteroids, which contain some water, may have also seeded it onto our home planet during collisions (Science 2014, DOI: 10.1126/science.1261952).

So great is the anticipation of answering some of these questions for Dworkin that he can easily recite the precise time and place OSIRIS-REx will return its samples from Bennu to Earth: 8:53 AM MDT on Sept. 24, 2023, at the Utah Test & Training Range, west of Salt Lake City. He’s been working on the project since 2004. “That’s what it takes,” he says, recalling the years of preparation, testing, and fundraising.

OSIRIS-REx aims to collect its sample in much the same way as Hayabusa2. After picking a suitable landing spot rich with organics using spectrometer readings, the spacecraft will conduct a touch-and-go collection process. Instead of using a projectile to extract material from Bennu, OSIRIS-REx will use a jet of nitrogen gas to stir up the surface material, which will then be blown into the collection head that’s part of the Touch-and-Go Sample Acquisition Mechanism, or TAGSAM. With three bottles of nitrogen gas, OSIRIS-REx is capable of making three sampling attempts and obtaining samples between 60 and 2,000 g.

Credit: NASA
Upon contact with Bennu, OSIRIS-REx will use a jet of nitrogen gas to stir up the surface material and catch samples with its collection head.

To study Bennu’s millions of organic compounds as well as the inorganic ones that will be present is going to be “a monumental analytical task,” Dworkin says. Because the samples from OSIRIS-REx will be “virtually irreplaceable materials,” he adds, it will be vital for scientists to maximize the amount of information they get from the rocks, meaning they’ll need to use the most sensitive instruments possible.

Archiving and storing the precious samples will also be key.

When OSIRIS-REx returns to Earth in 2023, one of the first things the team will do is store 75% of its sample as a legacy. “So future generations or other scientists or those who are not even yet born can come up with new ideas, new ways to look at the sample,” Dworkin says.

Efforts against sample contamination began even before the launch of OSIRIS-REx. Dworkin’s team meticulously documented the molecular background signals of the spacecraft, especially amino acids. So after its return, the researchers will know what compounds are new. In addition, the canister that will hold the samples is protected by an air filter consisting of activated carbon and electrostatically charged fibers. This setup will prevent contaminants from entering before and after collection.

A gowned engineer adjusts OSIRIS-REx sample collection mechanism in a clean room.
Credit: Space Sci. Rev.
An engineer adjusts the sample collection mechanism of OSIRIS-REx in a clean room.

In tests, Dworkin and colleagues found the filter’s trapping efficiency for particulates can be as high as 99.996% (Space Sci. Rev. 2017, DOI: 10.1007/s11214-017-0439-4). The filter will also be able to trap any volatiles evolving from captured asteroid sample. This way no precious material will be lost.

“We are still learning now new things about the moon from Apollo samples that we collected 50 years ago,” says Simon Green, an asteroid scientist from Open University. If the new asteroid samples are properly collected, stored, and preserved, the same thing will happen, he says.

Hazard and opportunity

Many hope that missions like $800 million OSIRIS-REx will offer more than just information about the origins of life. They’re hoping for priceless information about whether asteroids might ever need to be diverted from colliding with Earth.

In 1908, an asteroid around 50 meters in diameter exploded in the air about 5–6 km above the Tunguska area in remote Siberia. Although the celestial body didn’t make it all the way to the ground intact, the blast wind it generated destroyed about 2,000 km2 of forest.

If the Tunguska event were to happen over a city, says Lindley Johnson, head of NASA’s Planetary Defense Coordination Office, it would be a catastrophic disaster on par with a nuclear explosion.

Consider Bennu. NASA estimates it has a cumulative one in 2,800 chance of hitting Earth, with close encounters possible between the years 2175 and 2199. That risk might increase or decrease in 2135 when Bennu passes closer to Earth than the moon. At that point, Earth’s gravitational force could potentially change Bennu’s orbit.

The things we discover about [Bennu’s] composition, about how it’s affected by solar pressure, will allow scientists and engineers in 2136 to decide whether or not they need to defend against Bennu.
Jason Dworkin, project scientist for OSIRIS-REx, National Aeronautics & Space Administration

Scientists estimate that with a size of 500 meters in diameter, Bennu could have an impact energy equivalent to 1,450 megatons of TNT.

As OSIRIS-REx principal investigator and University of Arizona scientist Dante Lauretta explains in a 2016 blog entry, that figure is three times as much energy as “all nuclear weapons detonated throughout history.” The collision wouldn’t destroy Earth, Lauretta writes, but it would leave a 3.85-km-diameter crater and cause concentrated damage within tens of kilometers.

The information we’ll be able to get from the OSIRIS-REx mission, Dworkin says, will be useful for future generations to better predict Bennu’s orbital evolution and to make defense decisions. “The things we discover about its composition, about how it’s affected by solar pressure,” he says, “will allow scientists and engineers in 2136 to decide whether or not they need to defend against Bennu.”

Asteroids with a diameter of 50 meters hit Earth about once in a century or two, on average. For something that’s Bennu’s size, that frequency is only once every hundred thousands of years.

Asteroid impacts are much rarer than earthquakes or volcanoes, Johnson explains, but they have the potential to do a lot of damage.

His team is busy surveying the entire sky to find all near-Earth asteroids that are 120 meters in diameter and larger. “Our job is to find asteroids before they find us,” he says.

Of course, not everyone looks at asteroids and their proximity to Earth with defense in mind. Others see asteroids—and the precious elements they contain—as a business opportunity.

Tapping into space

On a vast plateau in New Mexico, a human-made wall of silver reflective panels sits ready to focus the unforgiving sunlight beating down into something even hotter.

The panels’ target is a piece of plain-looking charcoal-hued rock, prepared by scientists to mimic asteroids. Although it’s now sitting unassuming in a microwave-sized vacuum chamber, this rock could help demonstrate a technology that might one day tap into an immense well of extraterrestrial resources.

Joel Sercel is the CEO of TransAstra, a Los Angeles-based start-up that aims to turn asteroids into fueling stations for spacecraft. His firm is using these panels to test optical mining, which Sercel claims is the most practical way to extract water from asteroids.

The idea is simple, he says: You collect water vapor from the rock by heating it with intense energy from the sun, cool the vapor down to a liquid form, and then use that as the propellant to fuel spacecraft. In this way, asteroids could become fueling stations to support space missions.

But water is just one of the valuable commodities that investors see when they look at asteroids. Goldman Sachs once predicted that an asteroid the size of an American football field could contain up to $50 billion worth of platinum, a metal critical for many technologies on Earth and one that will eventually run out on our planet if not recycled. Looking to the future, the government of Luxembourg has funded asteroid mining start-ups with the goal of preventing such shortages.

Credit: C&EN/Shutterstock
The most cost-effective asteroids to mine
According to Asterank, a database of over 600,000 asteroids, Ryugu is the most cost-effective asteroid to mine. The site uses a measure called Δv to express cost-effectiveness. Δv describes the effort that would be needed to fly a spacecraft from Earth to the asteroid. The smaller the value, the better.
Sources: Ian Webster/Asterank, International Astronomical Union’s Minor Planet Center, Japan Aerospace Exploration Agency, NASA, European Space Agency
Credit: C&EN/Shutterstock
The most cost-effective asteroids to mine
According to Asterank, a database of over 600,000 asteroids, Ryugu is the most cost-effective asteroid to mine. The site uses a measure called Δv to express cost-effectiveness. Δv describes the effort that would be needed to fly a spacecraft from Earth to the asteroid. The smaller the value, the better.
Sources: Ian Webster/Asterank, International Astronomical Union’s Minor Planet Center, Japan Aerospace Exploration Agency, NASA, European Space Agency

As Sercel explains, once asteroid mining companies are mining water from nearby asteroids to use as a rocket propellant, going after other asteroids for precious metals will become affordable.

At an April workshop titled Asteroid Science Intersections with In-Space Mine Engineering (ASIME), Sercel and others gathered to discuss the knowledge gaps and technical hurdles that will need to be surmounted before asteroid mining can become a reality. The conference was a rare opportunity for scientists and asteroid miners to come together and learn from one another.

The picture of asteroids’ composition is still blurry. Scientists estimate from spectral data and meteorites that the majority of asteroids in our solar system, including Bennu and Ryugu, are C-type. About 17% of the asteroids are S-type—siliceous or stony. The asteroid Itokawa, sampled by the original Hayabusa mission, falls into this category. For asteroid miners, the most exciting type of asteroid may be the M-type, at least some of which could be metallic, like iron meteorites, containing predominantly iron-nickel alloy. The M-type asteroid 16 Psyche is the target of the NASA Psyche mission, scheduled to launch in 2023 and arrive in 2030.

Missions like OSIRIS-REx, Hayabusa2, and Psyche will help miners pinpoint which asteroids will be the most valuable targets.

Current Earth-based telescopes can’t gather data from asteroids that are just a few meters in diameter, but these could be targets of optical mining, Sercel says, especially those ranging from 5 to 50 meters. He recently coauthored a modeling paper with Robert Jedicke of the University of Hawaii, who is also a shareholder in TransAstra. They and colleagues concluded that there are at least a thousand asteroids with the proper diameter for optical mining that are also C-type asteroids that likely contain water (Planet. Space Sci. 2018, DOI: 10.1016/j.pss.2018.04.005).

The orbit of an asteroid is especially important for mining. Those with Earth-like orbits are the targets that are energetically easy to get to with today’s spacecraft technology, Sercel explains. “For at least a few decades, those are the asteroids that we will be mining, not the ones further into deep space.”

Back in New Mexico, Sercel succeeded in extracting water out of the faux asteroid: His optical mining method reduced the weight of the rock by 37%, or 130 g, releasing volatiles including water vapor.

Water in asteroids tends to be locked in the form of hydrated minerals. At least on Earth, Sercel’s demonstration shows that concentrated energy from the sun can extract water. More than ever before, he’s excited by the prospect of asteroid mining.

After attending the ASIME meeting with asteroid miners like Sercel, Open University’s Green says it’s hard not to catch some of that excitement. He’s no longer a skeptic after the meeting, but he says that he wouldn’t expect to see any real asteroid mining happen in the next decade.

“When there’s a gold rush, you either go dig for gold or you sell shovels,” Green recalls hearing an attendee at the meeting say. With asteroid mining, investors have shifted, at least in the short term, from the digging-for-gold mind-set to thinking about selling shovels—or, for now, technologies to support the shovels.

For instance, besides TransAstra’s optical mining technology, companies such as Deep Space Industries are working on building thrusters for water-fueled satellites. DPI’s sales revenue was $10 million in 2016.

Selling these products may help the companies stay in business while getting closer to the ultimate goal of mining. “If you design a rocket motor that can operate in space much more cheaply,” Green says, “then that would be of interest to the industry. They are more likely to get investment in designing that rather than the long-term goal of mining an asteroid itself.”

So asteroid miners will have to wait. Luckily, the wait won’t be as long for pals OSIRIS-REx and Hayabusa2.

On Sept. 20, as this story was going into production, OSIRIS-REx retweeted a message to make sure its followers wouldn’t miss its friend’s news.

“Hayabusa2 mission is now deploying a probe to the surface of the asteroid Ryugu. Follow along at @haya2e_jaxa.”

Of course we will.


This article has been sent to the following recipient:

Chemistry matters. Join us to get the news you need.