Early on Sunday morning, the skies above a secluded military complex in central Australia will be brightened by a fireball plummeting to Earth. It will be a flamboyant homecoming for the sample return capsule from Hayabusa2, a Japanese spacecraft launched almost exactly six years ago on a mission to shoot an ancient asteroid and steal some of its dirt. If the capsule survives its fiery descent, its payload of pristine space rock will help scientists understand the earliest days of our solar system, shed light on the mysterious origins of meteorites, and may even provide clues about the emergence of life on Earth.
By the time it lands under parachute in the Australian outback, the sample will have traveled more than 180 million miles from Ryugu, a diamond-shaped asteroid orbiting the sun between Earth and Mars. Scientists believe that Ryugu broke off from a larger parent body only a few million years ago, but the rocks that compose it are closer to 4 billion years old. Hayabusa2 camped out around Ryugu for more than a year and a half, studying the asteroid from a distance and sending robotic scouts to its surface to prepare for a sample collection. Its main mission was to collect just a few grams of dust and pebbles from this cosmic time capsule that has been preserved for eons in the frigid vacuum of space.
“We're hoping to learn a lot about how a giant cloud of gas and dust turned into planets 4.5 billion years ago in our solar system,” says Larry Nittler, a cosmochemist at the Carnegie Institution for Science and one of nine American scientists selected by NASA to participate in the Japanese mission. “Ryugu and other asteroids like it are basically the leftover building blocks that didn't grow into planets and have been floating around ever since.”
Ryugu looks like a piece of charcoal the size of several city blocks, and it spins like a top once every eight hours. It is one of the darkest asteroids ever discovered, its inky complexion a result of all the carbon trapped in organic compounds smeared across its surface. Some of these prebiotic compounds, such as amino acids, are the building blocks of life, and it may very well have been asteroids like Ryugu that seeded Earth with the molecular grist that kicked evolution into gear.
Carbonaceous asteroids like Ryugu are abundant in our solar system, but they mostly hang out around the outer planets. Every now and then, they bump into each other, break apart, and the pieces are sent on a trajectory toward the sun’s inner sanctum. If those pieces happen to collide with Earth, we call them meteorites. Almost everything we know about them is from the bits and pieces that make it to the surface. But by the time these stones have crash-landed on Earth, they have been cooked to a crisp and have been corrupted by terrestrial chemistry. Sending a probe to a still-orbiting asteroid is the best way to collect a clean sample. As the first spacecraft to visit a carbonaceous asteroid, Hayabusa2 can help determine the provenance of meteorites discovered on Earth and shed some light on the processes that formed the organic compounds in the early solar system.
“Are there samples of the organics that we don’t have in our collection because they didn’t survive going through the atmosphere? We don’t know,” says Harold Connolly, a geologist at Rowan University and a member of the sample analysis team for Hayabusa2 and NASA’s own asteroid sample return mission, OSIRIS-REx. But he hopes Hayabusa2 can help solve the mystery.
There is also a pragmatic reason to visit Ryugu. NASA researchers have identified it as a potentially hazardous asteroid, which means its orbit comes close enough to Earth to create a non-negligible chance of collision. While the risk is small, the complex forces acting on asteroids as they loop around the sun make it difficult to accurately predict their trajectory more than a few decades into the future. For example, when it’s exposed to the sun, an asteroid can release volatile compounds like water, and this outgassing can produce thrust that subtly changes its orbit. “We don't fully understand how asteroids move in detail, because we don’t fully know their composition,” says Connolly. “This will help us better predict hazardous asteroids and when they might hit Earth.”
Hayabusa2 is a follow up to Hayabusa, a Japanese mission launched in 2003. It was the world’s first asteroid sample return mission, but a failure with the collection mechanism meant that only a few micrograms of dust made it back to Earth. Like its predecessor, Hayabusa2 was designed to collect samples and deploy small robots on the asteroid’s surface. Hayabusa2 arrived at Ryugu in late 2018 after cruising through the solar system for three years, and a few months later the spacecraft deployed a lander called Mascot and the first of two small Minerva-II rovers. The cylindrical rover spent five weeks hopping around the surface collecting data and sent incredible pictures back to Earth. The shoebox-sized lander lasted just 17 hours before its battery died. During its brief life, Mascot used a suite of instruments to analyze the composition and structure of the asteroid’s regolith.
By the end of 2018, the rover and lander had completed their missions and set the stage for Hayabusa2’s descent to the surface. Scooping up some asteroid dirt is harder than it sounds, because Ryugu isn’t solid. Like most asteroids, it’s more like a rubble pile, a loose collection of dust and rocks held together by their own gravity. This makes it tricky to get down to the surface to collect a sample without stirring up a lot of rocks that could damage the spacecraft. Ryugu also turned out to be composed of more large boulders—some up to 10 stories high—than the mission’s scientists expected. “Safe landing locations were limited by the high abundance of rocks,” says Tomokatsu Morota, a planetary scientist at the University of Tokyo and one of the researchers who worked on Hayabusa2’s navigation camera. He says the team had to manually count more than 10,000 rocks and remotely measure more than 100 to narrow down suitable landing sites on the asteroid’s rough surface. “It was very hard work,” Morota says.
By early 2019 the team had a landing site picked out, and Hayabusa2 made its first descent. The spacecraft’s sample collection horn tapped the surface for only about a second before returning to orbit the asteroid. During that brief encounter, Hayabusa2 fired a small bullet into the asteroid to kick up some dust and trapped it in a collection chamber. A few months later, Hayabusa2 prepared for another collection run by dropping a small plastic explosive from its orbit to create an artificial crater more than 30 feet across, exposing the older rock beneath Ryugu’s surface. Once the debris around the asteroid had settled, the spacecraft made its second brief descent to take a sample from inside the crater. Just a few weeks before Hayabusa2 departed Ryugu, its second Minerva-II rover failed before deployment. But rather than let the rover go to waste, mission controllers released it and conducted a few gravitational measurements before it hit the asteroid.
Hayabusa2 will jettison its sample container when it’s about 100,000 miles away from Earth, roughly half the distance between our planet and the moon. Once the capsule has touched down, it will be recovered by a team of Japanese researchers stationed in the scorching Australian desert. It will immediately be brought to a temporary clean room built on site so it can be analyzed for any volatile compounds like water that may have been contained in the sample. Within hours of recovering the capsule, the researchers will puncture its hull and bottle any gases that may have been released by the sample and save them for analysis. After that, the sample will be returned to Japan where researchers at the Japan Aerospace Exploration Agency will distribute small portions to research teams around the world for further study.
“Exploring samples with laboratory instruments can tell us their composition, how much heating they have experienced, shock events, water flow events, and so on. You can get an entire history lesson from just a tiny sample,” says Bill Bottke, a planetary scientist at the Southwest Research Institute who was not involved with the Hayabusa2 mission. “Only a portion of this information can be determined by orbiting spacecraft. It is like the difference between seeing a mountain from a distance and studying one of its rocks in the lab.”
The Hayabusa2 team won’t know how much asteroid dirt the spacecraft collected until they pry the capsule open, but they’re optimistic that it will be around 10 grams. A significant portion of the sample will be given to NASA researchers, who collaborated closely with Japan on Hayabusa2 as well as OSIRIS-REx. In fact, NASA and the Japanese space agency each tapped a few of their own researchers to help the other agency. Connolly, who was one of the researchers working on both missions, is optimistic that the research done on the Hayabusa2 sample will improve the research done on the much larger OSIRIS-REx sample when it returns to Earth in 2023.
“We can apply the lessons learned in the analytical process and the actual information that we manage to tease out of these whispering rocks so that we can prepare better as a community for the analysis of OSIRIS-REx samples,” says Connolly. “My expectation is that they’re going to be complementary and will give us a better picture of the constraints on the earliest solar system processes.”
The Hayabusa2 capsule’s return to Earth marks a major milestone for the mission, but it’s not the end of the spacecraft’s journey. After it jettisons its sample this weekend, it will continue on a bonus mission to another asteroid that could last as long as 10 years. This time, it won’t collect any samples, but it will gather valuable data while it orbits the asteroid.
You can catch a livestream of the fiery finale of Hayabusa2’s main mission on Japan Aerospace Exploration Agency’s YouTube channel. The capsule is scheduled to begin its atmospheric entry at around 12:30 pm ET on Saturday (or 2:30 am on Sunday in Japan) and will land about 15 minutes later.