Enlarge /. The spaceship Hayabusa2 spots its shadow as it descends towards Ryugu to deploy two small rovers.
JAXA
Early on Sunday morning, the sky over a remote military complex in central Australia is lit by a ball of fire falling to earth. It will be an extravagant homecoming for the sample recovery capsule from Hayabusa2, a Japanese spaceship that was launched almost exactly six years ago to shoot an ancient asteroid and steal some of its filth. 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 possibly even provide clues as to the origins of life on Earth.
By the time it parachutes in the Australian outback, the sample will be more than 180 million miles from Ryugu, a diamond-shaped asteroid orbiting the Sun between Earth and Mars. Scientists believe Ryugu broke off from a larger parent body only a few million years ago, but the rocks it is made of are closer to 4 billion years old. Hayabusa2 camped in Ryugu for more than a year and a half, studying the asteroid remotely and sending robotic scouts to its surface to prepare for a sample collection. The main task was to collect just a few grams of dust and pebbles from this cosmic time capsule that has been kept in the cold vacuum of space for eons.
"We hope to learn a lot about how a giant gas and dust cloud in our solar system became planets 4.5 billion years ago," says Larry Nittler, cosmochemist at the Carnegie Institution for Science and one of nine selected American scientists from the NASA to participate in the Japanese mission. "Ryugu and other asteroids like this one are basically the leftover building blocks that didn't grow into planets and have been floating around since then."
Ryugu looks like a piece of charcoal the size of several city blocks and spins like a top every eight hours. It is one of the darkest asteroids ever discovered. Its dark complexion is the result of all the carbon trapped in organic compounds smeared on its surface. Some of these prebiotic compounds, like amino acids, are the building blocks of life, and it may very well have been asteroids like Ryugu that populated Earth with the molecular shot that started evolution.
Carbonaceous asteroids like Ryugu are common in our solar system, but they mostly hang around the outer planets. Every now and then they collide, break apart, and the pieces are sent on a trajectory towards the inner sanctuary of the sun. If these parts accidentally collide with the earth, we call them meteorites. Almost everything we know about them comes from the parts that make it to the surface. But when these stones crashed on earth, they were cooked until crispy and 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 origin of the meteorites discovered on Earth and shed light on the processes that formed the organic compounds in the early solar system.
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"Are there samples of the organics that we don't have in our collection because they didn't survive 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 Mission to return asteroid samples, OSIRIS-REx, but hopes Hayabusa2 can help solve the mystery.
There is also a pragmatic reason to visit Ryugu. NASA researchers have identified it as a potentially dangerous asteroid, which means its orbit is close enough to Earth to create a non-negligible probability of collision. While the risk is small, the complex forces acting on asteroids as they revolve around the Sun make it difficult to accurately predict their trajectory more than a few decades into the future. For example, when an asteroid is exposed to the sun, it can release volatile compounds like water, and this outgassing can create a thrust that subtly alters its orbit. "We don't fully understand how asteroids move in detail because we don't fully understand their composition," says Connolly. "This will help us better predict dangerous asteroids and when they might hit Earth."
The second time is the stimulus
Hayabusa2 is a sequel to Hayabusa, a Japanese mission that started in 2003. It was the world's first mission to return asteroid samples, but a failure of the collection mechanism resulted in only a few micrograms of dust returning to Earth. Like its predecessor, Hayabusa2 was developed to collect samples and deploy small robots on the surface of the asteroid. Hayabusa2 arrived in Ryugu in late 2018 after cruising the solar system for three years. A few months later, the spacecraft deployed a lander called the Mascot and the first of two small Minerva II rovers. The cylindrical rover hopped over the surface for five weeks, collecting data and sending incredible images back to Earth. The shoebox-sized lander only lasted 17 hours before the battery ran out. During his short life, Mascot used a number 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 created the conditions for Hayabusa2 to descend to the surface. Scooping up asteroid debris is harder than it sounds because Ryugu is not solid. Like most asteroids, it looks more like a heap of debris, a loose collection of dust and stones held together by their own gravity. This makes it difficult to get to the surface to collect a sample without stirring up many rocks that could damage the spaceship. Ryugu was also made up of more large boulders – some up to 10 stories high – than the mission's scientists expected. "Safe landing sites were limited by the abundance of rock," says Tomokatsu Morota, a planetologist at the University of Tokyo and one of the researchers who worked on the Hayabusa2 navigation camera. He says the team had to manually count more than 10,000 stones and measure more than 100 remotely in order to narrow down suitable landing sites on the asteroid's rough surface. "It was very hard work," says Morota.
In early 2019 the team had chosen a landing site and Hayabusa2 made its first descent. The spacecraft's sampling horn tapped the surface for only about a second before returning to orbit the asteroid. During this brief encounter, Hayabusa2 fired a small ball into the asteroid to dig up some dust and lock it in a collection chamber. A few months later, Hayabusa2 prepared for another collection by dropping a small plastic explosive from its orbit to create an artificial crater more than 30 feet in diameter that exposed the older rock beneath Ryugu's surface. After the debris settled around the asteroid, the spacecraft made its second short descent to take a sample from inside the crater. Just weeks before Hayabusa2 left Ryugu, his second Minerva II rover failed before deployment. But instead of wasting the rover, the mission leaders released it and took some gravity readings before it hit the asteroid.
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Hayabusa2 will dump its sample container when it is approximately 100,000 miles from Earth, approximately half the distance between our planet and the moon. As soon as the capsule is in place, it will be retrieved by a team of Japanese researchers stationed in the scorching Australian desert. It is immediately taken to a temporary clean room set up on site so that it can be examined for any volatile compounds, such as water, that may be in the sample. Within hours of recovering the capsule, the researchers will pierce the hull and fill in any gases that may have been released from the sample and save it for analysis. The sample is then sent back to Japan, where researchers from the Japan Aerospace Exploration Agency distribute small pieces to research teams around the world for further investigation.
“Examining samples with laboratory instruments can provide us with information about their composition, heating, shock events, water flow events, etc. You can get a full history lesson with just a tiny sample, ”says Bill Bottke, a planetary scientist at the Southwest Research Institute who was not involved in the Hayabusa2 mission. “Only part of this information can be determined by spacecraft orbits. It's like the difference between seeing a mountain from a distance and studying one of its rocks in the laboratory. "
The Hayabusa2 team won't know how much asteroid debris the spacecraft has collected until they pry open the capsule, but they are optimistic it will be around 10 grams. A significant portion of the sample will be made available to NASA researchers who have worked closely with Japan on Hayabusa2 and OSIRIS-REx. In fact, NASA and the Japanese space agency each tapped some of their own researchers to help the other agency. Connolly, one of the researchers who worked on both missions, is optimistic that research on the Hayabusa2 probe will improve research on the much larger OSIRIS-REx probe when it returns to Earth in 2023.
"We can use the insights gained in the analysis process and the actual information we can get from these whispering stones to better prepare ourselves as a community for the analysis of OSIRIS-REx samples," says Connolly. "I assume that they are complementary and give us a better picture of the limitations of the earliest processes in the solar system."
The return of the Hayabusa2 capsule to Earth marks an important milestone for the mission, but does not mark the end of the spacecraft's journey. After dropping its sample this weekend, it will resume a bonus mission to another asteroid that could last up to 10 years. No samples will be collected this time, but valuable data will be collected as it orbits the asteroid.
You can watch a livestream of the fiery finale of Hayabusa2's main mission on the Japan Aerospace Exploration Agency's YouTube channel. The capsule is scheduled to begin atmospheric entry on Saturday around 12:30 p.m. ET (or Sunday at 2:30 a.m. in Japan) and land about 15 minutes later.
This story originally appeared on wired.com.
