Author: Lisa Grossman / Source: Science News
Shogo Tachibana greeted asteroid Ryugu with dread.
The cosmochemist with the University of Tokyo had spent 10 years helping to design a mission to Ryugu’s surface. To touch down safely, the spacecraft, Hayabusa2, needs to find broad, flat stretches of fine-grained dust on the asteroid. But on June 27, when Hayabusa2 finally reached its target after a three-and-a-half-year journey (SN Online: 6/27/18), Tachibana got a rude awakening: Ryugu is covered in boulders. Big ones.
“We cannot find a 100 percent safe place to touch down,” Tachibana says. “It seems to be a very dangerous place.”
If Hayabusa2 can deal with the boulders — and any other challenges that arise — it will become only the second spacecraft to bring a piece of an asteroid back to Earth. And the mission will answer questions that its predecessor couldn’t. The original Hayabusa mission visited a sand- and rock-covered asteroid called Itokawa in 2005. But Itokawa has the wrong chemical makeup to address big questions about the origin of life that Ryugu, which is carbon-rich, is well suited for. And Hayabusa suffered a series of calamities that caused it to return to Earth several years late, with less than 2,000 grains of precious asteroid dust.
Tachibana and colleagues from the Japanese Aerospace Exploration Agency, or JAXA, are counting on Haybusa2 to return bits of Ryugu’s surface to Earth in 2020. And if a daring plan to blow a crater into the asteroid works, the spacecraft will get some subsurface grains as well.
A sister project from NASA, the OSIRIS-REx mission, arrived at an asteroid called Bennu in December to bring samples back in 2023 (SN Online: 12/3/18).
The two spacecraft face daunting challenges. The probes must investigate objects that have so little gravity that sunlight can knock them off their orbits. If the probes manage to pick up samples, the spacecraft must keep the dust pristine during the trip back to Earth. To get the most out of the missions, the Japanese and American teams are trying to work together across cultural and bureaucratic divides.
Ryugu and Bennu are small targets. Ryugu’s polar diameter is about 880 meters and Bennu’s is about 510 meters (left). Both asteroids orbit the sun on trajectories that sometimes take them relatively close to Earth (right).
But the uncertainties and anxiety are worth it. Asteroids like Ryugu and Bennu are among the oldest and most intriguing objects in the solar system. They could hold the keys to some of the most pressing planetary questions: What came before the planets? What are the origins of life? And how much of a threat do asteroids pose to life on Earth today?
Of course, planetary scientists already have tens of thousands of asteroid pieces to study. Such meteorites fall to Earth in the hundreds each year, offering researchers plenty of material to slice, grind and examine for clues to the solar system’s history.
Dante Lauretta of the University of Arizona in Tucson, the principal investigator of OSIRIS-REx, spent the first part of his career trying to coax meteorites into telling him whether molecules necessary for life — such as nucleic acids, amino acids and phosphorus, which are structural components of DNA — could have originated inside carbon-rich asteroids like Ryugu or Bennu.
Carbon-rich asteroids are thought to be mostly unchanged since their formation at least 4.6 billion years ago, which makes them perfect time capsules. A few grains of such an asteroid could reveal what the early solar system was made of.
Remote studies of asteroids also suggest that the raw ingredients for life, and maybe even the chemical processes that are necessary for life to begin, might have been present on carbon-rich asteroids even before the planets were done growing.
“We think an asteroid like this one may have delivered this material to the surface of the early Earth, providing seeds or building blocks of life,” Lauretta says. “If we can show the precursors [of life] started before the planet, I think the probability that there’s life elsewhere in the solar system goes way up.”
Studying meteorites to explore this notion falls short on two fronts, however: It’s hard to tell where they come from, and they’re contaminated. As soon as a space rock hits Earth’s atmosphere, it starts accumulating signs of Earth life. Therefore, any intriguing organic compounds in a meteorite could be from Earth, not native to the asteroid. There’s no way to tell.
“We needed samples of a carbon-rich asteroid to really answer the questions I was into,” Lauretta says.
Of more than 500,000 asteroids in the solar system, Ryugu and Bennu are two of only five with the right orbits, sizes and compositions for a sample-return mission.
Source: Univ. of Arizona
Break me off a piece
Getting to the origins of the solar system, and maybe life’s beginnings, makes bringing clean, carefully selected samples to earthly labs crucial. But spacecraft can’t just dig in with a shovel. There’s no grabbing a rock with a claw like in an arcade game. The asteroids are so tiny — Ryugu is about 880 meters from pole to pole and Bennu is about 510 meters — and their gravity is so weak that reaching out and grabbing something could push the spacecraft off course with the asteroid.
So instead of scooping or grabbing, the spacecraft will reach out with proboscis-like tubes, either touching down briefly or hovering above the surface. This tricky endeavor has been attempted only once before — and it was almost a disaster.
To sample Ryugu below the surface, Hayabusa2 will launch a projectile to blast open a crater, then take a lap around the asteroid to avoid damage. The spacecraft will then come close to the new hole in the ground and shoot a small bullet from its sampling horn. The bullet will splash on the surface, sending dust and sand into a catcher in the upper part of the horn.
The first Hayabusa spacecraft was supposed to use its three reaction wheels to stabilize itself as it hovered near Itokawa’s surface, stretched out a collection tube to touch the surface and fired a small bullet down the tube to stir up dust particles. Those dust grains would float up the tube into a sterile chamber for storage on the trip back to Earth.
Almost everything went wrong. Before Hayabusa even got to the asteroid, the biggest solar flare ever recorded damaged the spacecraft’s solar panels and one of its engines, slowing down the spacecraft and delaying its asteroid rendezvous by three months.
Once at Itokawa, two of the craft’s reaction wheels failed, making it hard for the craft to keep an even keel. A companion rover released by Hayabusa that was meant to land on Itokawa’s surface…
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