A brilliant fireball streaked above Australia's southern Outback early Sunday morning (Dec. 6) as a 16-inch metal capsule plummeted to Earth. Inside were fragments of a small, near-Earth asteroid painstakingly-gathered by Japan's Hayabusa-2 probe. As the capsule reached speeds of more than 26,000 mph (12 km/sec), the heat shield's temperature climbed to 5,400° F (3000° C),yet the samples remained cool and secure.

The glowing capsule and its luminous trail created a bright fireball across the Australian sky. The bright star at the bottom is Alpha Centauri; the Southern Cross is seen tipped on its side left of center. (JAXA)
The glowing capsule and its luminous trail created a bright fireball across the Australian sky. The bright star at the bottom is Alpha Centauri; the Southern Cross is seen tipped on its side left of center. (JAXA)

The pod landed safely on the red, sandy ground, its parachute hung up in a tree. A helicopter crew was dispatched and tracked its radio beacon until they made a visual sighting of the landing site. The treasure chest held two separate samples of Ryugu, a dark, "rubble-pile" asteroid 0.6 mile (1 km) across and currently about 7 million miles (11.5 million km) from Earth.

Asteroid 162173 Ryugu, the Japanese name for "dragon palace," is shaped a little like a spinning top and measures just 0.6 miles across. It's a dark, carbon-rich asteroid that occasionally passes close enough to our planet to be deemed potentially hazardous. We see both hemispheres in this panel. (JAXA)
Asteroid 162173 Ryugu, the Japanese name for "dragon palace," is shaped a little like a spinning top and measures just 0.6 miles across. It's a dark, carbon-rich asteroid that occasionally passes close enough to our planet to be deemed potentially hazardous. We see both hemispheres in this panel. (JAXA)

The first specimens were collected in Feb. 2019 by shooting a tantalum (a rare metal) bullet at the asteroid and gathering up the disturbed material into a horn-like instrument. In July the same year the spacecraft deployed a free-flying gun called the Small Carry-on Impactor (SCI). Once the probe maneuvered to a safe position, the SCI fired a larger, heavier copper projectile at Ryugu to expose pristine material from beneath the surface protected from the harsh space environment for billions of years. The impact excavated a 10-foot (33-m) crater in the process. When the coast was clear, the probe returned to sample the location.

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A Minerva rover, deployed by the spacecraft, took this photo of Ryugu's rocky surface. Astronomers believe the asteroid is basically a rubble pile of rock that formed in the wake of a collision with the asteroid's larger parent body. (JAXA)
A Minerva rover, deployed by the spacecraft, took this photo of Ryugu's rocky surface. Astronomers believe the asteroid is basically a rubble pile of rock that formed in the wake of a collision with the asteroid's larger parent body. (JAXA)

After safely stowing the goods, Hayabusa-2 fired up its engine in Nov. 2019 for the return trip to Earth. By the time the capsule landed it had traveled more than 5.1 billion miles (8.2 billion km).

The mission was Japan's second attempt to retrieve material from an asteroid. The first, Hayabusa-1, experienced several failures and retrieved less than a milligram (just 1,500 grains of dust). The current effort did far better: the team estimates that the capsule contains a full gram of asteroid soil.

Why go to all the trouble and expense of flying to asteroids when meteorites — fallout from asteroid collisions — are delivered to Earth every year for free? The reason is that falling fragments are less pristine. They're heated during their atmospheric plunge, which releases crucial volatiles, gases and compounds that easily vaporize. Often, meteorites are found days to weeks after they fall, during which time they become contaminated with earthly gases, water and chemicals in the soil.

The sample return capsule and its parachute are seen from above in the Australian desert. (JAXA)
The sample return capsule and its parachute are seen from above in the Australian desert. (JAXA)

When you go directly to the source and gather specimens in situ, you get true raw material. Among other things, Japanese scientists will also be looking for traces of ancient water in the samples. While comets delivered some of Earth's current water during its tumultuous youth 4.6 billion years ago, asteroids appear a more likely source because the kind of water they possess is a better match.

A water molecule is composed of two hydrogen atoms bonded to an oxygen atom. In most water, the nucleus of each hydrogen atom is a single proton. Rarely, at least on our planet, hydrogen in water possesses both a proton and a neutron. This form of hydrogen is called deuterium. Maybe you've heard of heavy water — that water with deuterium instead of the simpler form of hydrogen called protium. Turns out that comets are much richer in deuterium-laced water compared to water found in asteroids, which better match our lakes, seas and oceans.

The Hayabusa-2 spacecraft photographs the blue planet as it departs on its next mission Dec. 6. (JAXA)
The Hayabusa-2 spacecraft photographs the blue planet as it departs on its next mission Dec. 6. (JAXA)

Who knows what those precious bits of Ryugu will reveal, but I bet we' find surprises. Of course, that's one of the main reasons we explore distant worlds in the first place!

While the capsule journeys back to Japan, the spacecraft is already off on a second quest. 11 years from now Hayabusa-2 will pay a visit to the near-Earth asteroid 1998 KY26, which measures just 98 feet (30 meters) across and rotates incredibly fast, completing a spin in just 10.7 minutes. Great place to live if you like sunrises and sunsets. Onward!

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.