astronomy,  research,  space exploration

Peregrine falcon returns home: what secrets lie hidden in the Dragon Palace?

Six years after the Japanese spacecraft Hayabusa2 left Earth, we anxiously anticipate its return. On December 6, a small capsule will detach from the spacecraft and come to Earth. It will deliver an especially precious cargo: a piece of an actual asteroid.

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The successful missions to asteroids in recent years portray the feats as deceptively simple. Scientists build a spacecraft, give it a catchy name, and send it to a rock somewhere in the Solar system. After years of drifting through space, the spacecraft intercepts the rock and takes a few photos, sometimes a selfie. Landing seems to be a bit of a hassle, but we’re getting better at it. For some spacecraft, this sadly marks the end of their life. Others move forward, exploring the unknown. But not Hayabusa2. The Japanese spacecraft is returning home with an actual sample of an asteroid.

Bringers of life and destruction

Our Solar system is crawling with asteroids of all shapes and sizes. Millions of asteroids form a belt between Mars and Jupiter, others share orbits with planets or moons, and there are undoubtedly many more far beyond Neptune’s orbit, too dark to be seen even with the biggest telescopes. Most asteroids are orbiting the Sun in an almost circular orbit; not unlike Earth, but at larger distances from our star. But not all are well-behaved. Gravitational interaction with planets sometimes propels an asteroid into a highly elliptical orbit, moving it into the inner part of the Solar system and closer to Earth.

Asteroids that find themselves very close to Earth may pose a threat, as dinosaurs found out the hard way. However, such huge, extinction-causing space rocks are rare. The number of asteroids drops with their size. The larger the asteroid, the smaller the chance of collision with Earth. It is much more common for a smaller asteroid to drop by. Remember the Chelyabinsk event? In 2013, an asteroid with a diameter of 20 meters broke up over the Russian city of Chelyabinsk, generating a glass-shattering shock wave. Such asteroids impact Earth a few times per hundred years. Nevertheless, astronomers are vigilantly searching for and keeping an eye on possibly dangerous asteroids.

Did asteroids deliver water and carbon-based molecules—the building blocks of life—to Earth? Credits: NASA's Goddard Space Flight Center Conceptual Image Lab

Paradoxically, asteroids might have brought water and organic molecules to Earth, thus providing building blocks of life. Asteroid impacts were much more common when the Solar system formed; the constant bombardment during the first billion years would actually prevent life from forming. But eventually, the bombardment subsided, and subsequent impacts could deliver enough water and carbon-based molecules to give rise to life.

Whether we want to study asteroids to understand the origin of life or the origin of the Solar system—unlike planets, asteroids haven’t changed much since the Solar system formed, providing a unique view of the chemical composition in the early times—or to prevent a catastrophe, we need to get a closer look at the asteroids. And that is where the Hayabusa2 comes in.

Illustration of Hayabusa2 as it dives to collect a sample of the asteroid. Credit: Akihiro Ikeshita (C) Japan Aerospace Exploration Agency (JAXA)
Rovers, shoeboxes, and projectiles

Hayabusa2 launched on December 3, 2014, from Tanegashima Space Center and reached its target, asteroid Ryugu1The name Ryugu refers to Dragon Palace, a magical underwater palace in a Japanese folktale., three years and a half later. The mission’s primary goal was to collect a sample of material from the asteroid and bring it back to Earth. But the spacecraft had a few more tricks up its sleeve. Here are its most notable achievements in chronological order.

September 21, 2018. Two small rovers separated from the spacecraft and landed on Ryugu’s surface. This has been the first successful landing and autonomous movement on an asteroid’s surface. Ryugu’s small gravity prevented the rovers from moving with wheels. Instead, the two rovers moved by hopping across the surface.

October 3, 2018. A small shoebox-sized lander was deployed to the asteroid’s surface. The lander carried several instruments. It retrieved data that demonstrated a porous nature of the asteroid (Ryugu wouldn’t survive the entry into the Earth’s atmosphere), a composition consisting of two types of rocks, and an absence of a detectable magnetic field.

April 5, 2019. The spacecraft shot a 2 kg copper projectile into the asteroid, creating a small crater (ten meters in diameter). That way, scientists got a glimpse into the structure of the asteroid. The impact also ejected material that was later collected by the spacecraft.

February 22 and July 11, 2019. The spacecraft collected two separate samples; the second sample consists of material ejected by the projectile’s impact. Collecting the sample was reminiscent of a hunting falcon—it is not a coincidence that the spacecraft’s name translates as the peregrine falcon. Hayabusa2 approached the surface and, at the point of touchdown, fired a projectile into the surface to break the rocks apart. A sampler horn collected a part of the ejected material and stored it in the sample container. The two samples are stored in separate containers.

And that’s it! On November 13, Hayabusa2 started its voyage home. In a few days, the spacecraft will pass Earth, releasing the capsule with its treasure, and move on. This act will not be the end for the spacecraft. Its next target is a tiny asteroid 1998 KY26. Hayabusa2 will catch up with it in 2031, but that is another story.

The sample container will make a fiery entry into the Earth’s atmosphere and land somewhere within the Woomera Prohibited Area in Australia. If you happen to be in the area, check this resource to plan observations of the fireball. Once the scientists get their hands on the samples, they will analyze them in the labs in Japan and later independently in other places in the world. The time has come to meet (a part of) Ryugu in person.

Ryugu and Bennu: rocky twins?

The sample from Hayabusa2 will be only the second clean sample2Some asteroids survive the deathly cruise through the atmosphere. The remaining rock of the original asteroid that falls on the ground is called a meteorite. With a bit of luck, one can find meteorites and study them in a laboratory. However, these samples are not clean and unaltered. Their composition is affected by the heat of atmospheric entry, and they become contaminated when hitting the ground. The origin of their parent body also remains unknown. Having our hands on the original is obviously better. from an asteroid we have got our hands on. The first one was delivered by its predecessor Hayabusa. What is really exciting is that we have more than one iron in the fire right now. In a few years, the OSIRIS-REx mission will deliver another asteroid sample to Earth. In fact, OSIRIS-REx has already collected its sample from asteroid Bennu3Say what you will about 2020, it is a good year for asteroid science.. Hayabusa2 and OSIRIS-REx are similar in many aspects, and so are their targeted asteroids.

Asteroids Ryugu and Bennu are surprisingly alike. They both have a peculiar spinning-top shape, comparable sizes, and their orbits are similar to Earth’s. From what we know, they are both composed of carbonaceous material and have water locked within their minerals. The similarity suggests that the two are closely linked. Indeed, simulations show that Ryogu in Bennu might be fragments of the same parent body that collided with another space rock.

Asteroids Bennu and Ryugu

Both asteroids were discovered in 1999 by the LINEAR survey. Due to their proximity to Earth, they made perfect targets for Hayabusa2 and OSIRIS-REx missions. The two complementary missions have been developed and operated side-by-side by NASA and JAXA. By learning from each other, the two space agencies tried to improve the outcome of the missions. From what we have seen so far, successfully.

It remains to be seen if the two asteroids are indeed linked. The samples from Hayabusa2 and OSIRIS-REx will reveal a detailed chemical composition and structure of the material, helping to solve the puzzle. More importantly, the information we get of the water and carbon-based molecules may shed light on the origin of life on Earth. How about that!

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