Asteroids can tell critical stories about the birth of our Solar System and the processes that produced its planets. In some cases, they are time capsules for the planetesimals that went on to form our planets. In others, they've been through multiple rounds of catastrophic collisions and reformation, providing testimony of the violent processes that built our current Solar System. But figuring out what they tell us has been difficult, because their small size and generally large distance from Earth make them difficult to study using telescopes. And the bits and pieces we have been able to study directly have been altered by the process of plunging from space through the Earth's atmosphere.

All that's on the verge of changing in the near future, as we have not one but two missions that will return samples from asteroids over the next couple of years. In the case of JAXA's Hayabusa2 mission, the first sample retrieval has already taken place, while NASA's OSIRIS-REx arrived at its destination more recently. But since arriving, both probes have been studying the mini-worlds they were sent to, and the first results of those studies are now in.

Today, Nature and Science are releasing a large collection of papers that describe the initial observations of the two asteroids that these missions have targeted. The bodies have turned out to be remarkably similar, as you can see by visiting our Bennu coverage and then comparing it with what we now know about Ryugu, described below.

The rubble of Ryugu

Hayabusa2, the Japanese spacecraft, arrived at its destination, the asteroid Ryugu, in 2018. Since then, it has been scanning the asteroid from 20km above its surface and has done a number of closer approaches. This proximity means that Ryugu has been mapped at a resolution that's nearly unheard of: the position of every feature two meters and up in size is known. In addition, Hayabusa2 was allowed to free-fall toward the surface at one point, providing a measure of Ryugu's gravitational pull and thus its mass.

That mass, plus the careful mapping of Ryugu's shape, provides us with its density, which turns out to just be barely above that of water, at 1.19 grams/cubic centimeter. Even by the standards of water-rich bodies, that's an extremely low density. (Europa, with its global ocean, has a density nearly three times higher.) And all indications are that Ryugu doesn't have much water at all. While some indications point to a bit of water incorporated into minerals, spectrographic imaging suggests the asteroid is very water poor. Plus, its orbit is just a bit outside that of Earth's, which means its surface temperatures are high enough for any water to boil off into space.

The explanation for its low density is that it's a rubble pile, a poorly compacted collection of debris left over from a larger body that has been blasted apart in a collision. That's also consistent with its appearance: a large collection of boulders interspersed with dusty, granular materials.

Overall, that material is extremely dark. Areas of surface that are likely to have been exposed relatively recently are a bit brighter and tend to have a more bluish tint, suggesting that Ryugu's material gets darker and more red with age. That's typical of carbon-rich materials, though it's not clear whether these are picked up as it orbits or are already major components of its surface. These properties make Ryugu look similar to the asteroids Eulalia and Polana, and its orbit is consistent with it having been generated in the same region of the asteroid belt.

A bit of spin

Despite its relatively small size, Ryugu has plenty of craters. On average, bodies its size in this area of the Solar System are expected to experience a collision large enough to obliterate them every 108 years or so. But the cratering rate suggests that its surface is probably completely replaced roughly every 106 years. That said, there are some features that look like craters but might not be. That's because, given its low gravity, Ryugu can potentially spin fast enough that it would lose its grip on individual boulders, which would then fly off into space.

In fact, there's evidence that Ryugu was once spinning much faster than it is now. Its equator has a prominent ridge that's dotted with cone-shaped structures. The researchers suspect that these were formed as loosely packed granular material was driven toward Ryugu's equator by the force of its rotation. As it slowed down to its present rotational speed, some of that material has also slumped back down, a phenomenon that's visible near the edge of its boulders.

Right now, we're not sure what has caused those changes in rotation, and we don't know much about the material that's shifting around on Ryugu's surface. The researchers involved in these studies suggest several plausible explanations for how the asteroid ended up looking like it does, but more details may have to wait for Hayabusa2 to return samples from its surface to Earth, which is scheduled for 2020.