Space missions that changed the way we saw asteroids

In the last two decades, three robotic probes set foot on three different near-earth objects called asteroids. The probes conducted extensive observations of the ancient rocks and scooped up soil samples from the surfaces. JAXA (Japanese space agency) is a pioneer in exploring asteroids with two missions — the Hayabusa and Hayabusa 2 — investigating Itokawa and Ryugu, respectively. The missions released small rovers on the surface and collected a few grams of the soil. In contrast, US space agency Nasa’s OSIRIS-REx is on its way back from another asteroid called Bennu, carrying nearly 400 grams of Bennu’s sand, which it will deposit on earth in 2023.

Asteroids are billions-of-years-old vagabond space rocks. In the primitive stages when the solar system was forming, large rocks collided with each other, fused and formed the planets. Many of these rocks were mineral and water-laden. Asteroids are the remnants from these massive collisions, the flyaway that remained in space. Some fell to earth as meteorites, bringing life-forming elements like carbon which could have kickstarted life processes on earth.

Asteroid Itokawa 3D Model

Millions of asteroids of various sizes – specks to kilometres long – occupy the space between Mars and Jupiter in what is known as the Asteroid Belt. Held together by weak gravity, they wander around the sun in circular or elliptical orbits. Some come close to the earth’s orbit and are called near-earth objects (NEO). Sometimes, NEOs come too close to the earth, get pulled by gravity, and come hurtling down as meteors with potentially hazardous outcomes.

Ground-based observations have shown that several meteorite rocks have compositions similar to those of the asteroids. So, scientists came to believe asteroids could hold secrets of our origins, which naturally led to sending spacecraft to investigate the space rocks further. All three asteroids — Itokawa, Ryugu and Bennu — are NEOs. So the missions explored the topography, terrain, composition and trajectories of asteroids in great detail.

It turns out that whatever data that has been analysed so far from these missions have confounded our earlier understanding of asteroids. Each of these iconic missions has sprung surprises about the ancient space rocks, unlocking one mystery after another.

25143 Itokawa: The cosmic peanut

Itokawa belongs to the S-type of asteroids so far known to have dense rocks of silica minerals. However, Hayabusa, the robotic probe to Itokawa, was amazed by what it saw. Itokawa was not a single rock but two fused rocks, giving it the characteristic peanut shape.

From the mission data and the 1000 grains of the soil sample that Hayabusa could collect, researchers analysed that Itokawa’s original age was 4.6 billion years. However, between 1 and 2 billion years ago, a massive impact broke it into two. Eventually, the two rubble piles were drawn together by mutual gravity, which explains Itokawa’s dual terrain: smooth and rocky areas.

Another intriguing aspect was the presence of a 50 metre wide boulder on its terrain. As rocks are ejected after impacts, researchers used computer simulations to reconstruct the events that could have led to the formation of the boulder. They found that if such a big rock has to come out of a crater, then Itokawa should have been 20 kilometre long.

However, the longest side of the asteroid is a mere 0.5 kilometre implying the boulder is a fragment of another collision, sitting on Itokawa. These results again justify the two-part topography of the asteroid.

That was not all, Itokawa threw more surprises. The analysis showed that initially, the asteroid occupied a path in the main Asteroid Belt. But now its orbit has changed and possibly, it will cross paths with earth in the coming one million years. However, looking at how its terrain is changing, another possibility is that it could break apart before that.

162173 Ryugu and 101955 Bennu: The cosmic twins

Bennu and Ryugu belong to the C-type carbon-laden asteroids and are strikingly similar in composition and rotation dynamics. They are less than a kilometre long and have a diamond-like shape (called an oblate spheroid). They have a ridged equator filled with rubble and spin erratically like tops. The only observed difference between them is their hydration levels: Bennu is moisture-laden while Ryugu is dry.

For all we know, Bennu and Ryugu are likely celestial siblings. The similar composition and the preliminary data analysis from the two missions indicate that the asteroids may have broken off from a giant parent rock some 1.5 billion years ago.

A theory that emerged from computer simulations shows that the parent rock was hit by another large asteroid, shattering it. The fragments flew and re-accumulated (by mutual gravity) into the diamond-shaped bodies we see today. Also, Ryugu could have been closer to the parent rock and exposed to the intense heat of the impact, which explains its dehydrated composition. Bennu, on the other hand, would have formed farther away and hence did not undergo any thermal changes, retaining its moisture.

In a recently published paper on Bennu’s data, researchers report that earlier, they thought that Bennu had a smooth surface, like a sandy beach with a few rocks and boulders.

However, to their surprise, OSIRIS-REx (probe sent to Bennu) observed that Bennu is riddled with rocks and boulders. In fact, the original target site for the sample collection was so rough and devoid of a smooth stretch that the collection manoeuvre had to be aborted as it was hazardous for the spacecraft.

So, OSIRIS-REx spent several months circling Bennu, scouting for another suitable spot before zeroing in on a small crater called Nightingale. Even this attempt at scooping up the soil was a close call. Researchers say Bennu’s rocks are highly porous – like plastic balls and the asteroid’s surface offered no resistance for the probe to land. The sample collection operation labelled Bullseye TAG (touch-and-go) lasted just six seconds; in this time, OSIRIS-REx dived in, deployed a robotic arm to collect the soil and quickly pulled out. Any delay would have sunk the probe in the rubble – like a child diving into a plastic ball pit.

Another astonishing facet the probe found was that the asteroid was spitting out particles in a constant stream. So while some smaller rocks were pulled back by gravity, the bigger ones circled the asteroid.

On the other hand, JAXA reported that scientists who analysed Ryugu’s 5.4 grams of charcoal-coloured soil were thrilled to find carbonates, nitrogen and hydrogen (in the form of nearly 24 types of amino acids) in the asteroid’s fine sands. This is a huge find as these types of asteroids could be agents that brought life elements to earth.

Also, Ryugu’s soil has remained unaltered by external agents like heat and moisture over billions of years, making it a pristine sample collected so far.

These three iconic missions have given us a first-hand report of several intricate nuances of asteroids. However, researchers continue to investigate further and scrutinise the data deeply. Down the line, they will bring to light the dynamics of the top-like spinning of Ryugu and Bennu, what makes Bennu eject the particle streams and more details of the twins’ orbits. But, for now, the asteroids are unlikely to pose any hazard to earth.

Meanwhile, all eyes are on the 400-gram bounty from Bennu, which OSIRIS-REx will soon deposit. Researchers can’t wait to get their hands on the precious cargo.