The Hayabusa2 sample return capsule returning to Earth. The bright streak in the sky is the capsule, shock heated as it enters the Earth’s atmosphere. The bright lights on the ground are buildings. (JAXA)

In the early hours of December 6, 2020, what appeared to be a shooting star blazed across the sky above the Woomera desert in South Australia. The source was the sample return capsule from JAXA’s Hayabusa2 mission, which contained precious material from a near-Earth asteroid known as Ryugu.

Within 60 hours, the capsule had been retrieved and flown to the curation facility at JAXA’s Institute of Space and Astronautical Science in Japan. In vacuum conditions to prevent any trace of contamination, the capsule was opened to reveal over 5 grams of asteroid grains.

This material is expected to have undergone little change since the early days of the solar system some 4.5 billion years ago, and its highly anticipated analysis could provide new information about how the Earth acquired water and organics needed to begin life. The sample is the first ever collected from a carbonaceous (C-type) asteroid, which resemble primitive meteorites found to have a chemical composition close to that of the Sun.

Tet despite a rigorously planned and executed journey of over 5,000 million kilometers to bring back a pristine sample from space, concerns have remained. Chief among these are whether the rocky grains in the sample capsule were typical of the asteroid.

If the Hayabusa2 spacecraft had inadvertently gathered grains from an unusual spot, or if the grains had been altered during the collection and return to Earth, then deductions about the asteroid’s composition–and therefore our solar system’s past–could be wrong.  

The sample from asteroid Ryugu (from Yada et al. Nature Astronomy 2021)

The Hayabusa2 team had already gone to rather extreme lengths to mitigate this issue.

In addition to the rapid retrieval operation that ensured that the sample was not contaminated by our planet’s atmosphere, the spacecraft had performed the dangerous landing twice on the surface of asteroid Ryugu to collect samples from two separate sites.

One of these locations was close to where the spacecraft had made an artificial crater, ejecting material from beneath the asteroid’s surface to be gathered during the second collection operation. Rocky grains from below the top layer surface are expected to be particularly pristine, as they have been protected from the bombardment of sunlight, cosmic rays and micrometeorites.

And yet… could we be sure that the sample is definitely typical of the asteroid?

A paper published in the journal Science last week addressed this potential problem.

Led by the team manager for the initial sample analysis at JAXA, Professor Shogo Tachibana, the paper compared the grains in the returned sample with detailed images captured of the asteroid surface in multiple different locations.

If the sample grains appeared similar to the grains spread over the surface of Ryugu, then it would be a big confidence boost that the spacecraft had brought home a typical piece of asteroid. And as the paper concludes, the results are quite promising.

The images of the asteroid surface came from a number of different sources. One of the most valuable were photographs captured by the small monitor camera, CAM-H, that looked directly down alongside the sampler horn on the spacecraft where material was drawn from the asteroid surface into the sample capsule.

The camera had been installed by donations from the public, and had already provided some of the most exciting visuals from the mission. But in addition to the roller coaster view of the drop and ascent from the asteroid surface, the camera had captured a confetti-like storm of grains that were tossed upwards during the sampling operation. The team followed 67 of these particles through the images from the two sampling operations and examined their sizes and shapes.

Footage captured by the CAM-H camera onboard Hayabusa2, during the first sample collection (JAXA)

The appearance of these grains broadly fell into two different morphologies. Grains that had a rugged, irregular appearance and those with smooth faces. This variation was also seen in the boulders scattered across Ryugu in images captured by the spacecraft, and in those pictures taken by the German/French-developed MASCOT lander that dropped to the surface of Ryugu to analyze the rocky world on site. More detailed analysis of the shape of the grains at both sample sites revealed both elongated and flat structures.

Comparing this to the grains inside the sample container revealed a similar collection of shapes and sizes, with rugged and smooth surfaced grains and ones with similar elongation. Moreover the dark color and distribution of sizes—from nearly centimeter-sized pebbles to sub-millimeter-sized sand—fitted with the surface photographs returned by the spacecraft, the MASCOT lander and also the MINERVA-II1 rovers, which had explored yet another area of the asteroid.

(Left) The surface of Ryugu as photographed by one of the two MINERVA-II1 rovers. A flattened grain seems to be peeling off a larger boulder in the white box with the arrow. The rover’s shadow is seen in the center. (Right) A 1-cm-sized grain collected during the second sampling (JAXA).

In total, the observations covered a wide swath of the asteroid surface and detailed examination suggested that the grains now on Earth were representative of the asteroid as a whole, and had not undergone any substantial changes during the collection process and journey to Earth.

The properties of these grains, such as the color, shape and structure, are therefore an excellent guide to the geological history of an asteroid whose path through the solar system may tell us how life began on Earth.

Over the last year, the sample retuned from Ryugu has been catalogued to provide an initial description of the grains returned to Earth. This is free to browse, but will primarily be used by researchers proposing different analyses of the Ryugu sample.

A few early results from this initial description were also published at the end of last year (press releases here and here), hinting at a sample containing hydrous minerals (that have experienced reactions with water) and organics. It is a sample — some of which was shared with NASA — that promises a lot of excitement in the coming years.