The near-Earth asteroid Ryugu - a fragile cosmic 'rubble pile'

In the summer of 2018, the asteroid Ryugu, which measures only approximately 850 metres across, was visited by the Japanese Hayabusa2 spacecraft. On board was the 10-kilogram German-French Mobile Asteroid Surface Scout (MASCOT) - a lander no bigger than a microwave oven and equipped with four instruments. On 3 October 2018 MASCOT, operated by the control centre at the German Aerospace Center (Deutsches Zentrum fur Luft- und Raumfahrt; DLR) in Cologne, separated from its mother craft 41 metres above the asteroid.

It touched down on the surface for the first time six minutes after deployment, before coming to a halt 11 minutes later, like a dice on a board game moving in slow motion. Over the course of 17 hours, MASCOT carried out experiments in various places amid the large boulders.

Evaluation of the image data from DLR's MASCOT camera (MASCam) showing the descent and Ryugu's surface has now revealed a detailed view of a fragile 'rubble pile' made up of two different, almost black, types of rock with little internal cohesion. The scientific team, led by planetary researcher Ralf Jaumann from the DLR Institute of Planetary Research in Berlin-Adlershof, have now reported on this in the current issue of Science.

"If Ryugu or another similar asteroid were ever to come dangerously close to Earth and an attempt had to be made to divert it, this would need to be done with great care. In the event that it was impacted with great force, the entire asteroid, weighing approximately half-a-billion tonnes, would break up into numerous fragments. Then, many individual parts weighing several tonnes would impact Earth," says Jaumann, who is supervising the MASCam experiment, interpreting the observations.

The asteroid is very similar to carbonaceous meteorites found on Earth, which date back 4.5 billion years. With an average density of just 1.2 grams per cubic centimetre, Ryugu is only a little 'heavier' than water ice. But as the asteroid is made up of numerous pieces of rock of different sizes, this means that much of its volume must be traversed by cavities, which probably makes this diamond-shaped body extremely fragile. This is also indicated by the measurements conducted by the DLR MASCOT Radiometer (MARA) experiment, which were published recently.

Scientists writing space history
"The evaluation of the MASCOT experiments is yielding highly interesting results. To me, it is fascinating to see what this small, high-tech box has achieved on Ryugu, an asteroid 300 million kilometres from Earth," enthuses Hansjorg Dittus, DLR Executive Board Member for Space Research and Technology. "With MASCOT, we have written a small chapter of space history with our Japanese and French colleagues."

MASCOT 'hopped' across the surface using an internal swing arm. "Upon landing and arrival at the initial location, MASCOT had to perform a manoeuvre to correctly align the scientific instruments with the asteroid surface," explains MASCOT Project Manager Tra-Mi Ho of the DLR Institute of Space Systems in Bremen. "This was followed by three more changes in position, with additional measurements."

Two different types of rock, but no dust
The boulders that can be seen in the images acquired by the camera during MASCOT's descent and on the surface are mostly dark and measure between 10 centimetres and one metre across. Although most of them are angular, some are smooth. Boulders with level, fractured surfaces and sharp edges are slightly lighter in colour than those with more irregular, cauliflower-like and partially crumbly surfaces.

On average, Ryugu reflects only 4.5 percent of the incident sunlight, comparable with charcoal, making it among the darkest objects in the Solar System. MASCam was able to acquire images throughout the day and even at night. The camera system was equipped with light-emitting diodes for this purpose, which illuminated the immediate surroundings in different, clearly defined colour wavelengths in visible light and near-infrared, in order to record the reflective behaviour of their environment in different spectral channels.

The two types of rock observed are distributed approximately equally over Ryugu's surface. This suggests two possible origins: "Firstly," explains Jaumann, "Ryugu could have been formed following the collision of two bodies made of different materials.

As a result, it would have broken up, before the fragments came together under the influence of gravity to form a new body made up of the two different types of rock. Alternatively, Ryugu could be the remnant of a single body whose inner zones had different temperature and pressure conditions, thus resulting in the formation of two types of rock."

Ralf Jaumann and his team were particularly surprised by the lack of dust: "Ryugu's entire surface is littered with boulders, but we have not discovered dust anywhere. It should be present, due to the bombardment of the asteroid by micrometeorites over billions of years, and their weathering effect.

However, as the asteroid has very low gravity - only one-sixtieth of that experienced on Earth's surface - the dust has either disappeared into cavities on the asteroid or has escaped into space. This gives an indication of the complex geophysical processes occurring on the surface of this small asteroid."

Boulders reminiscent of materials from the primordial solar nebula
Until now, the MASCOT scientists believed that Ryugu was similar to two meteorites that fell to Earth in 1969 in Allende, Mexico, and Murchison, Australia. However, those meteorites barely contain bright particles, probably due to the weathering effect of water in the crystal grid of these minerals.

The bright inclusions that have now been observed have led the scientists to conclude that Ryugu's cauliflower-like rocks bear greater similarities to meteorites from Tagish Lake. On 18 January 2000, hundreds of small meteorites rained down on Earth following the explosion of a large fireball over Canada, and numerous fragments were found on the ice of the frozen lake.

These are very rare stony meteorites from what is referred to as the CI chondrite class. The C stands for the chemical element carbon, and the I for the similarity with the Ivuna meteorite found in Tanzania. They are among the oldest and most primitive components of the Solar System, remnants of the first solid bodies to be formed in the primordial solar nebula.

Ryugu is a 'Near-Earth Object' (NEO) - that is, an asteroid or comet that comes close to or intersects Earth's orbit. In some cases, these might be on a collision course with Earth. Ryugu's orbit around the Sun is almost coplanar to that of Earth and approaches it at an angle of 5.9 degrees to within a distance of approximately 100,000 kilometres.

Ryugu will never come within the immediate vicinity of Earth, but knowing the properties of bodies like Ryugu is of great importance when it comes to assessing how such Near Earth Objects (NEOs) could be dealt with in the future.

Preparing for the return to Earth
While the MASCOT sub-mission was being completed, Hayabusa2 carried out numerous manoeuvres, mapped the asteroid at high resolution and collected samples from various parts of the surface with a sampler horn.

These were then sealed in a transport container that will embark on its return journey in December 2019. The samples will then descend through the atmosphere and land on Earth in 2020.

Related Links
DLR MASCOT special
Asteroid and Comet Mission News, Science and Technology

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