We could have been mistaken about the nature of the asteroid Ryugu studied by the Hayabusa 2 mission. The data from the probe have been reinterpreted by a team of Japanese researchers who suggest that we are in the presence of a new class of object, that of extinct comets.
Revolutions in astronautics enabled us to observe better
It’s been about two centuries since we discovered the existence of asteroids but millennia for that of comets. However, it was not until the second half of the 20th century that we began to understand their nature and origin.
The revolution in astronautics even enabled us to observe some even at that time more closely, with in particular the spectacular mission of the ESA’s Giotto probe which approached Halley’s comet in 1986 . The ESA , as we know, was going to do much better with the Rosetta probe and comet 67P/Tchourioumov-Guérassimenko.
In recent years, the Hayabusa 2 mission has taken center stage for these small celestial bodies by providing not only close images of the asteroid (162173) Ryugu , but also because this mission has made it possible to collect samples from this potentially hazardous C-type Apollo asteroid discovered in 1999. These samples have arrived on Earth and are still being analyzed .
Three Clues to Ryugu’s Origin
The first data concerning Ryugu were however a bit surprising and had led to some hypotheses being proposed to account for it. The asteroid has the shape of a spinning top that can be explained well if it has adopted a figure of equilibrium in response to a rapid rotation on itself. All that remained was to account for this rapid rotation.
To also understand his form of balance one must know that Ryugu was discovered to be a heap of rubble made up of small pieces of rock and solid materials held together by gravity rather than a single monolithic rock. One way to explain it is to postulate that it is in fact the fragments produced by the collision between two asteroids which would then have slowly come together under the action of their own force of gravity (the size of Ryugu is 920 meters approximately).
Another data provided by the mission, the small celestial body is surprisingly rich in organic matter.
All of these pieces of the puzzle of (162173) Ryugu’s identity and origin have just been put together in a whole new way by a research team led by Hitoshi Miura of Nagoya City University , Japan, in a open access article in The Astrophysical Journal Letters .
Planetologists have built a new model that better accounts for all the sightings of Ryugu and come to the astonishing conclusion that it is a dead comet, i.e. what may be left of such an object in a periodic orbit when its repeated passes close to the Sun have almost exhausted its stock of volatile materials – essentially water ice.
A dirty snowball contracting
The scenario developed with equations by Hitoshi Miura and his colleagues Eizo Nakamura and Tak Kunihiro of Okayama University, Japan, is conceptually simple. If Ryugu was indeed a comet then we can compare it to a water ice with chocolate chips melting on the surface and evaporating several times.
The nuggets will sediment and eventually concentrate and come together at the bottom of the container containing the initial mixture.
Formed beyond Jupiter ‘s orbit in the initial protoplanetary disk about 4.5 billion years ago, a comet was to be just that famous “dirty snowball” model proposed long ago by the astronomer Fred Lawrence Whipple , having accreted blocks of varying sizes with ice encapsulating the whole.
As it evaporates, the comet decreases in size, which changes its moment of inertia as in the case of a skater bringing her arms together. The conservation of angular momentum will lead it to have its speed of rotation accelerate as long as its size decreases.
The researchers’ model then clearly accounts for this shape of top caused by the rotation speed calculated via numerical simulations and we also understand why, since it is initially a comet formed in cold regions and rich in matter. volatile organics, Ryugu seems much more carbonaceous than other asteroids studied from Earth by spectroscopy.
According to the statement from Nagoya University accompanying the published scientific work, objects in the form of spinning tops and piles of rubble with high organic content, such as Ryugu and Bennu (the target of the Osiris-Rex mission ), would be objects comet-asteroid transition (CAT). ”
CATs are small objects that were once active comets but have died out and are apparently indistinguishable from asteroids “, explains Hitoshi Miura who adds: ” due to their similarities with comets and asteroids, CAT could provide new information about our Solar System .”
