The bone in question is one “216 Kleopatra (A880 GB)” named colloquially as the “Dog Bone” because of its bone-like appearance. It is an asteroid (otherwise known as a minor planet) lying over 200 million kilometres from Earth in the Asteroid Belt, and measures only 270 kilometres in diameter .
However, how did such an asteroid come to be? It all comes down to the formation of our Solar System some 4.6 billion years ago (for comparison, the Universe is only 3 times older than that!). This asteroid, along with others (1,113,527 in all currently) ranging from just 10 meters to 530 kilometres across (Vesta), started their lives in the proto-planetary disk of gas and dust orbiting the young Sun. The planets coalesced themselves from this soup, along with many smaller objects such as dwarf planets (like Pluto — sorry!), during the first 5 million years of the Solar System .
They formed into larger and larger clumps due to the effects of gravity, whilst other material wasn’t so lucky, forming into small clumps and scattering into the Asteroid Belt and Kuiper Belt (due to the positioning and orbits of the planets) .
Kleopatra seen at different angles on different dates
Credit: ESO/Vernazza, Marchis et al./MISTRAL algorithm (ONERA/CNRS), CC BY 4.0
To differentiate them from comets (bodies with two bright tails), asteroids are defined as rocky bodies, as opposed to the comet’s icy composition. There are three classes: C-types, S-types, and M-types.
C-types (or Carbon-types), also known as chondrites, are the darkest colour, most common and oldest types of asteroid. Named because of their high carbon content (making them look charcoal-like), they’re made of clay and silicate rocks. Their age is testament to their distance from the Sun (in the outer reaches of the asteroid belt mainly), leading to them only heating up to below 50ºC .
You may have heard about the famous Winchcombe meteorite being called a carboniferous chondrite (or CM2-type). It’s similar to the pure chondrites I’m talking about, though these are also similar in composition to the Sun (without the volatiles like hydrogen and helium), providing an unprecedented view into the Solar System’s history (considered to be the best preserved bodies from the very beginnings of the Solar System) .
The display of the Winchcombe Meteorite in the Natural History Museum
Credit: Amanda Slater, CC BY-SA 2.0
Then there are the the S-types (or Siliceous-types), which are the 2nd most common in the Solar System. They make up some of the largest known asteroids (some big enough to be seen through 10x50 binoculars) and are found a bit further in than C-types, in the inner asteroid belt. They’re made of mostly nickel-iron and magnesium silicate materials, leading to a brighter appearance than their C-type counterparts .
253 Mathilde. Credit: NASA
And finally there are the the M-types (or Metallic-types), which are some of the least studied asteroids, with only part of their composition known to us, though what we do know is many are made of nickel-iron sometimes mixed with stone. They’re found in the middle of the asteroid belt and can get up to 200km in diameter. This is in fact the type our Dog Bone is, metallic in composition (though predicted to be 50% empty space to make its density the low 3.6 grams per cubic metre it is) .