The Bone
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 [1][2][3][20].
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 [3][4].
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) [4][5].
Kleopatra seen at different angles on different dates
Credit: ESO/Vernazza, Marchis et al./MISTRAL algorithm (ONERA/CNRS), CC BY 4.0
Asteroid Composition
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 [6].
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) [7].
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 [8].
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) [9][10].
The Widmanstätten pattern seen within many M-type meteorites.
Credit: Daniel Baise, CC BY-SA 3.0
You may wonder how they can classify asteroids into such ambiguous categories, but there is in fact an easy way, through spectroscopy. More specifically, it’s through looking at the spectra of light reflected off the asteroid, as well as light produced by the asteroid (heat being given off as low-energy light). The shape of a spectrum (usually on a graph of the strength of light, or the amount reflected, against the wavelength or energy of that light) then dictates what elements are present in the asteroid, and therefore what category it fits into [11].
Graph showing the spectrum of an asteroid which orbits a different star: white dwarf GD 40. This shows that this particular asteroid is high in silicates.
Credit: NASA/JPL-Caltech/UCLA
The Dog
To find our bone scientists employed something which is used a lot on anything from ships to military bases to some cars: radar. Standing for “RAdio Detection And Ranging”, a radar instrument sends a pulse of microwaves (low energy light, like what you find in your microwave, but much stronger - though they won’t cook anything in their way!) to the object in question, before an instrument measures the signal which is reflected (otherwise known as an echo).
The Arecibo Observatory (was used partly to observe planets using radar technology).
Credit: JidoBG, CC BY-SA 4.0
Because scientists know the properties of the signal which was sent, they can deduce what the properties of the object are by using the idea of doppler shift (when the wavelength of the light is either stretched or squashed, making the light redder or bluer) by comparing the received signal to the transmitted signal. Unlike other astronomical observing methods, radar actively creates a signal, meaning measurements can be more precise.
In fact, it’s so useful it has been used to accurately map Mercury, the Moon, Mars and Venus, as well as many asteroids including our Dog Bone. In fact, on 14th August 2021, the 1,000th asteroid (2021 PJ1) was observed using radar, just over 50 years after it was first employed for this use to observe the first target: 1566 Icarus.
The North Pole of Venus as seen by the Magellan probe which used a radar to penetrate Venus' clouds and observe Venus.
Credit: SSV, MIPL, Magellan Team, NASA
However, it can’t be used for further away objects because of Inverse-Square Law: energy dissipates, spreading to 4 times the area when it’s twice as far from the source, leading to the intensity of light dropping to a quarter of the previous intensity. This means that the echos of signals can’t be observed if looking at far-off targets without pumping lots of energy into the signal [12][13][14][15][16][17][18].
Diagram to show inverse square law.
Credit: NASA/JPL-Caltech
Kleopatra’s Friends
The asteroid observed isn’t your normal target by any stretch, not only because of its ‘bone-like’ appearance, but because of the two moons that orbit it.
Named AlexHelios and CleoSelene (after Cleopatra’s twins: Alexander Helios and Cleopatra Selene II), the moons (or more correctly, the satellites - moons are only classed as bodies in orbit around the planets [19]) are explained by Kleopatra’s formation as a rubble pile held together by gravity: the collision of two asteroids to form one. Any time after the start of the Solar System the asteroid came to be after a collision with another asteroid.
Then, 100 millions years ago, the asteroid was impacted by another asteroid, causing it to spin much faster than it was previously. This caused it to elongate and eject what came to be the most distant of the two moons: AlexHelios. Then, around 10 million years ago, the inner CleoSelene was shed.
These moons aren’t just there for show though: they’ve helped scientists determine Kleopatra’s density by looking at their orbits [20][21].
Image taken by the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument on the VLT, showing the two moons and Kleopatra.
Credit: ESO/Vernazza, Marchis et al./MISTRAL algorithm (ONERA/CNRS), CC BY 4.0
Observing The Dog Bone
Recently, the European Southern Observatory’s (ESO) Very Large Telescope (VLT) observed the Dog Bone, obtaining the best images yet of it, creating a 3D model of its unusual shape. Not only does it show the Dog Bone itself, but also the two moons that orbit it, helping them calculate new and more precise estimates for Kleopatra’s mass and volume. It was 35% lower than previously thought, showing Kleopatra must be extremely porous and backs up the rubble theory. It was also found to be 270km in length, similar to that of the English Channel!
Also, they found its rotation was almost at critical speed: the speed at which the asteroid would fall apart. This then backs up the other theory that the asteroid shed the two moons it has today.
SPHERE Optical Bench (an instrument on the VLT to improve the detail and accuracy of images taken by the VLT, such as the one of Kleopatra).
Credit: ESO, CC BY 4.0
However, with the new telescope being built by ESO (the Extremely Large Telescope, or ELT) it’s hoped that even better measurements can be taken of 216 Kleopatra, along with the possible discovery of smaller satellites that orbit this strangest of asteroids [20].
by George Abraham, ADAS member.
Click here for the previous news article
Click here for the next news article
Click here to see the list of asteroids observed by using radar
Click here to look at where a visible C-type asteroid is in the sky (10 Hygiea, from an apparent magnitude of around 9 to 11)
Click here to look at where a visible S-type asteroid is in the sky (3 Juno, from an apparent magnitude of around 7 to 11)
Click here to look at where a visible M-type asteroid is in the sky (16 Psyche, from an apparent magnitude of around 9 to 12)
Click here to look at the news article about the Winchcombe meteorite by the Natural History Museum, who now has the meteorite on display.
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