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Friday, April 01, 2011

 

Kleopatra, a Bone Shaped Asteroid with Two Moons

216 Kleopatra and its moons as visualised using Celestia.

216 Kleopatra is a rather unusual asteroid. Not only does it look like one of those cartoon dog bones, it has two moons.

Now, if you ask some random person off the street, or a kid, to define what it means to be a planet, they will often say "it has a moon".

However, in 1993 the first asteroid moon was was discovered when the Galileo probe imaged Dactyl orbiting 243 Ida. Astronomers has suspected that asteroids might have had moons long before though, but the images of Ida and Dactyl were particularly amazing.

Since then we have been discovering asteroidal moons hand over fist and have described around 190 of them. We are not sure exactly how asteroids gain their moons, although chunks of asteroids knocked off by impact may explain some examples.

216 Kleopatra is one of the more unusually shaped asteroids (hey, there are ones that look like flying saucers!), and it's moons (Cleoselene and Alexhelios, named after the original Cleopatra's children) were discovered in 2008. While there are lots of asteroids with a known single moon, there are only 6 known asteroidal systems with two moons.

Not only is Kleopatra's shape unusual, but recently published paper by by Pascal Descamps et al, used measurements of the orbits of its moons to determine Kleopatra's mass and density. It turns out Kleopatra is between 30% to 50% empty space! You can read the original paper here, Emily Lakdawalla's take on it here and the Bad Astronomer's take here.

Naturally, I've made a Celestia file for you to add. Kleopatra is already in Celestia 1.6 so just copy the data between the lines into a file called kleomoons.ssc (or whatever you like), and copy that file into the Celestia extras folder.

========8< cut====kleomoons.ssc========8<====================
"Alexhelios:216 Kleopatra I:Kleopatra I:A905 OA 1" "Sol/Kleopatra"
{
# Data for Alexhelios taken from:
# Descamps et al.: Triplicity and physical Characteristics of Asteroid (216) Kleopatra
# (Icarus 2011: 211; 1022-1033)

InfoURL "http://arxiv.org/ftp/arxiv/papers/1011/1011.5263.pdf"

Class "asteroid"
Texture "asteroid.jpg"
# Use the same color as for Kleopatra--uncertain if this is correct
Color [ 1.000 0.901 0.802 ]
BlendTexture true
Mesh "roughsphere.cms"
Radius 4.45

EllipticalOrbit
{
Epoch 2454728.5 # 2008.715917981
Period 2.32 #
SemiMajorAxis 678.0 #
Eccentricity 0.13 # guess
LongOfPericenter 310 # guess
AscendingNode 166 #
#
Inclination 51 #
}

UniformRotation
{
MeridianAngle 123 # place prime meridian facing Kelopatra
}

Albedo 0.2
}

"Cleoselene:216 Kleopatra II:Kleopatra II:A905 OA 2" "Sol/Kleopatra"
{
# Data for Cleoselene taken from:
# Descamps et al.: Triplicity and physical Characteristics of Asteroid (216) Kleopatra
# (Icarus 2011: 211; 1022-1033)

InfoURL "http://arxiv.org/ftp/arxiv/papers/1011/1011.5263.pdf"

Class "asteroid"
Texture "asteroid.jpg"
# Use the same color as for Kleopatra--uncertain if this is correct
Color [ 1.000 0.901 0.802 ]
BlendTexture true
Mesh "roughsphere.cms"
Radius 3.45

EllipticalOrbit
{
Epoch 2454728.5 # 2008.715917981
Period 1.24 #
SemiMajorAxis 454.0 #
Eccentricity 0.13 # guess
LongOfPericenter 310 # guess
AscendingNode 160 #
#
Inclination 49 #
}

UniformRotation
{
MeridianAngle 123 # place prime meridian facing Kelopatra
}

Albedo 0.2
}
========8< cut====kleomoons.ssc========8<====================

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