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Moonday: A Bite-Sized Moon

The Asteroid Belt (white dots) covers a large region of space between Mars and Jupiter. Even though it has a lot of asteroids, most are tiny, so it's possible to travel through the belt without ever seeing an asteroid.

Can a tiny object have a moon?

In 1993, the Galileo space probe was passing through the Asteroid Belt on its way to Jupiter. Despite what you may have learned from The Empire Strikes Back, the Asteroid Belt is relatively empty (and likely contains no mynocks): the many known asteroids fill a huge region between the orbits of Mars and Jupiter, so you could travel through without ever seeing one. However, Galileo was the first probe to perform an asteroid flyby, passing by Gaspra and Ida.

The asteroid Ida and its moon Dactyl, as photographed by the Galileo space probe. Ida is only 54 kilometers (about 34 miles) at its longest, while Dactyl is only 1.6 kilometers (1 mile) across. Sightings of the Little Prince standing on its surface are currently unconfirmed.

Neither of these are very big compared to Earth, but while analyzing images from the Galileo probe, astronomer Ann Harch discovered that Ida has a moon: a tiny asteroid subsequently named Dactyl. (The name comes from ten mythological finger-like creatures—dactyloi means “fingers” in Greek—who lived on Mount Ida in Crete.) This was the first asteroid discovered to have a satellite, and the only one with a clear photo as of 2012.

Ida is potato-shaped, as are many other asteroids. Along its longest axis, Ida is 54 kilometers (34 miles), and its mass is 7×10-9 times Earth’s mass (or 0.0000007%)—very small! Yet gravity isn’t just a feature of massive planets like Earth or Jupiter: you could have your own satellite, if you parked yourself in deep space. However, Ida’s gravity is correspondingly weak: a little jump could launch you off. That means it’s unlikely that Dactyl was an independent asteroid captured by Ida’s gravitational pull: the moonlet would have to pass by Ida very slowly for the attraction to do anything other than deflect it slightly. Instead, Dactyl probably formed either from debris broken off from Ida (as with our own Moon), or they formed together when a larger asteroid fragmented.

Ida and Dactyl compared with Earth. Even showing only a small portion of Earth, Ida is tiny and Dactyl is invisible!

Dactyl itself is remarkably spherical, being about 1.6 kilometers (1 mile) across. You can imagine playing Super Mario Galaxy in real life on the surface, except as with Ida you could jump clear of it. (The moonlet’s mass is unknown; we know Ida’s mass primarily through Dactyl’s orbit.) While astronomers don’t know its exact orbital shape or duration, assuming a circular shape means Dactyl takes about 20 hours to make a complete orbit around Ida.

Despite my strong desire to seek similarities between objects (due no doubt to my bias as a theoretical physicist), I love the variety of our Solar System. Comparing Dactyl to Europa to Titan to our Moon seems to show very little resemblance…yet they are all satellites, and their history is the history of our Solar System. They reveal details not only about themselves, but about our own history. Tiny Ida and Dactyl displays the dance of gravity; how can we not appreciate that dance?

(On a prosaic note, I have added a “donate” button to the blog. If you like what you read on this site, please consider donating a little bit, to help support me. Thank you!)

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2 Responses to “Moonday: A Bite-Sized Moon”



  1. 1 If You Love a Flower Found on a Star « Galileo's Pendulum Trackback on March 8, 2012 at 13:49
  2. 2 Asteroid moon: moonsteroid! | Galileo's Pendulum Trackback on May 31, 2013 at 11:57
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