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Asteroid 121 Hermione's Unusual Companion
Asteroid 121 Hermione's Unusual Companion
7 Oct 2002
(Source: The Planetary Society)

by Melanie Melton Knocke
Planetary Society
October 3, 2002

121 Hermione has a companion, and it's not Harry Potter!

Instead of a fictional character in a book, this Hermione is an asteroid orbiting around our Sun. And, instead of having two steady companions (like the Hermione of Harry Potter fame), 121 Hermione has only one. However, it is this one small companion asteroid that makes 121 Hermione unusual.

Most of the thousands of asteroids that orbit around the Sun are single rocks traveling alone in space. They are also fairly small in size and lie millions of miles from Earth, making detailed observations difficult at best. However, as astronomers develop more sophisticated observing methods, they are discovering more and more about these space rocks, including the fact that not all asteroids are alone.

To date, astronomers have found almost two dozen asteroids that have companions. 121 Hermione is the latest to join this list.

In the case of 121 Hermione, there is one main asteroid that is about 217 km or 134 miles in size. Astronomers have known about this one since James Watson first discovered it in 1872 at the Detroit Observatory. The newly discovered companion is much smaller (only 13 km or 8 miles in size). It orbits its larger neighbor at a distance of 630 km (390 miles).

The discovery of this small companion is impressive, especially when you consider that 121 Hermione lies over 516,000,000 km (320 million miles) from the Sun or about half way between the orbits of Mars and Jupiter. William Merline with the Southwest Research Institute discovered the companion using the 10-meter Keck II telescope on Mauna Kea, Hawaii.

Perhaps they will name 121 Hermione's companion Harry.

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Last Updated: 7 Oct 2002