The Strange Twisted Tale of Asteroid Hermes

In 1980 the idea of an asteroid hitting the Earth was somewhat of a joke. This was a time before Jupiter was accosted by bits of comet SL9, and before movies like Armageddon. Earth was safe from space. In mid 1980 asteroid Hermes passed within 300,000 miles from Earth's orbit, barely more than the distance to the Moon. No one saw it. It wasn't the first time Hermes had come close to Earth's orbit and it wasn't the last. Hermes is visiting us now.

The story of Hermes the asteroid is a strange one. It is a large, easy to see asteroid and visits Earth's neighbourhood frequently. But astronomers just kept missing Hermes. It was discovered originally in 1937, but lost and not re-discovered until October 2003!

The time was October 28, 1937, in Germany just before the start of World War II. Astronomer Karl Reinmuth noticed an odd streak in a photograph he had just taken of the night sky. He figured it must be an asteroid, but it must also be close to Earth because of the speed it was moving. It was because of the asteroid's speed that Karl gave it the name Hermes, the herald of Olympian gods. At that crossing, Hermes was about twice the distance of the Moon.

In 1937 astronomers knew about many asteroids and thought them all to be slow moving bodies beyond the orbit of Mars. But Hermes was different, it came very close to Earth, and it came very fast. Perhaps if WWII had not happened, more attention might have been given Hermes. It certainly illustrated a rather grave concept about perils to Earth but just then, Earth had it's own perils going on and no one gave a distant asteroid much thought. Reinmuth observed Hermes for five days but then lost it. No one gave Hermes further thought.

Sixty-six years after Reinmuth discovered and then lost Hermes, an astronomer at Lowell Observatory found the asteroid again. On October 15, 2003, Brian Skiff re-discovered Hermes and astronomer have been carefully tracking it ever since.

With orbital data, astronomer can calculate Hermes orbit, and see where it's at, where its been and where its going. As it turns out, Hermes approaches Earth's orbit twice every 777 days. Most of those times Earth is far away, but in 1937, 1942, 1954, 1974 and 1986, Hermes came perilously close to Earth and we never knew it.

We now know that Hermes belongs to a class of objects we call near-Earth asteroids. These bodies' orbits can change over time because they come so close to us that their orbits are affected by Earth's gravity. Hermes has the most chaotic orbit of all. Hermes also passes close by Venus which also affects its orbit. Because the orbits change, it makes it difficult to predict the asteroid's orbit for more than about a century. The good news is that we're safe for the next hundred years!

Chaotic orbits aside - how was it that Hermes discoverer Reinmuth lost the asteroid in 1937? We think we know the answer to that question also. When Reinmuth first spotted Hermes, it was moving towards Earth with the sunlit side facing us making it easy to see. But asteroids are almost as dark as charcoal and when Hermes swung around and its night side faced us, it was easy to lose. In just six days Hermes had faded by a factor or 60,000!

In 1993, when pieces of comet SL9 pummeled Jupiter, it brought to everyone's attention that catastrophes can happen to planets. The largest fragments, about the same size as asteroid Hermes, exploded with such force when they struck Jupiter that dark clouds as large as Earth formed in the atmosphere..

Hermes passed by Earth on November 4, but it was not a close one, about 18 times farther than the Moon. Wanting to learn more about the asteroid, NASA pinged Hermes with radar pulses and was surprised to find that it is a double asteroid, two rocks orbiting each other. Each asteroid is about 400 meters across.

Hermes has been lost to astronomers for many years, but now that it is found again, they hope to learn more about it. They will certainly be monitoring Hermes' orbit and watching for close approaches to Earth!

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