Near-Earth asteroid turns out to be a comet

3552 Don Quixote betrayed himself by a tail in the sun

Recording of the Earth Cruiser 3552 Don Quixote © Kevin Heider / CC-by-sa 3.0
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Some things are not what they seem - that is also true in space. Because one of the largest known near-Earth asteroids has now turned out to be an active comet. The 19-kilometer-long 3552 Don Quixote betrayed himself when he suddenly developed a faint tail at the sunniest point of his orbit. Astromomen have now discovered this in the Space Telescope Spitzer, as they report at the European Planetary Science Congress 2013 in London.

The asteroid 3552 Don Quixote is about 19 kilometers tall and is considered the third largest earth orbit cruiser. Its orbit passes the chunks discovered only in 1983 by the astronomer Paul Wild at regular intervals close to the earth - but so far in relatively long distances. Compared to other near-Earth objects, the asteroid dances out of line: "Don Quixote has always been considered a nerd, " explains Joshua Emery from the University of Tennessee in Knoxville. "His orbit brings him close to the earth, but also reaches far beyond Jupiter."

Only a long dead comet?

Such an eccentric orbit is actually rather untypical for an asteroid. These usually have more circular paths around the sun. Unlike comets: their orbits are usually highly elliptical or even hyperbola. For 30 years astronomers therefore assumed that Don Quixote could be a dead comet: a comet nucleus that has already outgrown all its ice and all its volatile components and therefore only consists of rock.

Dead because he can not train a tail even when approaching the sun. For such a tail arises when the surface of the comet, which consists to a large extent of ice, heats up and it begins to lose water vapor, dust and gases. Around five percent of all near-Earth objects could be astronomers' dead comets.

Recording the coma of Don Quixote by the Space Telescope Spitzer. In the right shot, the coma was removed to reveal the weak tail. © NASA / JPL-Caltech / DLR / NAU

Tail and coma occupy activity

An astronomer team led by Emery and Michael Mommert from Northern Arizona University wanted to know more about it in the case of Don Quixote. So they took another look at the images that NASA's Spitzer Space Telescope made in 2009 from the sky region in which the cosmic chunk was traveling. At that time, Don Quixote was in the sunniest part of his orbit. And at the same time, they discovered something surprising: The supposedly dead comet had a tail and a coma - a gas and dust brine around the solid comet nucleus. display

"Through the Spitzer telescope, we were able to see the coma and tail clearly - which is not possible with optical telescopes from Earth, " says Emery. "We now assume that Don Quixote not only consists of rock, but still contains a lot of ice, including in addition to water ice, carbon dioxide or carbon monoxide."

Typical features of a comet: coma, tail and an ice core. Kelvinsong / CC-by-sa 3.0

Comets as waterbringer?

And Emery and his colleagues discovered something else: in comparative shots from 2004, when Don Quixote was in orbit near the sunniest point, coma and tail were missing - a clear sign that these two phenomena indeed, such as in comets typical, first by the solar heat and the charged particles of the solar wind had been triggered.

This discovery not only means that the status of Don Quixote must be changed from the asteroid to the comet. It may also be an indication that there are many more undiscovered comets among the near-Earth asteroids than previously thought. According to the astronomers, this also proves that in the past the earth could have been hit by comets - which in the past may even have brought some of the water to earth. After all, Don Quixote alone estimates their value after about 100 billion tons of water.

(University of Tennessee at Knoxville, 10.09.2013 - NPO)