Space & Innovation

This is Why Some Asteroids Act Like Comets: Photos

What's with those asteroids that are streaming dust, just like a comet? One researcher explains how this could happen.

It seems so simple a definition: comets close to the sun have long streams of material behind them, whereas asteroids are more quiet. But it turns out that's not always the case. Since 1996, we've known that some asteroids possess long dust tails similar to what you'd expect on a comet. The object 133P/Elst-Pizarro is thus a "prototype" of "active asteroids," says Jessica Agarwal, a scientist at the Max Planck Institute for Solar System Research.

Comets spent of the time since their formation in the cold outer solar system and started to enter the inner solar system only recently, meaning that much of their primordial ice was preserved. The typical asteroid, however, stays in a belt between Mars and Jupiter with nearly constant heating from the sun for billions of years. So one question researchers are trying to answer is how asteroids can be active when most of their ice is likely gone.

Agarwal spoke with Discovery News about the processes by which asteroids can behave by comets, and also of her team's recent observations of 288P/300163 with the Hubble Space Telescope.

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Even a casual fan of "Star Wars" may have a distorted view of what an "asteroid belt" looks like. In "The Empire Strikes Back" the Millennium Falcon dodges constantly to avoid space rocks while outrunning the Empire's Star Destroyers. In reality, asteroids between Mars and Jupiter are spaced hundreds of thousands of miles apart. It's rare for them to actually crash into each other, but when they do, that could produce a spurt of dust.

"The result of such a collision depends on the relative sizes, velocities and internal strengths, and range from catastrophic destruction to a simple impact crater," Agarwal wrote in an email. "In any case, the collision will likely create a dust cloud, and it is this cloud that we can observe from Earth and which gives a temporary comet-like appearance to the impacted object."

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One example of this was 596 Scheila, which was smacked by a much smaller asteroid in 2010 and created two temporary dust clouds. The scar on the surface remained visible afterwards, as the light curve -- the change of total brightness in a telescope -- had changed, Agarwal said.

If an asteroid rotates fast enough, sometimes dust and other objects will fly away from its surface. The fastest spinners may even break apart, depending on how they are shaped and how much strength there is to their insides, Agarwal said. The question is, however, how does an asteroid change its rotation rate? One thought is that an asteroid that has an irregular shape or irregular surface reflectivity (or albedo) emits different thermal radiation in different directions, changing its spin -- a phenomenon called the "YORP effect."

"In some cases, this effect can spin up an asteroid to the breaking point," Agarwal wrote. "A prominent example of a catastrophic break-up is P/2013 R3, which was observed to disintegrate into at least 10 fragments in late 2013. Also P/2012 F5 is strongly suspected to have been activated by fast rotation, as it has a near-critical rotation period of about 3h, and ejected fragments and dust in a short-duration event in the summer of 2011."

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We pointed out earlier that asteroids are in a constant orbit battered by the sun's radiation. Over billions of years, could any ice survive? Scientists suggest that there could still be some ice on their insides, which would then be exposed through a process such as an impact or a landslide, Agarwal said. Then the ice gets heated by the sun and carries out dust particles as it melts.

"Such activity will typically continue for several weeks or months, while the solar irradiation is strong enough, then cease, and resume when the asteroid returns to the same position on its orbit with the favorable irradiation conditions," she wrote. "Currently, four active asteroids are known to show such orbitally recurrent activity for a few months when they are closest to the sun. One of them is the prototype 133P/Elst-Pizarro."

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"If a material is heated fast, the resulting thermal expansion may not be uniform. The surface is typically more affected by solar heating than deaper layers of a celestial body, and different materials respond in different ways to heating," Agarwal wrote. "The result is that quick temperature changes can result in cracking of the surface, which can produce dust."

One possible example of this, she added, is asteroid 3200 Phaethon. It can get very close to the sun and heats up to about 700 degrees Celsius (1,292 Fahrenheit) very quickly. It also is a fast rotator, just taking 3.6 hours to spin once -- adding much thermal stress to its surface. Astronomers have also seen dust spurts from the asteroid during its close approaches.

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The huge advantage of the Hubble Space Telescope is it orbits above Earth's atmosphere. This allowed it to detect the dust tail of active asteroid 288P/300163 is very thin -- only 1,000 km thick (621 miles) at about 50,000 km (31,000 miles) away from the comet's heart or nucleus. This thin trail suggests the dust was ejected at low speeds, possibly requiring the asteroid to rotate to make it possible.

The researchers suggest that ice is melting on 288P's surface and carrying dust with it. This is supported by a few pieces of data -- the activity continued over several months and the speed combined with the particle size suggested that the dust was "accelerated by streaming gas", said Agarwal.

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"As an add-on, we found that the appearance of the nucleus changed between our two observation dates, separated by about a week," she said. "One possible explanation could be that 288P has a binary nucleus, but we really need more data to confirm this. Until now, no active binary asteroid has been discovered, but the formation of a binary is a frequent outcome of both collisions and rotational break-up."