Article by Armagh Astronomer Tolis Christou
Imagine being outdoors on a sunny day, following the Sun as it rises from the east, heading towards the south and then on towards the west where it sets. Now think back: when did you feel the hottest: when the Sun was highest in the sky at midday or later on in the afternoon? The answer to this question is why some asteroids come apart in space – heat!
During the past ten years or so, astronomers have come across a handful of odd cases of asteroids featuring comet-like tails. That’s perfectly normal for a comet, except that there is no evidence of the presence of water or other so-called volatile compounds that drive cometary activity as we know it. Instead, other mechanisms seem to be at work: one is collisions, where a small asteroid smacks on a larger one and releases dust and other debris that could be observed at the telescope.
The other is a subtle phenomenon called YORP, activated by sunlight that the asteroid receives from the sun and then re-radiates out into space as heat. This causes an imperceptible but relentless change in the asteroid’s spin and over millions of years, the YORP effect drives an asteroid to spin faster and faster until material at the equator begin to lift off and be lost to space.
This seems to be what happened to asteroid Gault. Though the same tail could have been caused by a collision, there is no evidence of the larger chunks that usually result when two rocks smack into each other: here there seems to be only Gault and dust. In fact, astrononers have found that the asteroid spins around its axis every 2 hours, exactly what one would expect for an object at the threshold of spewing bits of itself into space.
The spectacle that the asteroid presents to Hubble’s scrutiny is not likely to last. Soon, the dust will disperse and the asteroid will assume again its star-like appearance, making it instinguishable from hundreds of thousands of its asteroid siblings between the orbits of Mars and Jupiter. The fact that we see objects like Gault at all is because telescopes can now survey wide swaths of the sky frequently enough to catch these rare episodes in an asteroid’s life. Soon, new astronomical facilities such as the Large Synoptic Survey Telescope (LSST) will increase the detection rate of asteroids with tails many-fold. Then, when astronomers have gotten a good statistical handle on this remarkable phenomenon, the science can begin!
For further information about this remarkable discovery please take a look at the ESA/NASA press release at https://www.spacetelescope.org/news/heic1906/.
Finally, if you were wondering what YORP is, it stands for the Yarkovsky–O’Keefe–Radzievskii–Paddack effect and is described further in this wikipedia article.