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Thursday, 16 January 2014

The six-tailed comet, and other mysteries

Comets are one of the spectacles of the solar system and some only pass by in view of the Earth every few thousand years (Comet Hale-Bopp is only in view of Earth every 2,500 years). At the end of 2013, astronomers observed the marvels of both Comet ISON and a “pseudo” comet with six tails! On Monday, ESA's Rosetta mission will wake from hibernation to continue its mission to orbit and land on a comet. This week, TWDK's physics editor Cait has interviewed Nick Howes, the Pro-Am Programme Manager for the Faulkes Telescopes in Hawaii and Australia. Nick is also an active amateur astronomer, with a particular focus on comets and other solar system bodies.

The main tail of a comet that you see in the sky is caused by the ice of the comet subliming (turning straight from solid to gas) as the comet approaches the Sun. This sublimation of ice also lifts dust of the surface of the comet (this nucleus is generally only a few kilometres in size) which then streams away to form a dust tail that is millions to hundreds of millions of kilometres in length. Looking closer, astronomers also observe another ‘ion’ tail - a tail of ionised gas. It is thinner and has a slightly different colour. The dust tail often appears curved as it is a trail of dust left behind in the comet’s path whereas the ion tail is straight as the charged ions are pushed outwards in the solar wind. Both tails always face away from the Sun, so the tails can appear to proceed the comet when it is travelling out of the solar system.

C/2007 N3 (Lulin) imaged on January 31st (top) and February 4th of 2009.
With Comet Lulin here in 2009, we sometimes also see an “Anti-tail” which looks like it's facing towards the Sun, but this is an optical illusion caused by line-of-sight effects with the comet.  Then in some rare comets, we may also "see" a third main tail, which is a sodium tail. "See" being specialist filters on large telescopes. This was notable in Comet Hale-Bopp in the late 1990s. Image credit: Wikimedia commons
The majority of asteroids in our solar system orbit the Sun in a belt between the orbits of Mars and Jupiter known as the “main belt” and were once thought to be the leftover ingredients for a planet that failed to form, but have a combined mass much smaller than would be necessary for this to be the case. Exactly what asteroids are made of and how many are out there, are questions that scientists are still working on - but they’re mainly rocky or metallic bodies or piles of rubble, sometimes with an icy coating. Comets, on the other hand, are generally thought of as ‘dirty snowballs’, and are more ice than rock. Regular comets come from the outer solar system, in the Kuiper Belt/Trans Neptunian zone or from the Oort cloud, and in the latter case in long, looping orbits that take extremely long periods of time - hundreds or thousands of years, or even longer.

The six tailed “comet” observed last year is a completely new type of comet. Because it had a tail, it was originally classified by the International Astronomical Union (IAU) as a “main belt regular comet” (designation: P/2013 P5) but with many of the orbital characteristics of a typical main belt asteroid. There are only five other objects which are currently classified as both comets and asteroids. However, more recent studies[1] have suggested that P/2013 P5 might be a Flora class asteroid, a type which isn't believed to contain any ice, but this is still uncertain.

Hubble Space telescope image of the six-tailed comet P/2013 P5
The active asteroid P/2013 P5 was discovered as an unusually fuzzy-looking object with the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) survey telescope in Hawaii.
Image credit: NASA, ESA, and D. Jewitt (UCLA)
“We looked at this curious object with the Faulkes telescope, comparing our observations with those made with the Hubble and other instruments, and everybody involved was amazed by what we were seeing. When it was discovered it showed a distinctive tail, which is why it was initially classified as a comet (or Main belt comet/dynamic asteroid to be more precise). Since then more precise observations have given us a lot more data. We now know that it’s only around 250 metres in size, or maybe a little less, and it appears that the multiple tails detected all appear to be dust.”
So if we expect one dust tail on a comet, how do we get six?
“The multiple tails all seem, unlike the dust tail of a typical comet, to be associated with different ejecta dates, kind of indicating that the tail ejections were events caused not by the typical cometary methods of solar radiation and sublimation by ices, but more by rotational effects and disruption on the regolith or surface of the body.’ This is based on research by David Jewitt's team which has shown this as the most likely method for the multiple tail formation.”
Jewitt and his team have theorised that instead of the sublimation method that we expect, the tails are a result of the body spinning so fast that it throws part of itself off into space[2], where the dust is then stretched by the radiation pressure from the Sun. This radiation pressure may also have been what made the comet spin so fast that its gravity is insufficient to hold it together.
“Think of it like a garden sprinkler, where you see jets coming out from various points. Imagine that but where the rotation of the object is causing the dust to be thrown off, sending it out into space.”
Models give possible dates for these ejections, but we still don't know exactly what happened or when. Astronomers will know if this method is correct once another ejection is observed to see if the dust is ejected in the equatorial plane - if so, then this is a likely reason for the multiple tails. Jewitt's team have been using data from the Hubble Space Telescope to build their models but this data collection is still ongoing so we are yet to see the full picture.
“Our plan is to image this more in the coming weeks with various telescopes located all over the world, and are happy to share our data with any research scientists who would like access to it.”
Elliptical orbit from Oort Cloud or kuiper belt superimposed on planetary orbits around the sun
Comet ISON followed a highly elliptical orbit around the Sun from a region of the solar system well beyond the orbits of the planets called the Oort Cloud. Image Credit: Prof James Schombert (University of Oregon)

There are two main sources of comets - the Kuiper Belt and the Oort Cloud, both beyond the orbits of all the planets in our solar system. Comet ISON is an example of one that originated from the Oort Cloud, a sphere of icy bodies located about 1 light year from the Sun. It has been travelling in towards the Sun from the outer solar system for about a million years! Comet ISON has been popular in the news, with scientists and the public eagerly awaiting news as to its fate. Its trajectory took it very close to the Sun on the evening of the 28th November 2013. Unfortunately, most of the evidence suggests that the comet broke apart as it approached the Sun so it is unlikely that we will see it in the skies in the future. We have been attempting along with the Hubble and others to try to pick up any possible remnants, but so far nobody has had much luck.

Comet ISON by planetary photographer Damian Peach, 2009
This photograph of Comet ISON was taken with a 20cm telescope on 15 November 2009.
Image Copyright Damian Peach, used with kind permission.

This six tailed comet has been in the news and is a wonder for astronomers but it turns out that it is not rare for astronomers to find other ‘odd’ comets and asteroids:
“133P/(7968) Elst-Pizarro is one example, which is believed to be a true main belt comet with ice driven outflow, caused by small impacts exposing underlying ices to solar radiation at certain points in its orbit. There are a growing number of these objects and it's an exciting time for anyone to be studying them.”
Outbursts that are similar to those occurring from the six tailed comet are also happening to C/2012 X1, and Elst-Pizarro is another blip in our understanding of these objects. In the late nineties, scientists observed a trail of dust emanating from Elst-Pizarro and as with the six tailed comet, this trail could not be as a result of impacts. Elst-Pizarro is a main belt object and so the presence of volatiles on its surface is a mystery.
“The images drawn of the great comet of 1744 show a remarkable similarity to modern CCD images of comet McNaught C/2006 P1 which developed an astonishing tail, showed wonderful synchronic formations (and syndynes) - a term which came about in the mid 1800s to describe the parallel bands which are sometimes seen in comet tails.”
This painting of the Great Comet of 1744, or "Comet de Cheseaux-Klinkenberg", was painted by Amedee Guillemin in 1877
The Great Comet of 1744 (C/1743 X1)was also a comet that appeared to have six tails that filled the skies.  The six tailed effect in this case is believed to be a result of magnetic fields interacting with the charged particles in the tails. Image credit: public domain.

So there have been a few of these comets in the past but what does that mean for the future?
“The more we study outbursts, the more we can work out what’s going on and possibly predict them with more accuracy in future. One area of research I am looking into is whether or not we can spot any tell-tale signs an outburst or disruption event is about to occur.”
Artist’s impression of the nucleus of comet 67P/Churyumov–Gerasimenko, portrayed far from the Sun with little to no activity.
Artist's impression of Rosetta's final destination.
Image credit: ESA–C. Carreau
The European Space Agency’s Rosetta spacecraft is currently dormant but is programmed to wake up on Monday morning. Rosetta is flying towards Comet 67P/Churyumov-Gerasimenko, which it will rendezvous with in May. Rosetta will then go into orbit around the comet, and join its journey towards the Sun. Along with sending a lander, named Philae, to sit on the surface of the comet, the Rosetta spacecraft will be accompany the icy nucleus as it travels in towards and then back away from the Sun, monitoring all the changes the comet goes through, with the mission expected to end in December 2015. You can follow the wakeup of the Rosetta spacecraft on twitter, through the hashtag #wakeupRosetta
“The ESA Rosetta mission, to me, is one of the pinnacles of ESA’s work in the past few years. I’m also very impressed with the team who have worked on it. Whilst NASA tend to get a lot of credit for many missions, ESA do tend to pull off spectacular ones. I'm thinking of Giotto, and the first flyby (close) of a comet in 1986, then Huygens and the farthest landing ever. With Rosetta they are trying something radical, audacious and risky in the extreme. I think from this mission we'll have such a phenomenal amount of data on cometary bodies, how the jets form, their composition in greater detail than ever and our first really high resolution look at the surface of one for a prolonged period with the Philae lander.”
If you're a fan of space images like these, you can find more of Damian's photographs in the wonderful book Planets: Planetary Systems: 1 and on Nick's Flickr photostream.

why don't all references have links?

[1] Licandro, J et al. "Intermittent dust mass loss from activated asteroid P/2013 P5 (PANSTARRS)." arXiv preprint arXiv:1312.5895 (2013).
[2] Jewitt, David et al. "The Extraordinary Multi-Tailed Main-Belt Comet P/2013 P5." The Astrophysical Journal Letters 778.1 (2013): L21.