When a comet’s not a comet after all

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Back in January the Lincoln Near Earth Asteroid Research (LINEAR) survey saw something a bit odd amongst the asteroids in the main asteroid belt (found between Mars and Jupiter). Initially the mystery object, P/2010 A2, was designated as a main-belt comet (a rare object found within this region of the Solar System, unlike the majority of comets which orbit a lot further away) because of its elongated fuzzy appearance. However, two sets of results published in Nature today suggest that this thing is actually the aftermath of a collision between two asteroids that occurred some time in February or March 2009.

A comet generally has a fan shaped tail, topped with a dust-enshrouded nucleus; when the first team looked at P/2010 A2 with the Hubble Space Telescope though they saw that it has a more rectangular shaped tail, beginning in an X-shape:

The second team also saw a distinctly un-comet like structure (shown left) when they imaged the object with the OSIRIS camera onboard the Rosetta spacecraft. This had a really good view as it was approaching the asteroid belt at the time, in preparation for its flyby of the asteroid Lutetia.

Modelling the structures seen in both images by the two teams independently revealed that the main body of P/2010 A2 is about 120 metres across, that it was formed from a collision with a much smaller body, and that all this occurred about a year before we first saw it. Discovering all this isn’t possible from Earth though, as ground based telescopes, such as the one used by LINEAR, can’t see it from the right angle.

This sort of collision between asteroids of this approximate size are only predicted to occur roughly once every 12 years, so its likely that P/2010 A2 will remain unique for a few years yet.

Images credit NASA & ESA

ResearchBlogging.orgJewitt, D., Weaver, H., Agarwal, J., Mutchler, M., & Drahus, M. (2010). A recent disruption of the main-belt asteroid P/2010 A2 Nature, 467 (7317), 817-819 DOI: 10.1038/nature09456

ResearchBlogging.orgSnodgrass, C., Tubiana, C., Vincent, J., Sierks, H., Hviid, S., Moissl, R., Boehnhardt, H., Barbieri, C., Koschny, D., Lamy, P., Rickman, H., Rodrigo, R., Carry, B., Lowry, S., Laird, R., Weissman, P., Fitzsimmons, A., Marchi, S., A’Hearn, M., Angrilli, F., Barucci, A., Bertaux, J., Cremonese, G., Da Deppo, V., Davidsson, B., Debei, S., De Cecco, M., Fornasier, S., Gutiérrez, P., Ip, W., Keller, H., Knollenberg, J., Kramm, J., Kuehrt, E., Kueppers, M., Lara, L., Lazzarin, M., López-Moreno, J., Marzari, F., Michalik, H., Naletto, G., Sabau, L., Thomas, N., & Wenzel, K. (2010). A collision in 2009 as the origin of the debris trail of asteroid P/2010 A2 Nature, 467 (7317), 814-816 DOI: 10.1038/nature09453

Correcting Hubble images

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I’m going to be upfront here: this post is about CCDs and readout electronics. Wait, come back, it’s going to be interesting I promise*. It involves the Hubble Space Telescope. Everyone likes that, don’t they?

Many astronomical instruments, including the ones on Hubble, use Charge Coupled Devices or CCDs. These detectors are like containers for electrons, and they’re divided up into grids of boxes, sort of like ice-cube trays. When a photon of light falls on the a particular part of the CCD it drops an electron into the corresponding box on the grid. To make an image the number of electrons in each grid section need to be counted as they represent the distribution of the detected light. To do this each row in the tray is emptied in turn, with the electrons shifted box-by-box towards the edge of the grid, as illustrated by this handy animation:

I’m glossing over a lot of the details here, but that’s basically how a CCD works (for more details try this How Stuff Works page on digital cameras, which use the same technology).

The CCDs onboard the Hubble Telescope are continually bombarded with cosmic rays which gradually damage them. In particular pockets can form which trap electrons and prevent them from being read out at the correct time. When they’re eventually released, they’re erroneously counted and included wrongly in the image, leaving distinctive trails behind bright objects.

This is a major problem for people who need to accurately measure the shape of galaxies in their Hubble images. One such person is Richard Massey who studies weak lensing – the small distortion in the shapes of galaxies caused by their light being bent by a massive foreground object, like a galaxy cluster. Obviously, the presence of little trails behind these galaxies is going to mess things up. Good news though, Richard, and his collaborators have come up with a way to fix this issue. They model the CCD system and manage to figure out where all the trapped electrons have come from and therefore can put them back where they belong at the initial stage of the data processing. This means that degraded images can be corrected back to how they would have been if they’d been taken years earlier, before the cosmic radiation had damaged the CCDs. As you can see in the images below, this will hopefully make things easier for the weak lensing people.

The before-correction (left) and after-correction (right) images from Massey et al 2010, demonstrating how successful this technique is at removing the trails seen behind many objects on the left

*interest levels not guaranteed

ResearchBlogging.orgMassey, R., Stoughton, C., Leauthaud, A., Rhodes, J., Koekemoer, A., Ellis, R., & Shaghoulian, E. (2010). Pixel-based correction for Charge Transfer Inefficiency in the Advanced Camera for SurveysMonthly Notices of the Royal Astronomical Society, 401 (1), 371-384 DOI: 10.1111/j.1365-2966.2009.15638.x

ResearchBlogging.orgRichard Massey (2010). Charge Transfer Inefficiency in the Hubble Space Telescope since Servicing Mission 4 MNRAS arXiv: 1009.4335v1

Happy birthday Hubble, from rita

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So Emma got there first, straight to my all time favourite Hubble moment. Then Stuart stole my second favourite! You know.. great minds..

But nevermind, because with Hubble everything comes a close 2nd, or 3rd, or 1st. But here’s an image that has been making its way into my presentations since the very first time I saw it:

It’s disk galaxy NGC 5866, which faces us pretty much edge on. I love the way this image shows a galaxy from a slightly less known perspective. You can see the striking dust lanes going along the disk, and the slightly yellowish bulge of older stars in the middle – just beautiful. And if you can, for a moment, take your eyes off NGC 5866 you can see a number of galaxies in the background and it always amazes me how many you can see. I can have endless fun with the zoomable version of this image – go on, have a go and have a good look around.

And I do mean a good look around. Spare as much time as you can. Because every one of those galaxies has such a complexity to it, such a history and a future that it almost pains me to dismiss it as ‘seen’ at any given moment. There are billions of stars there, with numerous planetary systems and civilisations that almost certainly surpass our imagination; endless physical phenomena that we don’t yet understand or probably even know exist; and – I like to think – endless renditions of the blues. This stuff is real and sometimes, for as beautiful as the Hubble legacy is and for all it has done for us, I can’t help thinking that no image and no finite amount of staring can ever do this Universe any justice.

Image credit: NASA

happy birthday Hubble from stuart

Happy birthday Hubble !  Thank you for many years of stunning views of the Universe.

I have to say Emma pretty much took the best image Hubble has ever taken as her favorite. Its a hard act to beat ! I am however very partial to this series of images taken by Hubble over the years.

As the glorious Hitchhikers Guide to the galaxy tells us “Space… is big. Really big. You just won’t believe how vastly hugely mindbogglingly big it is. I mean you may think it’s a long way down the road to the chemist, but that’s just peanuts to space…”. A lot of things in space are big as well and that normally means that the time scales over which they change are very long. So long in fact that its often very hard to see things changing at all, the universe appears timeless. What I love about these images of the variable star V838 Monocerotis is that over only a few years we can see the image of the star changing.

The star at the center of this image underwent a catastrophic outburst. In a short period of time it became roughly 1 million times brighter than our sun, giving out a huge amount of light in the process. The light which came directly towards us arrived first but then gradually over the years, light heading away from the star in a shell, reflected off of surrounding gas and dust eventually heading towards the earth. As we let time go on the light illuminates different regions around the star.

Despite the fact that the images look like they are showing a concave bubble around the star its actually convex with the material which appears to the the side of the bubble closer to us actually being behind the star.

I just love seeing something in the sky evolve in this way! Thanks again Hubble !

Happy Birthday Hubble!

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The Hubble Telescope as seen from the Atlantis Shuttle after the last, and final servicing mission

Twenty years ago today the Hubble Space Telescope was launched. Putting a telescope in space gets rid of all the distortions that come from our atmosphere, and means the images it takes are much sharper than equivalent instruments on Earth and, after a few initial teething problems, Hubble has demonstrated this magnificently, not just for astronomers but for the general public as well, through the excellent Hubble Heritage Project which turns data into art. Hubble has maintained its world-leading position because, unlike the new generation of space telescopes such as Herschel, it is close enough to get regular servicing missions from the Space Shuttle fleet.

In celebration of its 20th birthday today I thought I’d post one of my favorite Hubble images, the Hubble Ultra Deep Field.

This immensely deep image is of a tiny area of the sky in the Fornax constellation – its only a tenth the diameter of the full moon – and it took Hubble nearly a million seconds (though not all at once) to make it. Part of the observations were taken over Christmas 2003/2004 so maybe the astronomers wanted to leave it doing something useful while they went off to celebrate! Nearly everything you can see in this image is a distant galaxy, there are roughly 10,000 visible. More than anything else it was this image, and the Deep Field that preceeded it that really brought home to me how vast the Universe is, and how much there is for us to learn about it. I find it simply amazing.

If you want to take part in research using data taken with Hubble, the hundreds of thousands of archive images have just been incorporated into Galaxy Zoo for people like you to classify. From everything we’ve seen from the telescope so far, who knows what you might find!

Picture Credits: NASA