APOD: P2010 A2: Unusual Asteroid Tail ... (2010 Feb 03)

[Chris Peterson]

It was my notion that the Hubble photo resolved the tail, showing that it was comprised of thousands of individual points of light, and I assumed that each little point of light resulted from sunlight reflecting from an object like a rock or something. You seem to suggest that these objects could actually be quite small, perhaps centimeter-size pebbles or even smaller. Is that right? If so, I guess I was just WAY underestimating Hubble's ability to see tiny things that far away.

The tail is, indeed, resolved. What that means is that we are able to see structural detail, because the size of the entire tail is much greater than the minimum resolution of the optics and camera. But it doesn't mean that the individual particles that make up the tail are resolved (any more than you resolve the molecules in the dyes that make up pixels on your computer screen, even though you clearly resolve each character of my response). In the case of nebulous objects like comets or asteroidal debris trails, we are able to image (or even see visually) structure because there is so much material. The Hubble camera certainly would not be able to detect a single 1cm particle at that distance. But each pixel is actually recording the light reflected off of thousands, millions, or billions of particles. At the distance to the object, each pixel is subtending a 20,000 km area (and an infinite depth, of course, but we really only care about the depth through the tail). That's a very large volume of space. Hope that makes sense.

[Oldfart]

Chris, yes, that makes sense. I was interpreting the pixtelization (is that a word?) plus maybe some high-contrast processing of the image as showing a bunch of big rocks hurtling through space. I feel better now...

Thanks for being so patient!

Don

[bystander]

Well, Oldfarts deserve a little coddling. There are quite a few of us here. But orin is still the Resident Geezer.

[Rubie]

I think it looks like it totally just changed pathes, for some unknown reason. enjoy the inverted view I was diggin it. hah

[bystander]

Mystery "X" Stands For "X-traterrestrial" to Some Believers
Discovery Space News - 2010 Feb 07

When Hubble Space Telescope first photographed the strange comet-like object p/2010 a2 I just knew it would get attention among the X-Files crowd.

The "X" pattern of dust debris, presumably from an asteroid collision, is simply arresting.

Last week's release of the Hubble picture spurred a lot of spirited speculation on Internet discussion boards that the "X" was something other than a natural phenomenon:

[Rubie]

It looks more like a V to me.. I dont see the X.

[BMAONE23]

It reminds me of those things hanging from the trees in "Blair Witch"

[Lasse H]

... At the distance to the object, each pixel is subtending a 20,000 km area ...

I don't understand this (but I hope I will if I get an explanation): In the tail 'each pixel' = '20,000 km' ? That would make the tail of this object billions of kilometers long. Not to mention the small nucleus itself, which is said to be only 140 meters in diameter, and yet seems to be composed of maybe hundreds of pixels.

Lasse

[Chris Peterson]

... At the distance to the object, each pixel is subtending a 20,000 km area ...

I don't understand this (but I hope I will if I get an explanation): In the tail 'each pixel' = '20,000 km' ? That would make the tail of this object billions of kilometers long. Not to mention the small nucleus itself, which is said to be only 140 meters in diameter, and yet seems to be composed of maybe hundreds of pixels.

It looks like I switched meters for kilometers when I did my calculation. The UVIS WFPC3 camera used for this image has a pixel scale of 0.04 arecseconds, and the distance to the object was 144 million kilometers. So each pixel subtends an actual distance of 28 km at the distance to the object. That makes the imaged portion of the tail some 70,000 km long. (Even if the scale were 20,000 km per pixel, however, the tail would only be tens of millions of kilometers long, which is very typical of small comets.)

At 28 km per pixel, no individual object in this image is resolved. The remaining fragment, estimated at 140 meters in diameter, is completely unresolved- much smaller than a single pixel. It only appears to cover tens of pixels because of diffraction in the optics. The size of the nucleus was estimated based on its brightness, and an assumed albedo. That's how the sizes of most asteroids are determined- very few are large enough to be resolved optically.

[Lasse H]

Thank you very much! Very educational.
Lasse

[bison]

A good deal has happened with the P/2010 A2 (LINEAR) story since early February. For starters, the tail and the peculiar X-shaped pattern at its end have held their shape remarkably well, refusing to be scattered by the the force of the explosive collision supposed to have created them, or by the Solar wind. The tail, which was supposed to be driven away from the Sun, like that of a comet, has recently been photographed pointing almost directly AT the Sun. Dr. David Jewitt, who has now photographed the object several times with the Hubble Space Telescope has declined to release any of the images, beyond the first one. No explanation has been offered for why the debris of a collision should not be spherically distributed, how a narrow tail could have formed, why the supposed debris has not smeared out the complex structure it contains, or why the object's brightness has gone up, down, and up again, rendering it recently as bright as it was at the time of its discovery, over two and a half months ago. Ross