APOD: Comet Siding Spring Passes Mars (2014 Oct 20)

[APOD Robot]

Comet Siding Spring Passes Mars

Explanation: Yesterday, a comet passed very close to Mars. In fact, Comet C/2013 A1 (Siding Spring) passed closer to the red planet than any comet has ever passed to Earth in recorded history. To take advantage of this unique opportunity to study the close interaction of a comet and a planet, humanity currently has five active spacecraft orbiting Mars: NASA's MAVEN, MRO, Mars Odyssey, as well as ESA's Mars Express, and India's Mars Orbiter. Most of these spacecraft have now sent back information that they have not been damaged by small pieces of the passing comet. These spacecraft, as well as the two active rovers on the Martian surface -- NASA's Opportunity and Curiosity -- have taken data and images that will be downloaded to Earth for weeks to come and likely studied for years to come. The featured image taken yesterday, however, was not taken from Mars but from Earth and shows Comet Siding Spring on the lower left as it passed Mars, on the upper right.

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[Ann]

Wow, the comet is certainly faint compared with Mars. Well done getting such a good portrait of the comet, in spite of the glare from Mars.

And is that a galaxy to the left of the comet?

Ann

[alter-ego]

Wow, the comet is certainly faint compared with Mars. Well done getting such a good portrait of the comet, in spite of the glare from Mars.

And is that a galaxy to the left of the comet?

Ann

It's definitely an artifact as it does not show up on the Sloan or STScI sky surveys. It could be related Mars.

[Boomer12k]

Awesome image...What damage would it have done to Mars on a good direct hit???

Going to add "Collision Prevention" companies to my portfolio....and Comets on my home and auto insurance....
:---[===] *

[Nitpicker]

That's a lovely image.

I was busy recording the approach to the near-collision last night, too, albeit in a much less deep and aesthetically pleasing way. (And I've finally figured out how to process the result to my satisfaction, after a few horrible attempts.) Here is slightly more that an hour's worth of approach, showing the paths of Mars and Comet C/2013 A1 against the stars of Ophiuchus, around the same time as this APOD and about eight hours before closest approach. Note that the tail, which points away from the Sun in the APOD, does not show in my image, and is not at all aligned with the direction of the comet's motion, which is very close to northward. (Mars is moving eastward to the left.)

Maximum stack of 13x30s subs, ISO 3200, 6" SCT.

[bmesser]

Its a shame that you couldn't have created an HDR image, where Mars had a shorter exposure to make it less bright and exposed, and the comet could have received a longer exposure to make it visible, or would that be impossible to do?

[MargaritaMc]

http://asterisk.apod.com/viewtopic.php? ... 09#p233594

[isoparix]

Has anyone calculated how much this close encounter will alter the comet's trajectory? After all, this is like the 'slingshot' techniques used to accelerate spacecraft...

[Nitpicker]

Its a shame that you couldn't have created an HDR image, where Mars had a shorter exposure to make it less bright and exposed, and the comet could have received a longer exposure to make it visible, or would that be impossible to do?

Mars is several thousand times brighter than the comet and neither are resolved into discs, so both are point sources of light, just like all the stars. If Mars were shown at its true angular size in the APOD, it would be smaller just bigger than a pixel (by my rough calculation, please check for yourself). Personally, I think Mr Peach has made the glare look rather pretty; he may have even exaggerated it. I don't have much experience with HDR techniques, but I'd be surprised if they could make a significant or even desirable impact in an extreme case like this. As it is, I suspect that the image has been contrast-stretched to enhance the detail in the tail and other dusty bits, which are much dimmer than the comet nucleus.

[Chris Peterson]

If Mars were shown at its true angular size in the APOD, it would be smaller just bigger than a pixel (by my rough calculation, please check for yourself).

The angular size of Mars in this image (the full image you get when you click on the home page image) is 2.2 pixels. While that's visually very close to unresolved, the optics of an extended disc like that are very different than for stars.

I don't have much experience with HDR techniques, but I'd be surprised if they could make a significant or even desirable impact in an extreme case like this. As it is, I suspect that the image has been contrast-stretched to enhance the detail in the tail and other dusty bits, which are much dimmer than the comet nucleus.

It would certainly be possible to composite images (in an HDR fashion) so that Mars wasn't overexposed. Not real easy, though. And of questionable value. Sure, we'd (barely) see the size of Mars in comparison with the comet, and the color of Mars would be a little more accurate. But I think the image is better as it is, giving a much better indication of the true difference in intensity between the two objects.

[Psnarf]

Judging by the inbound trajectory, if the orbit is changed at all, it would be slightly closer to the Sun on the way out. Looking at the Mars-Earth-Sun plane, Sliding Spring comes in from above left and will return below left. It might be slightly closer to 1 Mars Astronomic Unit, but the change would be insignificant. Since Earth is farther around the plane, less than 90-degrees Mars-Sun-Earth, I don't think it will be visible from Earth with unaided eyes. We rely on these expert photographers with their telescopes to see it at all. Waiting with anticipation for the images from the four nearby cameras (two in orbit, two landers), I remain Yours Truly, Amazed.

[Chris Peterson]

Has anyone calculated how much this close encounter will alter the comet's trajectory? After all, this is like the 'slingshot' techniques used to accelerate spacecraft...

The orbit was significantly altered by the pass. That's because just about any perturbation of a hyperbolic orbit is significant. While I know that the path has been estimated by numerical modeling, the easiest thing will be to compare the past elements from future ones (these are determined by observation).

In practice, "significant" doesn't mean much with this sort of comet. The deviation of its path through the inner system is very small, and we're not going to see this comet again (or at least, not for millions of years if perturbations close the orbit).

[Nitpicker]

If Mars were shown at its true angular size in the APOD, it would be smaller just bigger than a pixel (by my rough calculation, please check for yourself).

The angular size of Mars in this image (the full image you get when you click on the home page image) is 2.2 pixels. While that's visually very close to unresolved, the optics of an extended disc like that are very different than for stars.

Interesting, thanks Chris. If you record an extended object at its true angular size (obviously more than a pixel), is that sufficient to say it has been resolved? I'd always just assumed before now that you'd need a few more pixels to span across the true size. How do the optics of a just-resolved extended object differ from a point source?

[Chris Peterson]

If you record an extended object at its true angular size (obviously more than a pixel), is that sufficient to say it has been resolved?

That's a surprisingly tricky question, given that there are multiple criteria for "resolved". Most actually depend on the ability to distinguish a close pair of point objects from a single point object. But on a purely theoretical level (from information theory) we might reasonably say an object is resolved if the Nyquist criterion, which would be the case if the image width is greater than two pixels. Of course, this assumes that the seeing and optics are good enough that a point source is smaller than that, which doesn't seem to be the case here- I measure the FWHM of stars at around 2.5 pixels.

How do the optics of a just-resolved extended object differ from a point source?

They have different diffraction structure.

[Nitpicker]

Thanks Chris.

I measure the FWHM of stars at around 2.5 pixels.

I assume that's more complicated than counting the pixels on the smallest recognisable stars? Edit: nevermind, I found the answer is yes.

[Nitpicker]

I measure the FWHM of stars at around 2.5 pixels.

I assume that's more complicated than counting the pixels on the smallest recognisable stars? Edit: nevermind, I found the answer is yes.

Having just noticed that the stacking software I typically use (Deep Sky Stacker) calculates the FWHM for me in pixels, I did a quick comparison between my reduced image and the APOD (which I assume has also been reduced) and was surprised that they both had similar FWHM values just under 7 arcsec. And if I check my full resolution images at 0.6 arcsec/pixel, raw and stacked/stretched, things improve (as I expected, though not linearly) to between 4 to 5 arcsec. But I'm struggling to relate these values with the FWHM values typically quoted for astronomical seeing, where 3 arcsec is rather average and anything above that is worse. I considered the seeing to be better than average on the 19th when I took my images. Might the discrepancy in the numbers be mainly due to tracking errors, or something else I haven't considered? Edit: perhaps the low elevation angles too.

[Chris Peterson]

Having just noticed that the stacking software I typically use (Deep Sky Stacker) calculates the FWHM for me in pixels, I did a quick comparison between my reduced image and the APOD (which I assume has also been reduced) and was surprised that they both had similar FWHM values just under 7 arcsec. And if I check my full resolution images at 0.6 arcsec/pixel, raw and stacked/stretched, things improve (as I expected, though not linearly) to between 4 to 5 arcsec. But I'm struggling to relate these values with the FWHM values typically quoted for astronomical seeing, where 3 arcsec is rather average and anything above that is worse. I considered the seeing to be better than average on the 19th when I took my images. Might the discrepancy in the numbers be mainly due to tracking errors, or something else I haven't considered? Edit: perhaps the low elevation angles too.

And you're using a DSLR, right? So you're going to see a loss of resolution from the color sensor. And depending on the model, there may be internal processing (Nikon cameras, for instance, aren't great for astroimaging). And the raw images from a DSLR typically don't have the same dynamic range as from a good astronomical camera, which can mess up FWHM measurements somewhat. And different FWHM algorithms give different results, especially if the data isn't linear, which is often the case with DSLRs. I'd say there's a good chance that a measured value of 4 arcsec in a DSLR raw equates to what I'd call 3 arcsec seeing.

A good test of the quality of the image is to measure the FWHM of multiple stars. They should all be the same to within a few tenths. If you're seeing more than that, it means there are nonlinearities in the underlying data.

[Nitpicker]

Having just noticed that the stacking software I typically use (Deep Sky Stacker) calculates the FWHM for me in pixels, I did a quick comparison between my reduced image and the APOD (which I assume has also been reduced) and was surprised that they both had similar FWHM values just under 7 arcsec. And if I check my full resolution images at 0.6 arcsec/pixel, raw and stacked/stretched, things improve (as I expected, though not linearly) to between 4 to 5 arcsec. But I'm struggling to relate these values with the FWHM values typically quoted for astronomical seeing, where 3 arcsec is rather average and anything above that is worse. I considered the seeing to be better than average on the 19th when I took my images. Might the discrepancy in the numbers be mainly due to tracking errors, or something else I haven't considered? Edit: perhaps the low elevation angles too.

And you're using a DSLR, right? So you're going to see a loss of resolution from the color sensor. And depending on the model, there may be internal processing (Nikon cameras, for instance, aren't great for astroimaging). And the raw images from a DSLR typically don't have the same dynamic range as from a good astronomical camera, which can mess up FWHM measurements somewhat. And different FWHM algorithms give different results, especially if the data isn't linear, which is often the case with DSLRs. I'd say there's a good chance that a measured value of 4 arcsec in a DSLR raw equates to what I'd call 3 arcsec seeing.

A good test of the quality of the image is to measure the FWHM of multiple stars. They should all be the same to within a few tenths. If you're seeing more than that, it means there are nonlinearities in the underlying data.

Good advice. I'm using a Nikon D5100. I've read about older Nikon DSLRs which had something of a reputation as "star crunchers" (relating to how the Nikon converted the raw image to its own NEF raw format, I think). That reputation seems difficult to shake, but the newer models have been improved with a better conversion to a 14 bit NEF format, which is not a problem with my not-so-deep astrophotos, anyway. Certainly, the software and controls available for astro imaging with Canon cameras leaves Nikon for dead, but other than that I'm very happy with the D5100. (We had the Nikon before I ever developed an interest in astronomy. I've also bought an inexpensive planetary imaging video camera since, which is a pretty good match for my modest 6" scope.)

[moonshiner]

In terms of celestial events, the debut of comet Siding Spring 2013 A1 with Mars was a disappointment. With a diameter of nearly 500 meters and a mass of approximately 50 million tons, the comet could have yielded Mars with precious elements in the event of an impact with the planet. Xenon - nearly 5000 tons - could have helped to stabilize the atmosphere; neon - approximately 100000 tons - could have contributed to the hydrodynamic saturation of the atmosphere; olivine and metal crystal - approximately 10 million tons - could have supplied material for future astro labs; and water - approximately 20 million tons - to quench the thirst of martian rodents.
Perhaps future missions should be aimed at nudging comets and asteroids into a friendly orbital decay around a terrestrial planet such that the body gently sheds mass over an extended period of time as it plows through the planet's upper atmosphere. But clearly, this cannot be the domain of rocketry; a more sophisticated propulsion system is needed to engage in celestial construction projects capable of tractoring comets and asteroids on a relatively large scale. There is reason to believe that such technology has always existed - but the relevant question is: what should be the extent of such celestial engineering and what celestial objects should be involved.
Someone once said that if man had sufficient faith he could move mountains. But, can man move asteroids..or comets.
Man may have to wait some time to gain access to such technology. In the meantime, physicists can figure out what it takes, in terms of force and vector, to move a 1 trillion ton asteroid into a decay orbit about Mars.

[Chris Peterson]

In terms of celestial events, the debut of comet Siding Spring 2013 A1 with Mars was a disappointment. With a diameter of nearly 500 meters and a mass of approximately 50 million tons, the comet could have yielded Mars with precious elements in the event of an impact with the planet.

Comets this size have struck Mars before. It is nowhere near large enough to have any significant impact on the resources of the planet.

[Ann]

Aren't the last two posts somewhat misplaced here?

Ann

[Chris Peterson]

Aren't the last two posts somewhat misplaced here?

Now, the last four.

[Beyond]

[bystander]

and now they're not

[BMAONE23]

Just the headers are misplaced