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Comet observation from Mars, comets close encounters to Mars in 2013 and 2014
Greenish
post Mar 5 2013, 08:33 PM
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Per http://ssd.jpl.nasa.gov/?horizons_doc#ca the Max/Min values are "formal 3 standard-deviations with linearized covariance mapping" so pretty much inclusive given the HORIZONS error model.

For fun and knowing as above that it's probably entirely wrong, and is certainly not right given the unknowns, here's how the current nominal orbit of C/2013 A1 could look from MSL's position.
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MaG
post Mar 5 2013, 09:42 PM
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As far as I have read, nobody else said something about size of nucleus, only Leonid Elenin. I think, it's very hard to predict it now, but Jakub Cerny (my friend) wrote an article in czech, that the size of nucleus must be much smaller. For example if you remember comet Hale-Bopp, it was much much brighter in the same distance. And we hope, that it's nucleus is 40 +/- 20 km. So comet's Sidding Spring nucleus is maybe much smaller, Jakub said about 3 to 8 km (derived from their nucleus brightness).

I hope there will be no collision in the future, but if so, even 5 km nucleus is very much. There is no doubt..

Thank you for information about possibilities of imaging by orbiters and rovers.


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JRehling
post Mar 5 2013, 09:45 PM
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To SFJCody's interesting thoughts about the unlikelihood of seeing a very large impact in our lifetime, I think the key points are:

1) Near misses, if you define them as having about 250x the cross section of the planet, will happen almost 250x more often than actual events. If Chicxulub events occur every 50 million years at Earth, there will a Chicxulub-sized near miss every 200K years.

2) If you count four bodies (Venus, Earth, the Moon, and Mars) as targets of interest, you divide that roughly by four, so every 50K years.

3) The martian cratering rate, per unit surface area, is about 2x the Moon's. But for near misses of a given distance, surface area is irrelevant, and we get a martian rate of about 8x the Moon's. So for Mars alone, we expect one such event every 25K years.

4) The pass is not known to be this close and we'd have to integrate over the possible trajectories to rate it fairly, but this may be considerably further, and so we get further reduction.

5) A generalization/extension of (3): There's a logical fallacy of taking the most unlikely thing that you observe/forecast and noting how weird it is that it happen. What was the probability that you would receive the phone number you were assigned at random? Near zero for that precise number, but the probability that you would get A number about which you could make that comment is near unity.

The solar system allows for many weird catastrophic events that could happen, and cumulatively the probability of one of them happening is much higher than the probability of any one of them. We could have seen a collision form Saturn's rings, but we didn't. We could have seen volcanic activity repave Venus's whole surface during our lifetimes but we didn't. We could be seeing a nearby red giant go supernova but we're not.

Given that the Mars event is looking like a once-every-few-thousand-years event and there are innumerable comparably striking (no pun intended) events we could make the same comment about, it doesn't seem so strange that we're seeing one of them.
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fredk
post Mar 5 2013, 09:46 PM
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Thanks for pointing out that documentation, Greenish. So 3-sigma means that the odds are only about 0.3% for the actual value to be greater than 300 000 km. This assumes gaussian errors and that the linearized error propagation is accurate.
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tolis
post Mar 5 2013, 09:47 PM
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The view from Earth may offer an interesting twist; assuming that the coma is extensive enough, Mars itself may project its shadow on it.
In addition, a particle in the coma moving at 56 km/s would clear the shadow in, say, 100 sec so small enough particles should cool down considerably
during that time. Thermal infrared, anyone?
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akuo
post Mar 6 2013, 07:22 AM
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Even if the nucleus of C/ 2013 A1 doesn't hit Mars, it will still have an effect on the Martian atmosphere. The coma and maybe the tail will cause a great meteor storm on the Martian sky. Most of the meteors would be very tiny specks of dust, but they would still be visible as shooting stars.

Meteor showers on Earth are caused by trails of comets long past, so the approach of an actual cometary coma must be a much more significant event in this sense.


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volcanopele
post Mar 6 2013, 08:34 AM
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QUOTE (MaG @ Mar 5 2013, 02:42 PM) *
As far as I have read, nobody else said something about size of nucleus, only Leonid Elenin. I think, it's very hard to predict it now, but Jakub Cerny (my friend) wrote an article in czech, that the size of nucleus must be much smaller. For example if you remember comet Hale-Bopp, it was much much brighter in the same distance. And we hope, that it's nucleus is 40 +/- 20 km. So comet's Sidding Spring nucleus is maybe much smaller, Jakub said about 3 to 8 km (derived from their nucleus brightness).

I am doubting the 50 km size as well, though for a different reason. While most comets we've seen upclose are very dark, they have also been close to the sun repeatedly. This is likely this comet's first trip close to the sun and it probably retains a significant amount of ice on its surface. So it probably much higher albedo. Assuming, an albedo of 0.5, its diameter should be around 16 km. Assuming a diameter close to 1, its diameter would be closer to 10 km.


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Greenish
post Mar 6 2013, 12:50 PM
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http://neo.jpl.nasa.gov/news/news179.html
Comet 2013 A1 (Siding Spring) will make a very close approach to Mars in October 2014
NASA/JPL Near-Earth Object Program Office, March 5, 2013
QUOTE
...
Currently, Mars lies directly within the range of possible paths for the comet and we can't exclude the possibility that the comet might impact Mars. Our current estimate for the impact probability is less than one in six hundred and we expect that future observations will allow us to completely rule out a Mars impact.
...
Although the current heliocentric orbit is hyperbolic (i.e., eccentricity greater than one), the orbit is elliptic when expressed in the frame of the solar system's barycenter. After more than a million year journey, this comet is arriving from our solar system's distant Oort cloud. It could be complete with the volatile gases that short period comets often lack due to their frequent returns to the sun's neighborhood.

During the close Mars approach, the comet will likely achieve a total visual magnitude of zero or brighter as seen from Mars-based assets.
...

Also includes illustration of brightness curve.
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fredk
post Mar 6 2013, 03:31 PM
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October 2014 will be well into southern spring, around solar longitude 210 or 220 degrees, a time when tau at Oppy is typically getting up around 0.8 or 0.9. So unfortunately opacity will limit visibility of the tail and coma somewhat. On the other hand, Oppy won't be in the dead of winter and so may have power to do limited nighttime observations. I mention Oppy because her pancam L1 appears to be the fastest optical system on the ground. Of course if the comet gets insanely bright, opacity and optics won't matter so much...
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DDAVIS
post Mar 12 2013, 02:52 AM
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I wonder if it would be worth having Curiosity try to shoot a minute of 720P video near the zenith at the time of maximum local exposure to the coma. Success or failure to see bright enough meteors would be informative. It could be a unique look at a super meteor storm.
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ugordan
post Mar 12 2013, 12:55 PM
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I'm not sure the video-capable Mastcam is well-suited for catching meteors on account of having fairly narrow FOV. Navcam would be a better candidate, but unfortunately it can't capture high frame rates.


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machi
post Mar 12 2013, 01:26 PM
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MAHLI is video capable and has relatively large FOV (39.4°×31.1°). It's not so big as the NavCam's FOV (45°), but it's bigger than FOV of the M-34 (20°×15°).


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fredk
post Mar 12 2013, 04:28 PM
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The more important thing is the overall camera system sensitivity (ie optical system speed plus CCD sensitivity). MH (and presumably MC, and definitely navcam) are quite slow, as the recent night sky imaging demonstrated. So if you see nothing, you can't be sure that you didn't miss many meteors below your limit.

It might be worthwhile, though. We'd need to estimate the faintest meteor that MH could detect.
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Explorer1
post Mar 27 2013, 11:25 PM
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Press release about Siding Spring:
http://science.nasa.gov/science-news/scien...6mar_marscomet/

Looks like MAVEN won't be fully commissioned until 2 weeks after CA, but there could be lingering effects. Everything in orbit and on the surface should get a show for the cameras. First I have heard about the aurora possibility.


The odds are a 1 in 2000 chance of impact at this point.
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xflare
post Mar 28 2013, 09:17 AM
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The nucleus has shrunk a lot. 1-3 km now? It was "upto" 50km before.
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