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Comet observation from Mars, comets close encounters to Mars in 2013 and 2014
Greenish
post Feb 28 2013, 07:45 PM
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QUOTE (fredk @ Feb 28 2013, 11:31 AM) *
Of course this is still early days, and brightness estimates will be improved. Most crucially, as mcaplinger said, orbital elements will be improved. The big question now is what is the uncertainty in the close-approach Mars distance? The blog quoted above stated that the comet "might pass just 41,000 km... from the planet's centre". Is that a lower limit? What's the upper limit? That would give us a better sense of what the Mars close-approach distance will be.

Well, based on the earlier observations (thru 20 Feb, giving nominal 105000 km close approach) JPL estimated miss distance could be anywhere from 0 to 1.18 million km with a 3-sigma time uncertainty of 260 minutes. If I'm reading it right the 3-sigma error ellipse at close approach is 735000x139000 km across, with the intersection around the 1-sigma contour... so it will take a bit more refining indeed! Of course,we have some time; that link should continue to be updated whenever they update the orbit.
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stevesliva
post Mar 1 2013, 12:38 AM
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QUOTE (fredk @ Feb 28 2013, 12:31 PM) *
but Giotto survived the Halley encounter at about 600 km


Mostly. With a Whipple shield.
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SFJCody
post Mar 2 2013, 06:13 AM
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http://tech.groups.yahoo.com/group/mpml/message/28195

Some new astrometric data has been collected. There should be a recalculation of the closest approach distance from Leonid Elenin soon.
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fredk
post Mar 2 2013, 06:43 PM
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A brief discussion of imaging prospects with MRO in this S+T story.
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SteveM
post Mar 3 2013, 01:19 AM
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QUOTE (SFJCody @ Mar 2 2013, 01:13 AM) *
http://tech.groups.yahoo.com/group/mpml/message/28195

Some new astrometric data has been collected. There should be a recalculation of the closest approach distance from Leonid Elenin soon.


Mont Carlo solution of approach to Mars at Minor Planets Mailing List .
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SFJCody
post Mar 3 2013, 03:40 AM
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I just find it extraordinary that an object of 10 to 50km diameter has a non-negligible possibility of hitting any of the terrestrial planets. The very idea seems in complete opposition to the principle of mediocrity. This thing would dig a crater at least twice as big as Copernicus on the Moon, and even that is so old that it predates the evolution of complex life on Earth. That we should be here at just the right time to see something like this slam into our neighbour just strikes me as astronomically unlikely.
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volcanopele
post Mar 3 2013, 04:42 AM
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Playing around with the current trajectory (from MPEC 2013-E06 issued a few hours ago) in Celestia:

The nucleus of C/ 2013 A1 rises above the local horizon at Curiosity at around 19:30:42 UTC @ 334 deg. (NNW) when the comet is 54,387 km from the rover. That's a couple of minutes after closest approach. This is in the late afternoon for the rover. The comet continues to rise in the northern sky. When the comet is 100,000 km from the landing site at 19:53:40, the nucleus is 46.5 degrees above the horizon at an azimuth of 344 deg. When the sun sets at 20:26:49 UTC, the comet's nucleus is 202,840 km away (AZ=338 deg, ALT=61 deg). The nucleus sets in the western sky on October 20 @ 02:12:08 UTC when it is at at distance of 1,359,200 km.

Opportunity gets to look at the comet on the inbound leg with the comet setting at 19:31:53 UTC @ 28.5 (NNE) when the comet is 55,283 km from the rover. That's three minutes after closest approach during mid-morning for Oppy. At 19:26:44 UTC, the nucleus will make its closest pass to Opportunity when it is 52,539 km away (AZ=20.75 deg, ALT=16.5 deg). When the sun rises at 17:48:17 UTC, the comet's nucleus will be 334,450 km away (AZ=56.9 deg, ALT=87.3333 deg). The comet rises in the eastern sky on October 19 at 11:34:19 UTC (middle of the night) when it is 1,590,000 km away. It rises again on October 20 at 23:45:26 UTC (ENE) when the comet is 867,230 km from Oppy.

Closest approach to Mars according to JPL Horizons data ported into Celestia is on October 19, 2014 @ 19:27:37 UTC. The distance from the surface is 50,130 km. The Comets C/A distance to Phobos is 53,326 km and to Deimos it's 51,618 km. From Mars' perspective, the comet is inbound over the planet's dawn hemisphere and outbound over its dusk hemisphere.

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Don1
post Mar 3 2013, 06:42 AM
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Shoemaker-Levy 9 passed within 70,000 km of the center of Jupiter and collided with the planet. Even in the past 20 years there is another case of a kilometer scale object coming this close to a planet. I think that the rate of large impacts at Mars and beyond may be a great deal higher than people had previously assumed.

This thing is a planet killer. If it hit Mars, it would make at least a Gale Crater size hole.

The other comets to be well imaged are all short period comets with an aphelion of under 6 AU. C/2013 A1 comes from the Oort cloud, so it may look significantly different from the others. Hopefully HiRISE can get a 40m/px images of this thing, and CRISM can detect the minerals on the surface. THEMIS may be able to resolve the nucleus in the thermal IR and measure its temperature. MAVEN can do fields and particles science on the coma. Mars Express may also get low resolution images, spectra and particles data.
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Explorer1
post Mar 3 2013, 07:09 AM
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Yes, that's a good point. First ever Oort object to be seen even remotely close up. Observations from Mars surface/orbit should be planned for basically that reason alone, even ignoring the visuals....
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rlorenz
post Mar 4 2013, 06:11 AM
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QUOTE (stevesliva @ Feb 28 2013, 07:38 PM) *
Mostly. With a Whipple shield.


The VEGAs actually fared less well in some respects, even though they were 10 times further out than Giotto (maybe encountered a jet?)
http://www.lpl.arizona.edu/~rlorenz/solararraydamage.pdf
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SFJCody
post Mar 4 2013, 06:24 AM
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Belatedly realised that what I said above was pretty silly. Was trying to think of young (<100Ma) craters in the inner solar system that are around 200km in diameter or greater, and for some reason the word 'Chicxulub' never occurred to me...
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Astro0
post Mar 4 2013, 10:22 PM
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Just because it's pretty smile.gif

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SteveM
post Mar 5 2013, 03:17 PM
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Checked the flyby on NASA's Horizons and found that the elongation of the comet from the Sun remains in the range 84° to 90° throughout the encounter.

According to the current model, Mars will never be "downstream" from the comet so won't be as strongly impacted by the outflow of the coma as it would be if it passed directly through the tail.

Steve M
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Phil Stooke
post Mar 5 2013, 04:42 PM
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"This thing is a planet killer." (Don1)

It might not kill Mars but it might be a rover killer. Depending on location it could darken the skies long enough to really hurt Opportunity. But the chance of that happening is very small - luckily for us rover-lovers.

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fredk
post Mar 5 2013, 06:57 PM
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The jpl database now lists the "nominal" close-approach distance at a bit over 50 000 km. But they list the "maximum" distance as over 300 000 km. I can see no definition for the "maximum" distance, but elsewhere they refer to 1-sigma uncertainties. So if that "maximum" distance corresponds to the 1-sigma upper limit, then it will be a much better indication of the likely close-approach distance than the "nominal" value, if the errors used in the calculations aren't overestimated. In fact, for a gaussian error distribution, the odds are something like 32% that the actual value will be greater than 300 000 km!

The "minimum" distance is listed as zero, which presumably means the error ellipse still intersects Mars. But the area of Mars is an incredibly tiny fraction of the area of a 300 000 km (or 600 000 km?) error ellipse, so the chance of collision is extremely small.

QUOTE (SteveM @ Mar 5 2013, 04:17 PM) *
the elongation of the comet from the Sun remains in the range 84° to 90°

That's interesting - it would mean that the comet would be viewed perpendicularly to the tail throughout the encounter, which is great news for getting good tail views (or at least views that aren't very foreshortened). But if the error ellipse still includes Mars, the comet could as easily be on one side of the planet as the other during encounter. It could also be sunwards or outwards, presumably by as much as something like the 1-sigma value, 300 000 km. So all options still seem to be open. It looks like we'll need lots more observations before we can pin down the encounter specifics.
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