[X] My Assistant

[djellison]

It'll get more and more accurate as time goes on - I'm sure we'll have a firm hit/miss in plenty of time.

Doug

[Stu]

My money's on a miss... it would be just too greedy to expect an impact as well as everything else we've enjoyed on or around Mars recently. I'll be happy to see it hit, of course, but equally happy to see it sail by harmlessly.

Would be sooooo funny if it hit a certain face-shaped mesa in Cydonia tho... wow, imagine those conspiracy theories!

[nprev]

Would be sooooo funny if it hit a certain face-shaped mesa in Cydonia tho... wow, imagine those conspiracy theories!

..."The Great Acne Scar on the <Bleep> on Mars!"

[Guest_PhilCo126_*]

Some 'real' images
http://www.lpl.arizona.edu/css/

[nprev]

Thanks, Phil.

Gotta say that these NEO searches really are paying off serendipitous dividends, here. Really did not expect that we'd ever pick up a potential Mars impactor (fantasized about it once or twice, though). For sheer SFX, would love to find a good-sized body slamming into Venus, preferably near inferior conjunction, preferably on the night side (hey, it's Christmas...I can ask for stuff!!! )

[rogelio]

If this asteroid did impact Mars on its sunward side, how easily would the impact be visible from earth?

Here are some back-of-the envelope calculations (they may be WAY off):

A similar-sized impactor hitting the moon would probably release energy in the kiloton range (at least?) and would probably be easily visible and even spectacular on earth with the naked eye... let’s be optimistic and say a magnitude of -10.

Mars is currently roughly 250 times farther than the moon, so all things being equal the magnitude of a similar-sized impact on Mars would be about 250 squared = 62,500 times fainter, that’s a 12 magnitude difference down to +2.

That would contribute only a few percent increase to Mars’ current brightness (and then for only a few seconds).

Bottom line (if these VERY rough calculations are in the ballpark): here on earth, the impact would not be visible to the naked eye, nor directly in a 6” reflector like mine; but might be recordable by advanced amateur scopes and more than likely visible to Hubble.

Another question: Would having an atmosphere (like Mars) make the impact brighter than if it impacted the airless moon? Or would a Mars impact be “instantaneously” dimmer, but with a longer “afterglow” due to ionized gases in its atmosphere (energy being conserved and all)?

[dburt]

Not until we witnessed freshly created hematite spheres raining from your impact surge cloud.

Sorry, I haven't been monitoring this speculative thread over the weekend. But hey, EGD, the blue-gray hematitic spherules are the Meridiani exception, not the martian rule - you don't find them everywhere on Mars, even though Mars was virtually saturated with impact craters. To form the hematitic spherules via impact, there must have been something unusual about the target, or something unusual about the impactor, or both. So you can't very well expect hematitic spherules from every martian impact. Also, keep in mind that Mars at the time of the Meridiani-forming impacts (if that's what formed Meridiani's uppermost layers and blueberries) presumably had a somewhat thicker atmosphere (although by then, about 3.8 billion years ago, Mars may have already lost 99 per cent of its atmosphere owing to erosion by the Late Heavy Bombardment). And, if not formed by impact (or other high temperature process), why are the spherules apparently the blue-gray or high temperature (specular) form of hematite (i.e., why aren't they the same color as the rest of Mars)? And if they're indeed sedimentary concretions, why do the blueberries so little resemble actual sedimentary concretions in terms of their distribution, size, shape, degree of aggegation (clumping), and host rocks?

BTW, I much enjoyed the two competing artistic impressions of distant meteorite impacts as seen from Victoria. One, the nuclear bomb-type "mushroom cloud" looks like what I would expect of a small impact on any planet with an atmosphere (i.e., Earth or Mars). The other looks like virtually all textbook illustrations of impacts, and shows the early-formed ballistic ejecta curtain only. It is what I might expect for impacts on the Moon or other planet with no atmosphere (although even there the rocks themselves are vaporized, at least ephemerally). Keep in mind that, AFAIK, there are absolutely no photos of actual large impacts to use as a basis for comparison (and that any such photos might well be the photographer's last ever). All the more reason to wish for Oppy to supply such an actual impact image, and survive.

-- HDP Don

[ugordan]

One, the nuclear bomb-type "mushroom cloud" looks like what I would expect of a small impact on any planet with an atmosphere (i.e., Earth or Mars). The other looks like virtually all textbook illustrations of impacts, and shows the early-formed ballistic ejecta curtain only. It is what I might expect for impacts on the Moon or other planet with no atmosphere (although even there the rocks themselves are vaporized, at least ephemerally).

I myself am very interested in how a convective updraft cloud would behave in a low-density atmosphere. On Titan I'd expect a mushroom cloud very similar to one on Earth, varying in head/stem dimension, but it would be interesting to see how one would look on Mars. For one, the buoyancy would be greaty reduced, also reducing the updraft which creates the characteristic mushroom stem. The two artistic views could be regarded as the same scenario - mine would be the initial impact (I was actually aiming at that) throwing ejecta up and Stu's would be the aftermath several minutes later when ejecta has settled down and the dusty "fireball" rose up and cooled down.

I believe the initial moments of an impact that couples energy to the ground well (something surface-detonated nuclear weapons don't do well, in addition to releasing over 50% energy as thermal, not kinetic) would be dominated by the ejecta blanket, screening the (small) incandescent fireball at the point of impact.

Here's a not-so-good analogy with ground penetrating weapons: http://www.youtube.com/watch?v=KF8gnokHrWw (Caution: VERY LOUD!)

There are better videos out there, but I can't seem to dig them up now.

Would you really like Opportunity to be anywhere near this shower of debris (and note Mars' thin atmosphere would basically allow ballistic emplacement of debris)? I know I wouldn't...

Here's a more classical fireball on the other hand, typical of atmospheric nukes, not resulting in large excavation as much as a large fireball and atmospheric shockwave: http://www.youtube.com/watch?v=PwuMdNg9oWQ (ignore the F-words)

[Stu]

My latest entry for the "I know it would look nothing like this, but just imagine...!" Martian Asteroid Impact Gallery...





Christmas Day now here in the UK and I'm off to work... have a great day everyone, wherever you are!

[MarsIsImportant]

Honestly, I don't think the stem of the mushroom cloud on Mars would be that tall. The air is too thin, so it would probably spread horizontally much quicker than on Earth. In fact, the mushroom like cloud might not even have a stem on Mars. Most or All the energy might go toward forming the cap almost immediately. The cap itself might simply grow larger as the blast wave expands.

I don't know. I'd like to see one in action on Mars for real.

[Guest_PhilCo126_*]

Indeed, the impact would be a 'minor' one...
Please check this video of an impact on the Moon:
http://science.nasa.gov/headlines/y2006/13...narsporadic.htm

[Stu]

Honestly, I don't think the stem of the mushroom cloud on Mars would be that tall.

... which is probably why I said...

"I know it would look nothing like this..."

[nprev]

One thing puzzles me. JPL is saying that if it does hit, the impact area would be in northern Meridiani. Given the fact that the margin of error is still about five times the diameter of Mars itself, how can they make such a statement?

Seems much more reasonable to assume that if (a damn big if) it hits, the location at this point would be defined as a huge ellipse projected on Mars spanning almost 180 deg of longitude. I'm sure that JPL's statement has something to do with the output of one or more models, but I can't really lend it any credence. Anybody have better info?

[ugordan]

One thing puzzles me. JPL is saying that if it does hit, the impact area would be in northern Meridiani. Given the fact that the margin of error is still about five times the diameter of Mars itself, how can they make such a statement?

I'm curious, where did you come up with such a small margin of error? As I understand it, the uncertainty ellipse is very stretched out with the semimajor axis still probably some 100 000 km (a guess out of thin air), but the semiminor axis is much, much smaller so the point where it intersects Mars is just a band covering a portion of Martian surface. In other words, we know the trajectory with enough certainty even now to rule out for example near-polar hits.

[nprev]

I'm puzzled as well, Gordan; here's a quote from the JPL press release (21 Dec):

"If this unlikely event were to occur, it would be somewhere within a broad swath across the planet north of where the Opportunity rover is located."

Full article: http://www.jpl.nasa.gov/news/news.cfm?release=2007-152

I think that I mis-stated Meridiani as the stated potential impact area, though pretty sure I've seen that in other articles (derived from this press release, no doubt). This may be getting out of control.

EDIT: Whups, didn't answer your question. The last published uncertainty I saw for a miss distance was around 21,000 miles, so rounding Mars' diameter to 4000 miles gave a bit more than 5 Mars diameters. Very simplistic, uniplanar, manifestly inaccurate; I withdraw the comment.