Starship Asterisk*

quigley wrote:

geckzilla wrote:Did you see this composite image which showed the coma and Mars together?
http://hubblesite.org/newscenter/archiv ... 5/image/a/

Given the immense size of the comet, I am surprised at how unimpressive the view of the comet is from the planet . Explanations of dust storms and impending sunrise taken into account, I still expected more from such a close proximity fly-by.

geckzilla wrote:Did you see this composite image which showed the coma and Mars together?
http://hubblesite.org/newscenter/archiv ... 5/image/a/

Nitpicker wrote:Is there room for a "moon unit" in astronomy for near misses like these? Say 1 moon unit equals 384,399 km (the semi-major axis of the Moon's orbit). I'm sure (near and dear Miss) Moon Unit Zappa wouldn't mind.

Nitpicker wrote:I suppose I've just demonstrated the bias inherent in an engineer. We tend to ignore the fluffy bits, sometimes to a fault.

Nitpicker wrote:I did see that, and I was busy calculating things from it while Chris was busy posting about the 8 degree size (2.5x Mars diameter) of the coma. The Hubble image makes the coma appear smaller than our Moon (< 0.5x Mars diameter), but there were a lot of different exposures going on in the Hubble composite, so I wasn't sure about my numbers.

geckzilla wrote:Did you see this composite image which showed the coma and Mars together?
http://hubblesite.org/newscenter/archiv ... 5/image/a/

Chris Peterson wrote:

Nitpicker wrote:So, about 2.5 times bigger than Mars in absolute diameter?

That's pretty believable. The coma of an active comet is usually many orders of magnitude larger than the nucleus. Remember Holmes? Its coma was the largest thing in the Solar System, bigger even than the Sun.

Nitpicker wrote:So, about 2.5 times bigger than Mars in absolute diameter? :shock:

Chris Peterson wrote:

Nitpicker wrote:I see (in my mind I assumed coma = nucleus at great distance). I wonder, then, just how big the coma would have appeared to our imaginary observer on Mars? And how one might fairly compare its brightness?

At closest approach, I believe the coma as seen from Mars was over 8° across. Figuring apparent brightness is further complicated by the fact that the coma is brighter in the middle.

Nitpicker wrote:I see (in my mind I assumed coma = nucleus at great distance). I wonder, then, just how big the coma would have appeared to our imaginary observer on Mars? And how one might fairly compare its brightness?

Chris Peterson wrote:

Nitpicker wrote:I assume the brightness comes mainly from the nucleus of a comet. This comet nucleus was too small to be considered an extended object at closest approach, at least to our imaginary human observer with unaided eyes at the North Pole of Mars. But if the quoted magnitudes of comets routinely cover any extended area of tail, then yes, that might make my comparison less useful (which I only made to indicate that I thought the comet wasn't fantastically bright, anyway). Given how variable comet tails can be, it would surprise me to learn that estimates of comet magnitude typically include the extended tail area.

The tail isn't factored in, but the coma is. Most of the reflected light would be from the coma. I don't know that the nucleus was even visible.

Nitpicker wrote:I assume the brightness comes mainly from the nucleus of a comet. This comet nucleus was too small to be considered an extended object at closest approach, at least to our imaginary human observer with unaided eyes at the North Pole of Mars. But if the quoted magnitudes of comets routinely cover any extended area of tail, then yes, that might make my comparison less useful (which I only made to indicate that I thought the comet wasn't fantastically bright, anyway). Given how variable comet tails can be, it would surprise me to learn that estimates of comet magnitude typically include the extended tail area.

Chris Peterson wrote:

Nitpicker wrote:The dark, wintry, North Pole of Mars would have been one of the (geometrically) better places to observe the closest approach of C/2013 A1. An observer there in clear skies could have seen the comet for at least 4 hours before and more than 4 hours after closest approach. In that 8 hour time interval, the comet would have brightened from about magnitude -2 to a peak of about -6 and dimmed back down to -2. A magnitude of -6 is about as bright as one ever sees the ISS from Earth.

But the magnitude of an extended object doesn't translate in an obvious way to apparent visual brightness. Consider Andromeda galaxy, with a magnitude of 3.4, but which most people have trouble seeing (while you need to be nearly blind to miss a star of the same magnitude).

Nitpicker wrote:The dark, wintry, North Pole of Mars would have been one of the (geometrically) better places to observe the closest approach of C/2013 A1. An observer there in clear skies could have seen the comet for at least 4 hours before and more than 4 hours after closest approach. In that 8 hour time interval, the comet would have brightened from about magnitude -2 to a peak of about -6 and dimmed back down to -2. A magnitude of -6 is about as bright as one ever sees the ISS from Earth.

ta152h0 wrote:So there are no images taken from the vicinity of Mars ?

overwhelmed wrote:The near-collision encounter between Mars and the comet was overwhelming.
There seems to be an absence of large meteoritic activity associated with the comet; a meteor storm following the encounter with Mars would have been expected but there was none.
This would suggest that comets such as Siding Spring, which originate in the Oort Cloud, are composed of mainly fine dust and gas.