Starship Asterisk*

The Day After Mars

Explanation: October 31, 1938 was the day after Martians encountered planet Earth, and everything was calm. Reports of the invasion were revealed to be part of a Halloween radio drama, the now famous broadcast based on H.G. Wells' scifi novel War of the Worlds. On Mars October 20, 2014 was calm too, the day after its close encounter with Comet Siding Spring. Not a hoax, this comet really did come within 86,700 miles or so of Mars, about 1/3 the Earth-Moon distance. Earth's spacecraft and rovers in Mars orbit and on the surface reported no ill effects though, and had a ringside seat as a visitor from the outer solar system passed by. Spanning over 2 degrees against stars of the constellation Ophiuchus, this colorful telescopic snapshot captures our view of Mars on the day after. Bluish star 51 Ophiuchi is at the upper right and the comet is just emerging from the Red Planet's bright glare.

<< Previous APOD

This Day in APOD

Next APOD >>

[/b]

I loved the facts about 51 Ophiuchi. What a fascinating star. The link says it is a B-type star, and Simbad database identifies it as a Be star, an emission-line B-type star. Project Pluto calls it an A0V star, and Bright Star catalog, quoted by Project Pluto, gives its spectral class as B9.5Ve. I think it is really remarkably bright to for an A0 or B9.5 main sequence star, particularly if it is a very young star. Young stars are typically comparatively faint and become brighter as they age. But 51 Ophiuchi is about three times brighter in visual light than Vega, another main sequence A0-type star, and it is about equal in brightness to Alkaid, a much hotter star of spectral class B3V.

Fascinating!

It's a fine picture, too.

Ann

Cool....to see the comet coming around....

Soon a new song...."She'll be coming a-round MARS, when she comes..."

:---[===] *

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.

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. Neither was there a shower of water like the one that rained on Earth for forty days and forty nights some time ago. A rainbow failed to materialize after the passage suggesting that perhaps the comet was deficient in watery components.

This is really quite a lovely photograph.
The uncorrected overexposure of Mars emphasizes the planet's color beautifully, and makes a great contrast with 51 Ophiuchi's blue.
The comet image is among the best I've seen.
Nice work!

While it would have interfered with our study of Mars, its too bad it didn't hit Mars, it could have used the water.

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.

Comets are not composed of gas. They are composed of dust and rock, and of volatile ices. The reason this comet failed to produce a meteor shower on Mars (this year, at least) is because it was so small and because there wasn't enough time for the debris stream to expand.

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

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

There are, but you know those artistic renderings earlier that made you think, "Wow, I can't wait to see a real pic of the comet from a rover!"

Well, here you go. http://mars.nasa.gov/comets/sidingsprin ... ageID=6679

Mars Reconnaissance Orbiter caught the comet too: http://mars.jpl.nasa.gov/mro/news/whats ... ewsID=1739

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.

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).

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).

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.

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.

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.

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?

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.

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.

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

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

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

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.

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.

It is quite amazing, but I feel like my perception of this event has suddenly been altered by better knowledge of the coma size.

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

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.

Edit: unless the coma includes the fluffiest parts opposite the tail (which it must), in which case all the numbers add up.

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.

Yeah, that's where the radial brightness change gets tricky. Where exactly is the outer edge of a fuzzy, difuse object? You might have a rigorous rule, like 1/e, but it's still going to look bigger given a longer exposure. I'd say the Hubble image shows the coma as at least a couple times larger than Mars, but maybe what I'm calling coma you're calling tail.

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

Fluffy bits are nice, especially if they belong to a huggable person.