APOD: The Day After Mars (2019 Nov 01)

[Chris Peterson]

You'd wonder how all the moons in our Solar System came to be there then ?
Obviously there is some mechanism whereby they can be captured...

Indeed. It just requires a three-body interaction. In our solar system, that's almost always between the small body, Jupiter, and the Sun.

[Chris Peterson]

If my little pocket camera can take an image of Mars that shows (more or less) the
sunlit crescent, and the surrounding starfield are mostly mere pinprick dots, then there
is no way that in the telescopic view here of Mars and comet is Mars merely one pixel ?
That defies common sense even ??? The comet head itself may be a different matter.

Googling for existing earthbound telescopic views of Mars shows quite some detail,
and one pixel they are certainly not. Isn't that how the "canals of Mars" came into
being, with some of the earlier telescopes ?

I'm not sure what you mean by your "little pocket camera". Or how anything on Earth can show a crescent Mars, given that it's not between us and the Sun.

This image is 3.5° wide. That's more than 2000 times the apparent diameter of Mars. Which is why Mars is an unresolved point. A good image of Mars made from the Earth requires a much longer focal length telescope than the one used here, to achieve a finer pixel scale. And nobody is going to get a very good image when Mars is near its greatest distance from Earth, as in this case. The good shots you've seen were all taken when Mars was much closer- subtending closer to 24 arcseconds, not the less than 6 in October 2014. And they were taken at a focal length that resulted in an image with a FOV of no more than a few arcminutes, not a few degrees.

[neufer]

You'd wonder how all the moons in our Solar System came to be there then ?
Obviously there is some mechanism whereby they can be captured...

Indeed. It just requires a three-body interaction. In our solar system,
that's almost always between the small body, Jupiter, and the Sun.

Most of the pro-grade moons in our Solar System started out from the original cir-cum-planetary disk.

Most of the retro-grade moons in our Solar System started out from collisions with the pro-grade moons.

[Chris Peterson]

You'd wonder how all the moons in our Solar System came to be there then ?
Obviously there is some mechanism whereby they can be captured...

Indeed. It just requires a three-body interaction. In our solar system,
that's almost always between the small body, Jupiter, and the Sun.

Most of the pro-grade moons in our Solar System started out from the original cir-cum-planetary disk.

Most of the retro-grade moons in our Solar System started out from collisions with the pro-grade moons.

Sure. I was just addressing the captured moons.

[TheOtherBruce]

If Mars is one pixel, why is the comet so many pixels. ??
That doesn't quite add up..
Even my pocket camera can do better than that.
& surely this is telescopic view ?

What you see of the comet's head is a big cloud of dust and gas (usually at least thousands of miles across) ejected from the actual solid nucleus, which is a tiny dirty snowball in the middle. Think back to the Giotto probe in 1986, which flew right through the head of Halley's Comet and took photos of the nucleus (less than ten miles across) from only a few hundred miles away.

[neufer]

If Mars is one pixel, why is the comet so many pixels. ??
That doesn't quite add up..
Even my pocket camera can do better than that.
& surely this is telescopic view ?

What you see of the comet's head is a big cloud of dust and gas (usually at least thousands of miles across) ejected from the actual solid nucleus, which is a tiny dirty snowball in the middle.

• Mars is thousands of miles across.

What you see of the comet's head is a big (think Jupiter sized) cloud of dust and gas (usually at least TENS of thousands of miles across) ejected from the actual solid nucleus, which is a tiny dirty snowball in the middle.

<<The coma is the nebulous envelope around the nucleus of a comet, formed when the comet passes close to the Sun on its highly elliptical orbit; as the comet warms, parts of it sublime. This gives a comet a "fuzzy" appearance when viewed in telescopes and distinguishes it from stars.

The coma is generally made of ice and comet dust. Water composes up to 90% of the volatiles that outflow from the nucleus when the comet is within 3-4 AU of the Sun. The H₂O parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization. The solar wind plays a minor role in the destruction of water compared to photochemistry. Larger dust particles are left along the comet's orbital path while smaller particles are pushed away from the Sun into the comet's tail by light pressure.

On 2 June 2015, NASA reported that the ALICE spectrograph on the Rosetta space probe studying comet 67P/Churyumov–Gerasimenko determined that electrons (within 1 km above the comet nucleus) produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma.

Comas typically grow in size as comets approach the Sun, and they can be as large as the diameter of Jupiter, even though the density is very low. About a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun. The Great Comet of 1811 also had a coma roughly the diameter of the Sun. Even though the coma can become quite large, its size can actually decrease about the time it crosses the orbit of Mars around 1.5 AU from the Sun. At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, enlarging the tail.

Space probe Giotto detected hydrogen ions at distance of 7.8 million km away from Halley when it did close flyby of the comet in 1986. A hydrogen gas halo was detected to be 15 times the diameter of Sun. This triggered NASA to point the Pioneer Venus mission at the Comet, and it was determined that the Comet emitting 12 tons of water per second. The hydrogen gas emission has not been detected from Earth's surface because those wavelengths are blocked by the atmosphere. The process by which water is broken down into hydrogen and oxygen was studied by the ALICE instrument aboard the Rosetta spacecraft.

A hydrogen gas halo three times the size of the Sun was detected by Skylab around Comet Kohoutek in the 1970s. SOHO detected a hydrogen gas halo bigger than 1 AU in radius around Comet Hale–Bopp. Water emitted by the comet is broken up by sunlight, and the hydrogen in turn emits ultra-violet light. The hydrogen atom are very light so they can travel a long distance before they are themselves ionized by the Sun. When the hydrogen atoms are ionized they are especially swept away by the solar wind.>>

[RocketRon]

I'm not sure what to make of the discussions here of the size of Mars in this APOD photo.
Sure the comet head/nucleus is going to be tiny, but Mars in this telescopic should big enough to be in some detail,
IF THE EXPOSURE TIME HADN'T BEEN STRETCHED OUT to capture the comet

At the view shown, Mars is going to be somewhere in the range of this comparative pic. ???
https://www.agenaastro.com/media/images ... uide_3.jpg

Some of these show the crescent phase of Mars.
While limited by our angle of viewing, it is apparent given the viewing angles sometimes.
https://www.cloudynights.com/uploads/mo ... _thumb.jpg

[Chris Peterson]

I'm not sure what to make of the discussions here of the size of Mars in this APOD photo.
Sure the comet head/nucleus is going to be tiny, but Mars in this telescopic should big enough to be in some detail,
IF THE EXPOSURE TIME HADN'T BEEN STRETCHED OUT to capture the comet

At the view shown, Mars is going to be somewhere in the range of this comparative pic. ???
https://www.agenaastro.com/media/images ... uide_3.jpg

Some of these show the crescent phase of Mars.
While limited by our angle of viewing, it is apparent given the viewing angles sometimes.
https://www.cloudynights.com/uploads/mo ... _thumb.jpg

Again, each pixel in this image spans 7 arcseconds of the sky. Mars spans 6 arcseconds. How do you expect to see anything other than an unresolved point?

The comet, both coma and tail, are much larger than Mars.

[alter-ego]

I'm not sure what to make of the discussions here of the size of Mars in this APOD photo.
Sure the comet head/nucleus is going to be tiny, but Mars in this telescopic should big enough to be in some detail,
IF THE EXPOSURE TIME HADN'T BEEN STRETCHED OUT to capture the comet

At the view shown, Mars is going to be somewhere in the range of this comparative pic. ???
https://www.agenaastro.com/media/images ... uide_3.jpg

Some of these show the crescent phase of Mars.
While limited by our angle of viewing, it is apparent given the viewing angles sometimes.
https://www.cloudynights.com/uploads/mo ... _thumb.jpg

Again, each pixel in this image spans 7 arcseconds of the sky. Mars spans 6 arcseconds. How do you expect to see anything other than an unresolved point?

The comet, both coma and tail, are much larger than Mars.

To get a better visualization, consider the Hubble image of the 1995, near-opposition Mars case in the first link:
  

700 x 700 pixels

It is ~14 arcsec diameter, or 2x larger (rounded up) than Mars' angular size in the APOD. The Hubble image is close to 28 arcsec square equivalent to 4 x 4 pixels in the APOD, but in the (reduced-resolution) Hubble image, Mars has a diameter of 330 pixels!. Now using the same imaging setup as Rolando, a zoomed in view of the 4 x 4 pixels containing an unsaturated, the near-opposition Mars would look like:
 

Zoomed-in view

4x4 Mars.png (16.78 KiB) Viewed 3005 times

You can see, Mars' larger angular size in 1995 doesn't help in Rolando's wider field image, and as Chris stated for the APOD image, only one pixel would be needed to show a well-exposed Mars:
 

Zoomed-in view

3x3 Mars.png (16.67 KiB) Viewed 3005 times

 

[Nitpicker]

The APOD has a rather wide field compared with typical planetary detail images from Earth. Chris is correct re the scale and apparent size of Mars.

(Edit ... Shoulda read the full thread. I've added nothing.)