Starship Asterisk*

Chris Peterson wrote: ↑Mon Nov 04, 2019 2:18 am

RocketRon wrote: ↑Mon Nov 04, 2019 1:36 am 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.

RocketRon wrote: ↑Mon Nov 04, 2019 1:36 am 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

TheOtherBruce wrote: ↑Sun Nov 03, 2019 5:51 am

RocketRon wrote: ↑Sat Nov 02, 2019 5:11 am
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.

https://en.wikipedia.org/wiki/Coma_(cometary) wrote:
<<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 wrote: ↑Sat Nov 02, 2019 5:11 am 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 ?

neufer wrote: ↑Sun Nov 03, 2019 12:19 am

Chris Peterson wrote: ↑Sat Nov 02, 2019 9:08 pm

RocketRon wrote: ↑Sat Nov 02, 2019 9:03 pm
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 wrote: ↑Sat Nov 02, 2019 9:08 pm

RocketRon wrote: ↑Sat Nov 02, 2019 9:03 pm
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.

RocketRon wrote: ↑Sat Nov 02, 2019 9:07 pm 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 ?

RocketRon wrote: ↑Sat Nov 02, 2019 9:03 pm 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...

orin stepanek wrote: ↑Sat Nov 02, 2019 5:37 pm Yup; that was really close! Maybe Mars could have had a third moon! :mrgreen:

https://en.wikipedia.org/wiki/C/2013_A1_(Siding_Spring)#/media/File:PIA18611-Mars-CometSidingSpringFlyby-20141009.jpg wrote:

Mars Orbiters 'Duck and Cover' (Artist's Concept)

---

Mars Orbiters 'Duck and Cover' for Comet Siding Spring Flyby

<<This artist's concept shows NASA's Mars orbiters lining up behind the Red Planet for their "duck and cover" maneuver to shield them from comet dust that may result from the close flyby of comet Siding Spring (C/2013 A1) on Oct. 19, 2014. The comet's nucleus will miss Mars by about 139,500 kilometers, shedding material as it hurtles by at about 56 kilometers per second [~12.6 lunar distances per day], relative to Mars and Mars-orbiting spacecraft. NASA is taking steps to protect its Mars orbiters, while preserving opportunities to gather valuable scientific data. The NASA orbiters at Mars are Mars Reconnaissance Orbiter, Mars Odyssey and MAVEN.>>

orin stepanek wrote: ↑Sat Nov 02, 2019 1:19 pm

alter-ego wrote: ↑Fri Nov 01, 2019 6:44 pm

orin stepanek wrote: ↑Fri Nov 01, 2019 10:40 am
Beautiful photo; wish one could have been taken from Mars!

There were, just none as visually stunning as earth-based images. Emily Lakdawalla's post 2 weeks later presents three amateur images (including today's) and mars-base images from rover Curiosity, Mars Reconnaissance Orbiter, MAVEN, Mars Express, and ISRO MOM (later post). Obviously, taking images from the orbiters involved timing and pointing adjustments. Of the (then) 5 orbiters capable of imaging the comet, only 4 were successful. The timings were spread out from minutes to hours before/after closest approach. From an ground-based observer point of view, my favorites are images from the two rovers, Curiosity and Opportunity. Curiosity showed Mar's horizon through a dusty atmosphere in twilight with a faint, point-source comet. Opportunity showed a moving, fuzzy comet trailing in a different direction than the stars.

Tnx; looks like the mars based cameras were a little bit off!

https://en.wikipedia.org/wiki/C/2013_A1_(Siding_Spring)#/media/File:PIA18611-Mars-CometSidingSpringFlyby-20141009.jpg wrote:

Mars Orbiters 'Duck and Cover' (Artist's Concept)

---

Mars Orbiters 'Duck and Cover' for Comet Siding Spring Flyby

<<This artist's concept shows NASA's Mars orbiters lining up behind the Red Planet for their "duck and cover" maneuver to shield them from comet dust that may result from the close flyby of comet Siding Spring (C/2013 A1) on Oct. 19, 2014. The comet's nucleus will miss Mars by about 139,500 kilometers, shedding material as it hurtles by at about 56 kilometers per second [~12.6 lunar distances per day], relative to Mars and Mars-orbiting spacecraft. NASA is taking steps to protect its Mars orbiters, while preserving opportunities to gather valuable scientific data. The NASA orbiters at Mars are Mars Reconnaissance Orbiter, Mars Odyssey and MAVEN.>>

RocketRon wrote: ↑Sat Nov 02, 2019 5:11 am 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 ?

alter-ego wrote: ↑Fri Nov 01, 2019 6:44 pm

orin stepanek wrote: ↑Fri Nov 01, 2019 10:40 am Beautiful photo; wish one could have been taken from Mars!

There were, just none as visually stunning as earth-based images. Emily Lakdawalla's post 2 weeks later presents three amateur images (including today's) and mars-base images from rover Curiosity, Mars Reconnaissance Orbiter, MAVEN, Mars Express, and ISRO MOM (later post). Obviously, taking images from the orbiters involved timing and pointing adjustments. Of the (then) 5 orbiters capable of imaging the comet, only 4 were successful. The timings were spread out from minutes to hours before/after closest approach. From an ground-based observer point of view, my favorites are images from the two rovers, Curiosity and Opportunity. Curiosity showed Mar's horizon through a dusty atmosphere in twilight with a faint, point-source comet. Opportunity showed a moving, fuzzy comet trailing in a different direction than the stars.

Chris Peterson wrote: ↑Fri Nov 01, 2019 7:57 pm

neufer wrote: ↑Fri Nov 01, 2019 7:18 pm

Chris Peterson wrote: ↑Fri Nov 01, 2019 1:32 pm
You can't even "see" Mars in this image. Just its light. At the scale of this image, Mars itself is smaller than one pixel. An unresolved point source. And yes, it's extremely bright. 36 times brighter than 51 Oph, 5000 times brighter than the comet. Far more dynamic range than our computer displays can present.

• "Mars itself is smaller than one pixel"

The Comet got as close as 21 Mars diameters away.

The pixel scale of this image (the full size click through image) is 7.1 arcseconds/pixel. At the time, Mars subtended 6 arcseconds. So it just fits inside a single pixel.

neufer wrote: ↑Fri Nov 01, 2019 7:18 pm

Chris Peterson wrote: ↑Fri Nov 01, 2019 1:32 pm

Ann wrote: ↑Fri Nov 01, 2019 5:38 am
Mars looks very bright in this picture. Was Mars actually close to Earth when this picture was taken? Or does Mars look so bright because it was just so much brighter than the tiny little comet?

You can't even "see" Mars in this image. Just its light. At the scale of this image, Mars itself is smaller than one pixel. An unresolved point source. And yes, it's extremely bright. 36 times brighter than 51 Oph, 5000 times brighter than the comet. Far more dynamic range than our computer displays can present.

• "Mars itself is smaller than one pixel" :?:

The Comet got as close as 21 Mars diameters away.

Chris Peterson wrote: ↑Fri Nov 01, 2019 1:32 pm

Ann wrote: ↑Fri Nov 01, 2019 5:38 am
Mars looks very bright in this picture. Was Mars actually close to Earth when this picture was taken? Or does Mars look so bright because it was just so much brighter than the tiny little comet?

You can't even "see" Mars in this image. Just its light. At the scale of this image, Mars itself is smaller than one pixel. An unresolved point source. And yes, it's extremely bright. 36 times brighter than 51 Oph, 5000 times brighter than the comet. Far more dynamic range than our computer displays can present.

• "Mars itself is smaller than one pixel"

orin stepanek wrote: ↑Fri Nov 01, 2019 10:40 am Beautiful photo; wish one could have been taken from Mars!