Ceres and Vesta (APOD 22 June 2007)

Comments and questions about the APOD on the main view screen.
FieryIce
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Ceres and Vesta (APOD 22 June 2007)

Post by FieryIce » Fri Jun 22, 2007 4:37 am

Can someone please explain why the Hubble Telescope can get great images of stars, nebulas etc. that are much sharper and clear than these two minor bodies that are much much closer?
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cosmo_uk
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Post by cosmo_uk » Fri Jun 22, 2007 9:18 am

Well for a start hubble has no "great images of stars", stars have such a small angular size on the sky that they are unresolved point sources. The fact that they can look big in images is due to there intensity, point spread, diffraction etc. To my knowledge there are no resolved stars therefore the images of "two minor bodies" are much better resolved than any star - you can see surface features for example. In the case of nebulae and galaxies these may be distant but they are huge and therefore subtend a resolvable angle on the sky. So basically the resolution is down to how wide the object appears on the sky.

makc
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Post by makc » Fri Jun 22, 2007 9:40 am

any reason to NOT merge with this? How is it relevant to "Ceres and Vesta, June 22, 2007"?

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BMAONE23
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Post by BMAONE23 » Fri Jun 22, 2007 2:09 pm

Think of it this way, The Moon covers approx 30 arc minutes of sky, (that is equal to 1800 arc seconds) given its relative size and distance. Ceres is approx 605 miles in diameter but at the closest point in its orbit, it lies 176million700thousand miles away. that translates to an angular size of .70623 arc seconds or approx 1/2548th the apparent size of the moon in the sky.

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Post by craterchains » Fri Jun 22, 2007 2:53 pm

I must admit that the colors of Ceres has my attention.
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iamlucky13
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Post by iamlucky13 » Sat Jun 23, 2007 12:27 am

In addition to what the others noted, which explains most of the reason, almost all of the pictures that Hubble takes of objects in our solar system are taken with the Wide Field and Planetary Camera. This is a slightly lower resolution, lower sensitivity instrument compared to the Advanced Camera for Surveys. The WFPC has a resolution of 0.043 arcseconds per pixel, while the ACS has a resolution of 0.025 arcseconds per pixel. It's not a huge difference, but it is notable.

If I understand right, the ACS is too sensitive for exposures of objects in the inner solar system, although the demand for using it for deep space observations may also be a factor.

The primary benefit of the Hubble, by the way, is not that it has better resolution than telescopes on the ground, but rather that it has better sensitivity, can take longer exposures (since it can remain pointed at the same object almost 24/7 as desired), and doesn't have to deal with portions of the spectrum being blocked by the atmosphere.


Irregardless, the Dawn mission will be blowing away everything we know about these two bodies in just a couple more years. Aside from Pluto and Eris, which are much further away, Ceres is the only major body in our solar system that hasn't had a decently close study by a probe. We should be able to figure out what's up with the funny colors on Ceres, too.
"Any man whose errors take ten years to correct is quite a man." ~J. Robert Oppenheimer (speaking about Albert Einstein)

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Ceres and Vesta

Post by JonInFL » Sat Jun 23, 2007 5:26 am

Somebody correct me if I'm wrong: I was curious about the statement "...950 kilometers and 530 kilometers in diameter - about the size of Texas and Arizona." and calculated the surface area of these bodies as if they were spheres, then compared them to the area of the states as given in Wikipedia. The sizes are way off; in fact, the area of the state in square miles are close to the area of the asteroids, which would be in square kilometers. Am I wrong, or did someone mix units when looking up the areas?
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Case
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Re: Ceres and Vesta

Post by Case » Sat Jun 23, 2007 8:34 am

JonInFL wrote:Am I wrong, or did someone mix units when looking up the areas?
It seems to fit quite nicely when you project them over each other at the same scale.

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Areas

Post by JonInFL » Sun Jun 24, 2007 6:31 pm

That may very well be true--except that the area of a sphere is not equal the the area of a circle with the diameter of the sphere. If this is the level of thought that people on this forum exhibit, there's no point in wasting my time in trying to ask questions on it.
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makc
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Post by makc » Sun Jun 24, 2007 7:29 pm

What level of thought made you post this in a separate thread? Now I have to spend ages to find where it really belongs.

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Pete
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Re: Areas

Post by Pete » Sun Jun 24, 2007 8:34 pm

JonInFL wrote:That may very well be true--except that the area of a sphere is not equal the the area of a circle with the diameter of the sphere. If this is the level of thought that people on this forum exhibit, there's no point in wasting my time in trying to ask questions on it.
Oh please. Don't you think astronomers would strive to make ballpark size comparisons as intuitive as possible? To give an idea of size, they'd logically reference dimensions of which most people have an idea, which in this case would be the apparent sizes of an asteroid and a U.S. state (okay, so not everybody has an idea of the size of Texas... I sure didn't). The statement "$HeavenlyBody is the size of $USState" compares the cross-sectional area of $HeavenlyBody to the area of $USState, as Case has done above. Cross-sectional area is easier to visualize than total area, as you may have realized during your calculations.

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Re: Areas

Post by Andy Wade » Sun Jun 24, 2007 11:46 pm

Pete wrote:
JonInFL wrote:That may very well be true--except that the area of a sphere is not equal the the area of a circle with the diameter of the sphere. If this is the level of thought that people on this forum exhibit, there's no point in wasting my time in trying to ask questions on it.
Oh please. Don't you think astronomers would strive to make ballpark size comparisons as intuitive as possible? To give an idea of size, they'd logically reference dimensions of which most people have an idea, which in this case would be the apparent sizes of an asteroid and a U.S. state (okay, so not everybody has an idea of the size of Texas... I sure didn't). The statement "$HeavenlyBody is the size of $USState" compares the cross-sectional area of $HeavenlyBody to the area of $USState, as Case has done above. Cross-sectional area is easier to visualize than total area, as you may have realized during your calculations.
That's what I thought too. Take an asteroid and place it (very very slowly of course :) ) on a US state and it is about the same size.
Seems logical to me as a reference guide.
I confess I usually scan over really technical bits until I get to a bit that doesn't follow. Then I read back through it again.
It's like the opening bit in the 'Hitch Hikers Guide to the Galaxy':
"Space is big. You just won't believe how vastly, hugely, mind- bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space".
Works for me. :lol:
Regards,
Andy.