Earth to Perform Asteroid 'Flyby', Radar imaging |
Earth to Perform Asteroid 'Flyby', Radar imaging |
Nov 8 2011, 12:06 AM
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#16
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Senior Member Group: Members Posts: 1419 Joined: 26-July 08 Member No.: 4270 |
This is going to be a newby question, but why are the asteroids in these radar images not fully lit (full phase)? If the radar antennas bounce radio waves off the asteroids, wouldn't the detector and the light source effectively be in the same place?
Secondly, I'm surprised Arecibo is to be used during closest approach. I thought that the asteroid was too near Earth for the telescope to be used, due to the time it took for it to switch from sending to receiving. -------------------- -- Hungry4info (Sirius_Alpha)
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Nov 8 2011, 12:36 AM
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#17
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Member Group: Members Posts: 699 Joined: 3-December 04 From: Boulder, Colorado, USA Member No.: 117 |
The answer is that radar images don't show the appearance of the asteroid in the plane of the sky, like a conventional camera. They are "delay/Doppler" images which means that the vertical distance in the image is the distance from the radar antenna (with the closest point to the antenna at the top of the image), and the horizontal distance shows the Doppler shift, which tells you the distance of each point from the spin axis of the asteroid.
So essentially you're looking at the asteroid from a direction perpendicular to the radar direction, in a plane that contains the spin axis, and with the asteroid being transparent so you can see both the "top" and "bottom" hemispheres simultaneously, superposed on each other. John |
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Nov 8 2011, 01:14 AM
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#18
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Senior Member Group: Members Posts: 1419 Joined: 26-July 08 Member No.: 4270 |
Interesting! Thanks! Much appreciated.
-------------------- -- Hungry4info (Sirius_Alpha)
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Nov 8 2011, 03:49 AM
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#19
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Solar System Cartographer Group: Members Posts: 10151 Joined: 5-April 05 From: Canada Member No.: 227 |
John's right, of course, but the same radar data could be used to create plane-of-sky images. Typically we do this for spherical objects like Venus (the most frequently imaged example)... and what you get is an image with north-south ambiguity and a blurry equator. For irregular objects like asteroids we don't know the plane-of-sky shape without further work, and the process gets too messy to work well. But plot it in delay-doppler coordinates and you get images like these, with the northern and southern sides superimposed. They are perfectly superimposed if the object is symmetrical, but in cases where the object is not symmetrical in delay-doppler space - like an elongated object tilted with respect to the incoming radar - most of the image is made only from one face of the object and there is only limited ambiguity around the sub-radar point.
Phil -------------------- ... because the Solar System ain't gonna map itself.
Also to be found posting similar content on https://mastodon.social/@PhilStooke Maps for download (free PD: https://upload.wikimedia.org/wikipedia/comm...Cartography.pdf NOTE: everything created by me which I post on UMSF is considered to be in the public domain (NOT CC, public domain) |
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Nov 9 2011, 12:46 AM
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#20
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Senior Member Group: Admin Posts: 3108 Joined: 21-December 05 From: Canberra, Australia Member No.: 615 |
Here's a tweaked six frame animation of the asteroid rotation, a little larger and slower than the official version.
Click to animate. |
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Nov 10 2011, 04:05 PM
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#21
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Senior Member Group: Members Posts: 2492 Joined: 15-January 05 From: center Italy Member No.: 150 |
This is my attempt to improve animation through de-pixalization (Gauss filter) and height reduction (better aspect ratio)
Thanks to Astro0 for source animation! -------------------- I always think before posting! - Marco -
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Nov 16 2011, 07:10 PM
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#22
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Senior Member Group: Members Posts: 1729 Joined: 3-August 06 From: 43° 35' 53" N 1° 26' 35" E Member No.: 1004 |
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