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Mars Orbiter Photographs another Mars Orbiter
rde writes "We're all familiar with blurry photographs of UFOs, but NASA have gone one better; the Mars Global Surveyor has photographed fellow satellite Mars Odyssey as it whizzed past. This is the first instance of one extraterrestrial satellite photographing another."
The two views of Mars Odyssey in this image were acquired a little under 7.5 seconds apart as Odyssey receded from a close flyby of Mars Global Surveyor. The geometry of the flyby (see Figure 1) and the camera's way of acquiring an image line-by-line resulted in the two views of Odyssey in the same frame. The first view (right) was taken when Odyssey was about 90 kilometers (56 miles) from Global Surveyor and moving more rapidly than Global Surveyor was rotating, as seen from Global Surveyor. A few seconds later, Odyssey was farther away -- about 135 kilometers (84 miles) -- and appeared to be moving more slowly. In this second view of Odyssey (left), the Mars Orbiter Camera's field-of-view overtook Odyssey.
Its the same frame exposed twice. Think about what happens when you take a picture, but set the exposure time too long.
They read their imaging array one line at a time. It saw the Odyssey once, which is the image you see on the left (I think). It continued to move, and then caught it again on the right. For the complete logistics of how it happened, we'd have to know more about their imaging array, and the relative speeds. Ya, I'd think there should be a blur in there somewhere, but aparently there isn't.
Think of a flat top copy machine. With the top open, put your hand at the left (if it scans from that side). After it passes your hand, put it on the right side. It'll see your hand again.
When I was in middle school, we took at trip to Washington DC. They did a panoramic picture of the class. The photographer had the girl on the left side of the picture move, as soon as she was out of the shot, and run around to the right side. She showed up twice, like twins. It was easier than editing her in later, or at least then it was. Now, it's a piece of cake in Photoshop.
Serious? Seriousness is well above my pay grade.
If you do the math its because for certain values of the exact geometry there are multiple solutions in the range [0,pi] for t in the equation k.t+phi = tan(v.t) (where k is the rotational rate of the camera, v is the velocity of the flyby probe, phi is the angle of the camera to the probe at time t=0).
Obviously there are other solutions as the camera rotates round again, and I've assumed the camera is static and the probe is in a constant speed linear path.
The above maths is pretty simple, every graduate should be capable of computing it. Its hardly rocket science. Oh hang on, maybe it is!
It is a simple question of dispersion in the atmosphere. Take telescopes for instance ...Ground-based telescopes can seldom provide resolution better than 1.0 arc-seconds while the Hubble's resolution is about 10 times better, or 0.1 arc-seconds. Not just because its a good camera but because it is up where it doesnt have to deal with the atmosphere.
And the cameras on the MGS do not rely on a good lens as much as they do rely on the electronics. It uses a linear array CCD which will scan the night sky one line at a time (much like a CRT actually). And it is not limited to the visible region of the spectrum. UV and near infrared have way too much information to give than just an optical picture.
Most the pics released finally are almost always digitally enhanced and represented in the visible region of the spectrum. The kids these days will not be fired up about astronomy if all they see is an output of wavelets in an array.
Except of course its not like TV at all which scans in horizontal lines.
The picture was scanned from the right hand side in vertical strips. As the picture is being scanned, the viewed Mars Odyssey is moving from right to left. The camera is also rotating from right to left.
Picture a car on the opposite side of the road moving in opposite direction flying by the side window of your car as you look out (90km) and then the camera rotating and "catching up" as it scans, with the same car out the back window (135km).
If you RTFA, you will see it mentioned that this picture and one other picture taken by the Mars Global Surveyor were the first, not that this particular photo was the first. The European Space Agency's Mars Express was the first extraterrestrial satellite imaged in this method. The Mars Express was imaged April 20, 2005, and it seems Mars Odyssey was imaged this month (can't see a date, I've looked several places).
My other sig is just as lame
The picture they posted here_ fig1.jpg
http://photojournal.jpl.nasa.gov/figures/PIA07941
does help a bit- the camera is rotating and because it builds up the image over time it sees (the relatively near by) object twice.
I guess when imaging the surface they have to rotate the orbiter/camera to account fo the fact the orbiter is moving relative to the planet surface faster than they can take the picture.
In a sense you are right, but there is a more obvious case. When the lander seperated from the command module during landing, they both filmed the other ship. Here it is not even on the ground but in mid space fligt.
-:) Oh no - not again.
www.rednebula.com
But hey.. What do I know..?
Remember, there are no stupid questions. But there are a lot of inquisitive idiots.
http://saturn.jpl.nasa.gov/news/press-release-deta ils.cfm?newsID=522
So I don't think that's exactly the first instance.
Well, I guess you could quibble.
-- ac at work (quibble, what a great word)
It was the Apollo 11 crew (Armstrong/Aldrin) that managed to land the Eagle with less than a minute of fuel remaining.
The Apollo 12 LEM was called Intrepid. It was quite a feat that Conrad & Bean set it down so close to the Surveyor probe. (Not to mention the amazing job done by the mission planners to place them so close to the target!)
Its the same frame exposed twice.
Score:5, Informative?? Score:-1, Factually Incorrect.
It is a single exposure, but the frame is sweep-scanned. The closest analogy I can think of is the way a fax machine sweepscans a page one line at a time. It's like scanning a fax and half way through you reverse direction of the rollers pulling in the paper. It would sweep-scan the same half of the page on the way out. The printed fax would be a single exposure, but the two halves would have the same thing twice.
A simpler (but really rotten) analogy would be a photo with a mirror across half a room. It's a single exposure, but everything appears twice.
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