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Hypothetically, if one were to point the James Webb at Earth:

  1. Could it be focused in such a way that the entire Earth was in it's FOV, or is it too close?
  2. If this were possible, what would be the smallest object that could be resolved in orbit?
  3. If this weren't possible, could it be focused in any way such that things in orbit could be resolved?

Bonus) If none of the above is possible, what satellite currently offers the best capabilities for imaging other things in orbit? Ideally with the widest possible FOV.

Yes, I know it's not optical, and yes I know it's not really designed for this, but I've got an idea and I'm wondering if it would even be plausible.

ScottishTapWater
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    If one were to point the (optical instruments at the) James Webb at Earth, we were to lose the telescope for good. – fraxinus Dec 01 '22 at 18:42
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    You may be interested in reviewing the JWST Target Viewing Constraints, which prohibit orienting it to view the Earth due to thermal requirements and the fact that it must forever be oriented to remain in the shadow of its own sunshield. See Video – Wyck Dec 01 '22 at 19:01
  • Near-infrared instruments could probably survive heating up from the Sun (the colder ones will almost surely break) and in theory could resume their operation after cooling them down and aligning the mirror again. On the other hand, if the instruments turn to Earth, both the solar panels and the communications gear will face the empty space. The telescope will lose power (it has no batteries) and any good advice it could otherwise get from Earth. – fraxinus Dec 03 '22 at 01:30
  • p.s. looking at Earth from where the Webb is right now is generally looking mainly at the Sun. One could do quite a lot of welding (and cutting) with a well-focused 40kW light beam. – fraxinus Dec 03 '22 at 01:44

2 Answers2

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Answer:

  1. No. JWST's NIRCam observes a 9.7 arcmin² field with a ~44" gap separating two 2.2' × 2.2' areas (or ~640 by 1350 miles at JWST's distance).
  2. Optical resolution is 0.1 Arcsec ( 0.5 miles at JWST's distance).
  3. No. ISS (the largest Earth satellite) is smaller than the optical resolution.

Figures for NIRCam from https://jwst-docs.stsci.edu/jwst-near-infrared-camera/nircam-instrumentation/nircam-field-of-view and https://webb.nasa.gov/content/about/faqs/facts.html

However, the NIRCam operates at 37K. If you swung JWST to look at Earth, the sensor would instantly warm to Earth nightside temperature of about 280K so you would get no pictures. If the nearby image of the sun fell on the sensor, it would be destroyed.

Woody
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    #3: I'd argue the moon is in orbit around earth, and a satellite... – vidarlo Dec 01 '22 at 17:16
  • @vidarlo I'd argue Earth and Moon are both in orbit around their mutual barycenter, and are effectively a double planet. /;0) – Woody Dec 01 '22 at 17:41
  • Not the worst world view... :) – vidarlo Dec 01 '22 at 17:58
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    @Woody - Isn't the barycentre of the earth-moon system well inside the earth? – ScottishTapWater Dec 01 '22 at 19:40
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    @ScottishTapWater You are right. I just feel sorry for the Moon. It don’t get no respect. The Earth/Moon system was formed by the collision of two planets, so I like to think of the system as a double planet. The earth is only 3.7X the diameter of the Moon, but weighs 81X as much because it got the metallic core in the collision. Like the favorite cousin who didn’t deserve the biggest inheritance. Not fair. – Woody Dec 01 '22 at 20:34
  • I wish it was just above the surface, would be way cooler to be a dual-planet system – ScottishTapWater Dec 01 '22 at 22:22
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    @ScottishTapWater Until you pointed it out, I didn't realize the definition of a double planet required the barycenter to be outside both planets. But Wikipedia agrees with you. Smarter every day. – Woody Dec 01 '22 at 22:35
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    @Woody Knowledge expansion does not equal more intelligent, which in turn does not equal smarter... I'm not saying that you are not smart, but gaining knowledge does not make you smarter by itself!</pedantic mode> – cmaster - reinstate monica Dec 02 '22 at 07:49
  • @cmaster-reinstatemonica I wasn't aware of that pedantic distinction ... smarter every day (this might become an endless loop) – Hagen von Eitzen Dec 02 '22 at 19:35
  • @cmaster-reinstatemonica It was an oblique reference to Destin Sandlin who runs the "Smarter Every Day" YouTube channel. I admire his almost child-like delight in learning new kewl stuff ... every day! – Woody Dec 02 '22 at 20:20
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Bonus) If none of the above is possible, what satellite currently offers the best capabilities for imaging other things in orbit? Ideally with the widest possible FOV.

Probably many military, weather and mapping satellites are capable of taking an image of another satellite, if the orbit is known and happens to align suitably.

However taking an image of the whole surroundings of Earth and zooming into satellites in that image is not possible with current technology. For example 1 meter resolution would mean an image of 12 000 000 x 12 000 000 pixels.

The Earth-based Vera C. Rubin observatory has a 3200 megapixel camera. This could cover up to 16 degree wide cone of the sky with 1 meter resolution at a 200 km distance. So I guess that would be the most capable instrument currently, while it is not a satellite.

The actual resolution needed will depend on what you want to do. If the camera is between sun and the satellite, it is possible to detect the reflection (also called satellite flare) with a much lower resolution camera. If you actually want to be able to identify the satellite in the image, you would need 0.1 meter or even better resolution.

jpa
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