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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.

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    $\begingroup$ If one were to point the (optical instruments at the) James Webb at Earth, we were to lose the telescope for good. $\endgroup$
    – fraxinus
    Dec 1, 2022 at 18:42
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    $\begingroup$ 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 $\endgroup$
    – Wyck
    Dec 1, 2022 at 19:01
  • $\begingroup$ 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. $\endgroup$
    – fraxinus
    Dec 3, 2022 at 1:30
  • $\begingroup$ 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. $\endgroup$
    – fraxinus
    Dec 3, 2022 at 1:44

2 Answers 2

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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.

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    $\begingroup$ #3: I'd argue the moon is in orbit around earth, and a satellite... $\endgroup$
    – vidarlo
    Dec 1, 2022 at 17:16
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    $\begingroup$ @Woody - Isn't the barycentre of the earth-moon system well inside the earth? $\endgroup$ Dec 1, 2022 at 19:40
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    $\begingroup$ @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. $\endgroup$
    – Woody
    Dec 1, 2022 at 20:34
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    $\begingroup$ @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. $\endgroup$
    – Woody
    Dec 1, 2022 at 22:35
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    $\begingroup$ @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> $\endgroup$ Dec 2, 2022 at 7:49
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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.

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  • $\begingroup$ The idea was to use it to detect space-debris by the silhuoettes $\endgroup$ Dec 1, 2022 at 19:45
  • $\begingroup$ @ScottishTapWater Silhouettes against what? The blackness of space or the sun? If the latter, diffraction might be quite limiting. $\endgroup$
    – jpa
    Dec 2, 2022 at 6:00
  • $\begingroup$ Well I was thinking against the reflected light off the Earth $\endgroup$ Dec 2, 2022 at 9:25
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    $\begingroup$ @ScottishTapWater Ah, well then the JWST is in completely wrong place to begin with, because the sun is always behind the Earth when viewed from L2. Satellites themselves reflect light also, so even from L1 it would be a bright point against bright background when viewed against the Earth. $\endgroup$
    – jpa
    Dec 2, 2022 at 9:49
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    $\begingroup$ "Probably many military, weather and mapping satellites are capable of taking an image of another satellite" See Could a spy satellite image another satellite? (has photos!) and At what magnitude would adjacent large geostationary communications satellites see each other? and Will WFIRST and JWST be able to resolve each other? $\endgroup$
    – uhoh
    Dec 3, 2022 at 10:53

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