10
$\begingroup$

JWST has a giant sunshield made from mostly parallel layers of metalized polymer film that are sufficiently tensioned so as not to touch each other, though I'm not sure how taut or flat they are:

I think it will be much, much more directionally reflective than white paint, but due to ripples and the metalization process itself, and the shape of the sail (is it nominally planar?) it will not behave optically as one giant perfectly flat mirror either.

Questions:

  1. How "specularly" and therefore spectacularly will JWST reflect sunlight?
  2. Will we be watching for JWST flares? If so, in what areas of the sky will JWST be looking at these times?
  3. and if so, with what will we be watching? Telescopes? Binoculars? Cameras? Naked eyes?

The first part is about the angular pattern of reflected light from the half-degree wide incident sunlight, the second is more about geometry; will and how often will the reflected light pattern intercept Earth.

The third part requires some quantitative treatment.


there's a discussion of calculations of reflections from wrinkly films on spacecraft in

also see:

$\endgroup$
3
  • 1
    $\begingroup$ For (3) at least, my answer to another question might be relevant... given its distance, the JWST is gonna appear Quite Small, so you'll need some serious optics to spot it. (2) is probably "no", but I can't answer (1) for you. $\endgroup$ Feb 4, 2022 at 13:48
  • $\begingroup$ @StarfishPrime Well if the sunshield were a 300 m^2 flat mirror, then it would be as bright as the Sun per square meter (ignoring $1/r^2$) and so $2 \times 10^{-16}$ as bright as the Sun, so I guess that makes it at most magnitude +12 when flaring? $\endgroup$
    – uhoh
    Feb 4, 2022 at 15:46
  • $\begingroup$ @StarfishPrime there's a new answer that seems to show a flare :-) $\endgroup$
    – uhoh
    Feb 18, 2023 at 22:34

1 Answer 1

9
$\begingroup$

How "specularly" and therefore spectacularly will JWST reflect sunlight? Will we be watching for JWST flares?

tl;dr Yes, some of us will, and it's up to the reader to decide if transition from invisible to +13.5 magnitude (estimated) in a minute or two spectacular.


Tracking the position of JWST using the Tycho-Tracker software for near-Earth objects, about half-way through a series of 60 second exposures between 03:10 and 04:07 UTC on 2023-02-15 there is a several minute-long flare in JWST's exact predicted location.

Screen shots are shown below and a video of all exposures is available in the Dropbox link. The flare can be seen about halfway through the video (which compresses the hour-long exposure into 9 seconds). It lasts for a few frames which corresponds to about 5 minutes, with a maximum in exposure 26.

Before/after the flare the JWST was not visible in these exposures using a 5 cm aperture. That's no surprise, this answer to What would JWST's apparent magnitude be? estimates +19.5 for JWST without flaring.

While both the flare and several stars in the field are significantly overexposed in these 60 second integration images, I did a visual comparison of the size of the overexposed spot against nearby stars, and I get an estimate of +13.5 magnitude at flare max.

comparison of the size of the overexposed spot to those of nearby known stars,


  • Telescope: 5 cm f4.8 SkyWatcher
  • Scale: 1.91 arcsec/pixel
  • Crosshair RA: 07h 53m 54.13s
  • Crosshair DE: +15° 19m 30.7s
  • Exposure: start: 2023-02-15 03:10:35
  • Exposure: stop: 2023-02-15 04:07:23
  • Exposure time: 60 sec/frame

Dropbox link to screenshots and video showing the flaring

Screenshots:

screenshot of observation of possible JWST flare

screenshot of observation of possible JWST flare

$\endgroup$
4
  • 3
    $\begingroup$ Having most of the relevant facts in a link is discouraged here; this answer would be greatly improved with an an image of the flare, estimate of its magnitude, etc $\endgroup$
    – Erin Anne
    Feb 18, 2023 at 18:42
  • 1
    $\begingroup$ This is great, thanks! You can also ask a follow-up question about the orientation of the JWST at this moment to help figure out what surface might have done the reflecting. I've added some of the information back into your answer post by clicking "edit". Feel free to update further. Welcome to Space Exploration Stack Exchange! $\endgroup$
    – uhoh
    Feb 18, 2023 at 22:06
  • 1
    $\begingroup$ Oh this is really cool! I noticed that there's a small drift in the star positions - about 1 arcminute per hour. Is your software tracking the real-time position of the JWST seen from your location on Earth? (orbital motion + parallax as Earth rotates) Also, are you able to estimate the magnitude? It may be over exposed, but perhaps the size of the blob can be compared to over exposed stars to get some rough estimate? $\endgroup$
    – uhoh
    Feb 18, 2023 at 22:32
  • 2
    $\begingroup$ Yes, the Tycho Tracker software tracks the position of the JWST in each exposure. I did a visual comparison against nearby stars, and I get an estimate of +13.5 magnitude at flare max. $\endgroup$
    – jhroof
    Feb 19, 2023 at 16:54

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.