2564: Sunshield

Explain xkcd: It's 'cause you're dumb.
Jump to: navigation, search
RIP the surface of Mars
Title text: RIP the surface of Mars


This is another comic with a Fact, though not a Fun fact - this time an Astronomy fact. The next comic with a fact, namely 2596: Galaxies, was also with an Astronomy fact. This is the first time that the field that a fact pertains to has been immediately repeated.

JWST stands for James Webb Space Telescope, a space telescope that was launched 12 days prior to publication of this comic, see more details here 2559: December 25th Launch.

It has a sunshield to protect its instruments from the heat of the sun and to keep them below 40 K (-233 °C/-388 °F). Deployment of the sunshield was completed the day before the comic was published. The JWST has to undergo a complex sequence of deployment steps to unfold parts that had to be packed tightly for launch. This sequence has 344 possible points of failure that would render the very expensive space telescope useless; 75% of them led up to the successful full deployment of the sunshield. Thus successful steps are widely celebrated, with this comic an example of such a celebration.

Ordinary cameras use a flash to take pictures in low-light situations. Outer space is very dark [citation needed] (one of the JWST's mission objectives will help astronomers calculate exactly how dark), so this comic posits that the JWST has a very powerful flash to compensate for this. Most astronomical cameras don't use flash photography [citation needed] -- they depend on the light either emitted by objects themselves (e.g., stars) or from nearby very bright objects (e.g., Solar System planets will reflect the Sun's light, while distant clouds of gas and dust may be largely illuminated by the light of supernovae or recently formed stars within or near them). A flash generally doesn't work for many reasons:

  • It would take too long for the light of the flash to return to the telescope - at least twice the time that it had already taken for the original image to arrive on its own.
  • The clicking sound indicates that the JWST is using a camera with mechanical parts that are moving in order to take a picture. This can be a mirror of a single lens camera, where the clicking noise is produced by the mirror moving away to allow the light to reach the sensor. Alternatively the shutter in front of the sensor produces a clicking noise when it is opening to start the exposure. In the latter case the opening of the shutter would happen before the flash is emitted, so light from the flash wouldn't even reach the camera's image sensor. It is however possible that the camera is using a time exposure and that the shutter was still open when the flash occurred.
  • Not enough 'flash' light would return due to it uselessly spreading in all directions. Instead, in a telescope mirrors and/or lenses focus the light, and long exposure times are used to collect enough of the current light to form a decent image.
  • A flash powerful enough to overcome the previous difficulty would have to be inordinately powerful. This would raise significant questions about powering it, its destructive effect on JWST, and its damage to (or at least disturbance of) many of the things the flash would be able to illuminate.
  • The objects and phenomena of interest of JWST are, by dint of their extreme distance, being seen as they were in the early stages of the universe, and emit light that potentially gives vital clues about that era, only marginally this side of the current visible-horizon of our apparently expanding universe. Should our flash ever reach them (assuming they still exist) and we have the patience to await the return (assuming we still exist), this will only reveal the much older versions of whatever they have become and only in the form of light that we have swamped them with.
  • Before this, any intervening civilization that possesses (or can still develop) the necessary capabilities will have at the very least responded, if not retaliated, to the original flash. Their response might be far less humorous.

There are some examples of astronomical research done using things similar to a flash. Radar astronomy involves emitting radio waves (microwaves) that bounce off distant planets, asteroids, comets, etc., and analyzing the returned waves. The Lunar Laser Ranging experiment uses lasers, which are loosely related to flashes for photography, to measure the distance between Earth and Moon. The outward light is concentrated upon the approximate area of the lunar target, which employs an optical trick to send most of that which actually struck it back to the approximate area of the source equipment.

The comic assigns the sunshield a new, comical purpose of shielding the Sun (and Earth ,which is roughly in the same direction as the Sun, due to the deployment at the L2 Lagrange point) from this flash, rather than the other way around. When the camera is taking a picture, the comic shows space in front of the shield lit up while there is a totally dark shadow behind the shield (in the direction of Earth and Sun).

The comic also has the camera making a "click" sound. In traditional mechanical cameras without a mirror, this sound comes from the shutter opening and closing, and mirror-less digital cameras mimic this sound so the user (and subject, when human) knows when the picture is being taken. JWST won't actually click -- it doesn't have a shutter, as it takes long-exposure digital images, and in space no one can hear you click.

The telescope also tells the universe to smile for the picture. The universe doesn't have a mouth to smile with[citation needed], although there are a number of features both on Solar System objects and in deep space that look like faces; this is a phenomenon called pareidolia. The most well known is the Man in the Moon, but there are numerous others both in the Solar system, most famous is probably the Face on Mars and out among the galaxies, like the Cheshire Cat galaxy group named after the Cheshire Cat from Alice in Wonderland.

The title text suggests that, due to the sunshield not being angled to shield Mars, Mars's surface has been badly scarred by the flash. This implies incredible strength of the flash, perhaps to ensure the light can return from its destinations, comparable to death-ray satellites in fiction.


[The James Webb Space Telescope (JWST) is floating through space, shown in white on a pitch black background. The two mirrors are seen in front of the sunshield, which is kite shaped. A white line goes from the telescope up to two lines of white text, connected with a small white line.]
JWST: Okay, universe-
JWST: Smile!
[Same setting, but now only a small thin white line goes up to a line of white text representing a sound made by the telescope.]
JWST: Click
[Same setting, but now it turns out that a small bulb on the front of the telescope is a flash light. A bright flash glows from the bulb, turning most of the panel white. A cone on the left side is blocked and kept pitch black by the telescope's sunshield. The light fades a bit towards the edges of the picture, giving the light cone a rounded appearance. Thus the image actually looks a lot like Pac-Man in the process of eating the telescope.]
[Caption below the panel:]
Astronomy fact: The purpose of the JWST sunshield is to protect the Sun and the Earth from the telescope's powerful flash.

comment.png add a comment! ⋅ comment.png add a topic (use sparingly)! ⋅ Icons-mini-action refresh blue.gif refresh comments!


i thought this is common knowledge. 15:00, 5 January 2022 (UTC)

It's normally supposed to block sunlight from reaching the telescope. This comic turns this on its head by suggesting the telescope emits light instead of collecting it. The emitted light is claimed to outshine the sun. The title text means that the flash is bright enough to scar the surface of Mars, which is unlikely. 15:12, 5 January 2022 (UTC)

According to wikipedia it is about preventing heat from reaching the telescope and only secondary about the light. -- 15:16, 5 January 2022 (UTC)
The comic mentions the role of blocking light which is why I mentioned it's light-blocking property. 15:34, 5 January 2022 (UTC)
Obviously, the comic distorts the facts to make it funny. -- 15:33, 5 January 2022 (UTC)
The point of the title text is that if the flash really were bright enough to serve its purpose, it would scar the surface of Mars (when Mars happens to be in that general direction). -- Barmar (talk) (please sign your comments with ~~~~)
SCAR? For the flash being bright enough, it would need to outshine supernovas. Mars would melt if not sublime. -- Hkmaly (talk) 00:24, 6 January 2022 (UTC)

Also, the JWST is drawn here to resemble the old Polaroid Land SX-70 Instant Camera (of just about 50 years ago.) Hmmmm. "Instant" raises issues of relativistic simultaeity....

Worse than that, shutter-click and then a flash (at least a frame of time apart) isn't that useful.
Trigger the flash and then the shutter around the time the flash returns. Depending upon distance and depth-of-field involved (also the duration of the flash and the movement of the subjects) you might have a longer shutter-open taking in passive light already, but only if you're happy mostly registering the 'bounce-back' from any given distance as the overwhelming flash gets there and back and don't mind the low amount of non-flash light leaching in, perhaps revealing motion.
(See various planetary nebulae photos where 'shells' of illumination, from subsequent flareups in the active centre, make it look like there's shells of matter, when it's more that these are volumes of debris that were in the right place at the right time to give us the current reflected glory. It makes for interesting mind-experiments.) 18:39, 5 January 2022 (UTC)

The JWST doesn't have a physical shutter, but makes a clicking sound so that it can't secretly take voyeuristic photographs of the universe. 21:24, 5 January 2022 (UTC)

Searching for facts I found three comics with facts in the title, and also one other using facts like this ones Astronomy Fact. I have thus added this: Category:Facts, with Category:Fun fact as a sub category. I have only found 5 comics so far. But where "facts" is only used a limited time, then "fact" is used more than 500 times. And the two with fact that are not in the title only used the word "fact". So if you can remember any other comic with facts that are not fun facts, then please add them to the category. As I wrote on the fact page this reminds me of the Category:Tips under which Category:Protip now belongs. --Kynde (talk) 08:48, 6 January 2022 (UTC)

I have also added all JWST comics to Category:Space probes and wrote a bit about it in the collapsed explanation there. But may be with 6 comics (and more on the way?) it should have it's own category. What do people think about that? --Kynde (talk) 09:05, 6 January 2022 (UTC)

Wait, the JWST is NOT a space probe. It is a space telescope for imaging purposes. So I disagree with putting it in that category. It already fits in the category for telescopes. 19:01, 6 January 2022 (UTC)

Concerning the "click"-part in the explanation: If the JWST is using a reflex camera, the clicking noise is the mirror moving out of the way for the exposure, is producing a clicking noise far louder than a shutter mechanism and obviously happens before exposure/flashing. 07:45, 10 January 2022 (UTC)