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July 4[edit]

Ironman vs. The Martian[edit]

At the end of The Martian, Mark Watney makes a hole in his glove (an implausibility on its own, as I assume spacesuits aren't really that easy to puncture) to reach the height of his passing ride. It seems to me that the escaping air wouldn't give a grown man+suit all that much thrust. Roughly how much would it actually impart? Clarityfiend (talk) 04:12, 4 July 2022 (UTC)[reply]

Note that in the original book, this is suggested but rejected as unworkable. It's not even about the thrust or lack thereof, it's the off-axis thrust and lack of controllability. Fgf10 (talk) 06:50, 4 July 2022 (UTC)[reply]

First a generic formula. Let

m

be the mass of the astronaut+spacesuit,

V

the volume of the escaping air and

v

the speed at which it escapes. The mass of the escaping air, using

1.2 kg/m 3

as its specific density, is then

1.2 V kg/m 3

, and if all escapes in the same direction, its total momentum equals

1.2 Vv kg/m 3

. By the conservation of momentum, the speed of the astronaut is then

1.2 Vv / m kg/m 3

. For the rest we can only do ballpark calculations. NASA gives the weight of a spacesuit as approximately 280 pounds.^[1] Let us assume dramatically improved technology and put

m = 120 kg

. If

V = 0.1 m 3

, which seems high, and the air escapes at

10 m/s

, the resulting speed for the astronaut equals

0.01 m/s

. --Lambiam 08:40, 4 July 2022 (UTC)[reply]

I've only seen the movie once and it's some time ago. Spacesuits are normally filled with pure oxygen at around 30 kPa of pressure, so that density is a bit lower. Estimating the velocity of the escaping gas is a bit hard. I don't expect Bernoulli will give us a very accurate number, but it suggests speeds on the order of 100 m/s are energetically allowed. Still, with such a poor nozzle, 10 m/s seems more plausible. The hero could only use part of the gas in his spacesuit, or he would loose consciousness due to oxygen deprivation.

With such a low thrust, controllability doesn't look like a very serious problem. There's enough time to compensate by moving your hand around. PiusImpavidus (talk) 09:24, 4 July 2022 (UTC)[reply]

About what I figured. Watney asphyxiates in orbit to the tune of "Don't Leave Me This Way" ("I can't survive, can't stay alive ..."). Clarityfiend (talk) 10:13, 4 July 2022 (UTC)[reply]

Millipede and centipede gaits[edit]

Millipedes walk with a gait characterized by waves of legs propagating from the back to the front, as can be seen in this 30-second video: https://www.youtube.com/watch?v=rlIwoiO24XI

Centipedes I've run across walk this way also.

However, some centipedes (such as those from the scolopendra genus) walk with a gait having leg waves propagating from front to back.

Here are simulations I made of both gaits. I modeled these with OpenSCAD. Each foot spends 50% of the time on the ground and each segment is 45° out of phase from the adjacent segment. The only difference between the two animations is the sign of the phase shift.

"Normal" back-to-front waves (like a millipede)
A centipede gait with front-to-back leg waves

I'm curious how these gaits evolved. I notice that even 4-legged creatures walk with the hind legs leading the front legs on the same side, as can be seen in the animations in the gait article. If I crawl on my hands and knees, I can crawl in either manner if I think about it, but it feels more natural for my legs to lead my arms on the same side, just like a four-legged animal.

I can see an advantage to each centipede gait:

The back-to-front gait has the feet more evenly distributed on the ground.
On the other hand, the front-to-back gait has feet more or less touching the same place, which would be useful for traversing over small obstructions.

An explanation I found in this paper from Nature, which studied scolopendra centipedes, suggests that neural impulses travel from the head to the tail, which makes intuitive sense to explain the front-to-back waves, but not the back-to-front waves. That paper also found that cutting the nerve connection between two segments impeded the propagation of impulses for a wriggling swimming motion but the legs still figured out walking from mechanical contacts in spite of being severed from the brain.

Has anybody seen any explanation for the difference? Unfortunately I was unable to find anything about this on Wikipedia. The gait article is good, but its main focus is on tetrapods. ~Anachronist (talk) 03:20, 4 July 2022 (UTC)[reply]

Wouldn't back-to-front be easiest to implement with a nerve to the rear and back that transmits at the speed of the wave? Then if it wants to stop it can't even start trying till an entire brain-to-butt signal has travelled while in front-to-back all the legs would've got the message already. My bad you mentioned that already. I've always wondered what gait cockroaches run. And if the milder waddlers hit some sort of vibrational resonance or something or maybe just didn't get enough copies of the evasive maneuvers gene. Which probably doesn't hurt it much vs no evasive maneuvers gene as a line doesn't have to be very straight to be almost as short. Sagittarian Milky Way (talk) 14:48, 4 July 2022 (UTC)[reply]

Web-searching "Centipede gait" throws up a number of references, the first one being here.

Perhaps this and other hits will have some insight on the question, but finding evolutionary explanations for complex anatomical details and especially behaviours (which do not fossilise well) run the risk of approaching 'Just-so stories.' {The poster formerly known as 87.81.230.195} 90.195.174.88 (talk) 17:53, 4 July 2022 (UTC)[reply]

Thanks. I had done the same search and found many papers about robotic walking applications, which wasn't really what I was looking for, but it seems I didn't look carefully enough because that paper you reference seems to derive the two gaits from sensory inputs rather than simply coming up with a way to reproduce the gaits mechanically. ~Anachronist (talk) 20:44, 4 July 2022 (UTC)[reply]

Our articles on centipedes and millipedes fail to mention that, when walking, they move their legs in a metachronal wave. YouTube videos I found of walking centipedes showed a front-to-back wave; are you sure you observed centipedes with a back-to-front wave? These nice animations are worth adding to the section Terrestrial locomotion § Number of legs. --Lambiam 22:01, 4 July 2022 (UTC)[reply]

@Lambiam: Yes, centipedes can be found that walk in either fashion. Here's one with a back-to-front wave: https://www.youtube.com/watch?v=LAYzFAHHntI - in fact the first videos I found were of centipedes walking this way. ~Anachronist (talk) 13:54, 8 July 2022 (UTC)[reply]

I think they do it to leave "slack" to allow them to rear up or lunge forward. Abductive (reasoning) 19:01, 5 July 2022 (UTC)[reply]

@Anachronist: Those simulations look really nice! I know nothing about OpenSCAD, but looking at this, I believe the source should be some textual code. Can I encourage you to copy the source in the description of the files, so that someone else can reuse it? (See WP:PIFU: If the image was generated from a script [...], the code should be included so minor improvements like labels and color adjustments can be made.) Tigraan^{Click here for my talk page ("private" contact)} 09:14, 8 July 2022 (UTC)[reply]
@Tigraan: I'd have to rewrite it as a single file to exclude the two libraries of my own making, for the geodesic spheres and hemispheres. But then if I did that, you wouldn't get exactly those images. ~Anachronist (talk) 13:54, 8 July 2022 (UTC)[reply]
@Tigraan: I have modified the source code to eliminate my libraries, and included the source code in File:Centipede forward propagation.gif. The source code is the same for both images; the only difference is the sign of the phase offset variable. ~Anachronist (talk) 15:50, 8 July 2022 (UTC)[reply]

Catalogued stars that went supernovae[edit]

Other than typical SN designations which are assigned after explosions, List of supernovae and a quick look into related category doesn't mention any catalogue names of related stars, including Bayer and Flamsteed designation. Why is that? 212.180.235.46 (talk) 19:39, 4 July 2022 (UTC)[reply]

It seems that thus far only two observed supernovae were earlier observed as regular stars: SN 2020tlf and SN 2020fqv. If they had no catalogue names, it may have been because none had been assigned. The Hubble Space Telescope has made over a million observations; I suppose the majority of observed astronomical objects has not been catalogued. --Lambiam 21:34, 4 July 2022 (UTC)[reply]

SN 2012Z, not on our list, has also an earlier observation as a regular star, specifically as a white dwarf.^[2] --Lambiam 00:00, 5 July 2022 (UTC)[reply]

But that 'observation' was merely its (extremely faint) presence in images of its host galaxy amongst millions of other stars (etc.) in the same images: it hadn't been individually catalogued, or probably noticed in any way.

As you probably know (but others may not), nearly all (observed) supernova precursors that we know about were white dwarfs; a tiny proportion were massive stars, and there are (currently) 2 suspected 'electron capture supernovae' that may have been on the verge of becoming white dwarfs.

White dwarfs are (by stellar standards) very small and therefore intrinsically faint, so only those quite close to the Solar system appear bright enough to have been conventionally observed and entered in "traditional" catalogues, though modern automated surveys will contain many within their data that have not (yet) come to any direct human attention. {The poster formerly known as 87.81.230.195} 90.195.174.88 (talk) 04:36, 5 July 2022 (UTC)[reply]

As I understand the paper, SN 2020fqv had prior observations of the same patch of the host galaxy, which were useful for constraining the properties of the progenitor star, but the progenitor star itself was not detectable. For 2012Z, the white dwarf was not detectable, but there was a "luminous, blue source that we have interpreted as a helium-star companion (donor) to the exploding white dwarf." SN 2020tlf does appear to have a pre-SN detection of the progenitor star, since it started to flare brightly before the SN. --Amble (talk) 18:25, 5 July 2022 (UTC)[reply]

Almost all the supernovae we see are in other galaxies, even distant ones; a supernova in the Milky Way has not been seen for hundreds of years. But star catalogs are generally for Milky Way stars. See Supernova#Milky_Way_candidates for some stars within the Milky Way that could produce a supernova in the future. --Amble (talk) 06:12, 5 July 2022 (UTC)[reply]

The one exception is SN_1987A, which was in a satellite galaxy of the Milky Way. The progenitor star, Sanduleak_-69_202, was detected and catalogued in 1970. I think this is the only one where the progenitor star itself was detected and catalogued before it began any activity related to the supernova. --Amble (talk) 06:27, 5 July 2022 (UTC)[reply]