Wikipedia:Reference desk/Archives/Science/2021 March 7

= March 7 =

Falcon 9 launch
In the wee hours of March 4, I was photographing the launch of a Falcon 9 from Cape Canaveral, from 180 miles north of there. About 5-6 minutes after launch, it became visible to me again and I took this 39-second exposure. I could see only a faint dot of the flame from the engines, but this photo shows some sort of purplish gas or something below it. I've gotten something like it once before and I've seen other photos showing it. What is it? Bubba73 You talkin' to me? 02:25, 7 March 2021 (UTC)
 * Twilight phenomenon perhaps? --jpgordon&#x1d122;&#x1d106; &#x1D110;&#x1d107; 06:01, 7 March 2021 (UTC)


 * Perhaps, but this happens when it is not too long after sunset or too long before sunrise (30-60 minutes), when sunlight is shining on it. This launch was at 3:24AM and sunrise was at 6:45AM.


 * I'm guessing that it is (1) dumping fuel or some gas, or (2) it is ionizing the gas in the ionosphere. But I'd like to know.  Bubba73 You talkin' to me? 06:36, 7 March 2021 (UTC)
 * Falcon 9 launches (and first stage landings) are continuously filmed from various cameras on the ground and several mounted on/in the first and second stages, and shown (initially live) on SpaceX's YouTube Channel, where they remain available thereafter (as well as on other space-related channels). Ordinarily it would be easy for you to watch the appropriate video and see what events and activities correlate with your exposure, but as it happens the Stage 1 downlink apparently failed this time around: however, you could watch some earlier Starlink missions to see similar Stage 1 events.
 * As a personal observation: the exhaust gases from such rocket launches spread more broadly as the rockets get higher because of the reducing atmospheric pressures they are expelled into, and in twilight as seen from those altitudes (per Jpgorden), the Sun's oblique illumination combined with a perspective unusual to ground watchers can give rise to some otherwise rarely seen phenomena.
 * Falcon 9 lanches do not generally involve 'dumping fuel', but after the Second stage has separated (around 2:40 from lift-off and 70km altitude on this occasion) and its single engine started (a few seconds later at about 75km altitude), the now-ballistic First stage adjusts its attitude using inert pressurised nitrogen jets and later (6:29 from lift-off at 55km altitude this time) re-ignites three of its nine engines in a re-entry burn (of about 20 seconds) to slow it down (and again for a low-altitude landing burn later). {The poster formerly known as 87.81.230.195} 2.125.75.168 (talk) 08:09, 7 March 2021 (UTC)


 * I've added a 198-second exposure, taken before the first one. It was taken about 180 miles north of the Cape.  It took more than 1 minute for it to become visible over the horizon.  It must to get to be hundreds of miles away in these photos and the cloud is perhaps miles wide.  Bubba73 You talkin' to me? 00:06, 8 March 2021 (UTC)
 * This was with a 20mm lens on a full-frame camera, so calculations can be done. Estimating that it is 500 miles away in the top photograph (5-6 minutes after launch from 180 miles away), that puts the width of the visible purple area about 5-6 miles wide.  Bubba73 You talkin' to me? 02:54, 8 March 2021 (UTC)
 * Clearly that bottom photo shows first stage (booster) flight through MECO and stage separation, followed by second stage flight.
 * Regarding the top photo, when the burn became visible to you again, did it last for about 20 seconds?
 * If so, that (and the ~ 6 min past launch timing) strongly suggests the second stage entry burn, as mentioned above.
 * (The stage one entry burn start-up and shutdown callouts were at T+6:28 and T+6:49 as heard on that video. The launch timeline on the SpaceX page (archived here) gave "1st stage entry burn complete" at T+6:43, but didn't give a start-up time; the burn probably started at about T+6:23.)
 * You say that the second exposure was for 39 seconds. Presumably you started after it became visible.  Did you continue for another 20 seconds or so after the dot of flame disappeared? -- ToE 15:58, 8 March 2021 (UTC)


 * As it was too early for the sun to be illuminating the exhaust plumes, I assume they were lit by the engine itself. The Merlin 1D when operating in a vacuum has a specific impulse of 310 s, which when given as an effective exhaust velocity is 3.0 km/s, so it will span your 5-6 miles width within seconds. Note that at the far right of the trail, where the photo started with the entry burn in progress, you can see an arc of illuminated exhaust plume which has already streaked away from where the stage had passed.
 * The three engines of the entry burn and the lower altitude explain its greater visibly compared to the fainter plume from the higher, single-engine second stage flight.
 * What I don't understand is why it is visible only below the line of flight. -- ToE 15:58, 8 March 2021 (UTC)


 * I can't recall about the 20 seconds. After MECO, I lost sight of it but I kept the shutter of the camera open.  After some time (I don't know how long) I spotted it again.  After it crossed over the palm tree, I knew it was out of the camera's field of view, so I swung the camera around for the photo at the top.  Knowing the details of the camera, measuring the photo, and estimating that it was 500 miles away, I calculated that it was 5-6 miles wide.  It seems to be that it would be extremely thin when spread out that much.  But also in the bottom photo (the first chronologically), it is getting wider as time goes by, perhaps because of lower atmospheric pressure?  Would the gas be glowing? Bubba73 You talkin' to me? 00:32, 9 March 2021 (UTC)


 * Bubba73, did you also photograph Thursday morning's flight? Are you planing to for Sunday morning's launch, scheduled for an hour and a half before sunrise?  As that flight gains altitude and heads to the northeast, at some point it is going to enter the daylight, though I don't know how far up the coast you will need to be to see that.
 * From you description, I now suspect that it wasn't the booster entry burn you photographed, but that it was just the upper stage which you reacquired. Proof would be in how long it was visible.  If the end of the streak on the left of that top photo corresponds to the end of the exposure, then it was the upper stage for sure.  But if it corresponds to the stage abruptly disappearing, and you continued the exposure for another 20 seconds or so, then that sounds more like the booster's entry burn.
 * As to why the purple glow? You pointed out that it's too early to be illuminated by the sun.  I mentioned that it could be illuminated by the engine(s), but I don't think that's correct.  I think it is simply the energized plasma in  the exhaust glowing as it spreads out.  Have you seen this photo of the June 2019 STP-2 Falcon Heavy mission? (Source tweet.)  It launched at 2:30AM, so it wasn't illuminated by the sun.  The more spectacular, top portion shows the interaction of the two booster boostback burns, but the bottom show the upper stage, and note the purple toward the outer arms of that exhaust plume.  (Those arms are a reflection of the combustion chamber construction!)  These flights are staging at about 80 km altitude where the atmospheric pressure is 10-5 bar, and while that's rather rarefied, the mean free path is still only half a centimeter, and I'm thinking that may quench the plasma quicker than later in flight.  (That half-centimeter is for interactions of the atmosphere with itself, and I think the higher temperature of the exhaust would increase it's mean free path length through the atmosphere, but the figures give a sense of scale.)  By 86 km (the maximum altitude accepted by this calculator) it is up to 1 cm, and by 100 km altitude it is up to 10 cm.  If you reacquired the upper stage at T+5m, it would have been at an altitude of 150 km where the pressure is another four orders of magnitude lower and the mean free path correspondingly higher, allowing it to retain its energy and glow longer as it spreads out.  (I'm going to feel really silly when you discover it is just some internal reflection in your camera!)
 * Question: Your upper photo was exposed for 39 s and the lower for 198 s, and both were taken at f/4 and 100 ISO, yet the palm tree appears similarly illuminated. I'd have thought a 5x exposure time would have been more noticeable.  Did you (or the camera) process them to achieve that similarity? -- ToE 14:55, 13 March 2021 (UTC)
 * To answer some questions, I did not photograph Thursday morning's launch (at 3:13AM). I was all prepared to go and do a similar shot from the same location, except about 10 degrees to the east to get more of it as it passed over the palm tree.  But around midnight I decided that I needed my sleep more.  I am not planning to do the one at 5:01AM tomorrow - bad time for me.  I usually do any night launch when it is clear enough and at a reasonable time. Another one is scheduled for about a week from now.  The previous one that I did also showed some of the purple, but not as well as this one.  I'll try to post it.


 * You are correct about the exposure. The lower photo (the first chronologically) is straight as it came out of the camera, except cropped. The upper photo (second chronologically) I had to raise the exposure in post processing.  Bubba73 You talkin' to me? 18:47, 13 March 2021 (UTC)


 * I have not seen that amazing photo. It must have been taken with a telescope.  Yes, the top (and the third) photos were after I reacquired the rocket visually. Bubba73 You talkin' to me? 05:53, 14 March 2021 (UTC)


 * BTW, the top photo is the full frame. The bottom photo is cropped, so it is enlarged with respect to the top one.  Bubba73 You talkin' to me? 00:34, 9 March 2021 (UTC)

I've added a third photo from an earlier launch. I quit exposing after I lost sight of it, then I saw the flame of the engine again and swung the camera around to take this time exposure. After I got it back to the darkroom, I saw the purple haze, which I did not see in person. This is with a 35-mm lens instead of a 20-mm, so it is enlarged by a factor of 1.75 w/r/to the top photo. Bubba73 You talkin' to me? 21:52, 13 March 2021 (UTC)

Can heavy water be used as a food preservative?
The stronger hydrogen bonds of deuterium compared to hydrogen derails biochemical processes, causing organism that take in too much heavy water to die. Can this be exploited to inhibit bacterial and fungal growth after harvesting of crops by watering crops with heavy water some time before harvesting? Count Iblis (talk) 03:08, 7 March 2021 (UTC)
 * That would depend on the relative toxicity of heavy water to bacteria and funguses vs. that to humans -- but even if this proves possible, it would still be far more economical to use salt for the same purpose (heavy water is very expensive!) 2601:646:8A01:B180:21E9:FB1D:6AAA:EDD3 (talk) 08:15, 7 March 2021 (UTC)
 * [Edit Conflict] Possibly, although its effects on the crop's ripening would also have to be taken into account. (I presume you've read Heavy water and the succeeding sections?) However, since (from cursory web searching) heavy water costs several tens of US dollars per pound to produce, I'd think it would be prohibitively expensive even if it did work. {The poster formerly known as 87.81.230.195} 2.125.75.168 (talk) 08:21, 7 March 2021 (UTC)
 * Swapping H for D in biological systems does not really "derail" processes, if by that you mean "stops them in their tracks". The kinetic isotope effect just means that the rate of reaction is (usually) slowed. This can have real consequences, as discussed in . One thought experiment (there are likely to be actual examples but I've not tried to find the refs) is to replace one or a few H for D in a drug or agrochemical with the intention of extending its half-life by altering the metabolic processes acting on it. For example, in a phenyl ring one point of oxidative metabolic attack is in the positions of highest electron density. Putting a D instead of H there would likely extend the useful lifetime. Or a methyl group subject to oxidation could be swapped for a CD3 group. The costs of doing that would not be prohibitive if the active ingredient was thereby given some useful improvement. Such tactics are well known where fluorine is the atom substituted for the H but one problem is that other properties of the fluorinated material may differ too much from the original material. Mike Turnbull (talk) 12:17, 7 March 2021 (UTC)
 * Found a reference: Mike Turnbull (talk) — Preceding undated comment added 12:49, 7 March 2021 (UTC)
 * feel free to expand our Deuterated drug article:) DMacks (talk) 07:37, 14 March 2021 (UTC)
 * You might be interested in Di-deuterated linoleic acid ethyl ester. This has primarily been studied as a drug, though. --47.152.93.24 (talk) 16:57, 10 March 2021 (UTC)

Charge a black hole
I understand that having a black hole with a significant charge is p~0 in practice. But suppose that you could dump somehow a few stellar masses of electrons into a black hole. Could you by adding enough charge overcome the black hole's gravitational cohesion? 93.136.167.229 (talk) 05:17, 7 March 2021 (UTC)
 * I don't think so. Gravity overcomes the strong force in black holes, so it should easily overcome the electromagnetic force. Mike Turnbull (talk) 12:39, 7 March 2021 (UTC)

Black holes become electrically charged when charged objects fall into them falls into them. Physicists calculate that black holes have an “extremal limit,” a saturation point where they store as much electric charge as possible for their size. When a charged black hole evaporates and shrinks in the manner described by Hawking, it will eventually reach this extremal limit. It’s then as small as it can get, given how charged it is. It can’t evaporate further. JumboWriter (talk) 12:53, 7 March 2021 (UTC)


 * Very interesting article! So it's conceivable that by adding enough electrons (which have Q > M) we might be able to create a Q > M black hole which wouldn't evaporate (at least via Hawking radiation), or we might not even be able to stuff those electrons into the hole? 93.136.3.148 (talk) 09:50, 8 March 2021 (UTC)


 * According to Reissner–Nordström_metric there is the maximum possible charge above which the black hole cannot exist. Ruslik_ Zero 18:07, 7 March 2021 (UTC)
 * If I have made no mistake, that maximum capacity is given by:
 * $$\left|\frac{Q}{M}\right|\leq\left(\frac{G}{K}\right)^{\frac{1}{2}}\approx 8.6175\cdot 10^{{-}11}\,\mathrm{kg}^{{-}1}\mathrm{C},$$
 * where $$G$$ is the gravitational constant and $$K$$ is the Coulomb constant. So suppose now we have managed to charge a black hole right to its negative limit, and try to dump one more electron in it. If that dump cannot succeed, how does this impossibility manifest itself? As some kind of resistive force? Or is it possible (as a gedankenexperiment) but does the event-horizon singularity then cease to be? What does theory tells us here? --Lambiam 11:30, 8 March 2021 (UTC)
 * Wow, that's a very small number - going from the data at the electron article, the electron carries about 176 GC/kg. That would mean we only need to dilute the black hole with 1 in 2 × 1021 parts by mass of electrons to get to this limit. If I didn't make any errors that'd be a million tons of electrons for every solar mass. 93.136.3.148 (talk) 11:50, 8 March 2021 (UTC)
 * For an electron $$e/m \sim 10^{12}$$ and therefore the electrical repulsion from the critical black hole will far exceed the gravitational attraction. So, it is impossible to add any electron to such a black hole. Moreover the article says: "Objects with a charge greater than their mass can exist in nature, but they can not collapse down to a black hole, and if they could, they would display a naked singularity". Ruslik_ Zero 18:23, 9 March 2021 (UTC)
 * Does the electric force appear to tend to infinity (for an outside observer) as the electron approaches the event horizon? Otherwise, one could try to aim a high-velocity electron at the black hole. --Lambiam 23:46, 9 March 2021 (UTC)
 * I suppose you can but the ratio of the charge to the relativistic mass of the electron will be much smaller than 1012. The end result will be that the charge-to-mass ratio of the black hole will actually decrease after such a fast electron is added. Ruslik_ Zero 12:43, 10 March 2021 (UTC)
 * If it is only possible to add a charged particle to a charged black hole (of the same polarity) if the absolute value of the repulsive electric force $$F_e$$ does not exceed the attractive gravitational force $F_m$, or, equivalently, $ we can do a simple naive calculation. Let $$q$$ and $$m$$ be the charge and mass of a particle, and let $$Q$$ and $$M$$ be the charge and mass of the black hole. As noted above, then $ Using the classical equations
 * $$|F_e|=K\,\frac{q\,Q}{r^2},~F_m=G\,\frac{mM}{r^2},$$
 * we then have:
 * $$\left|\frac{F_e}{F_m}\right|=\frac{K}{G}\cdot\left|\frac{q}{m}\right|\cdot\left|\frac{Q}{M}\right|=\frac{K}{G}\cdot\left|\frac{q}{m}\right|\cdot\left(\frac{G}{K}\right)^{\frac{1}{2}}$$
 * $$\,=\left|\frac{q}{m}\right|\cdot\left(\frac{G}{K}\right)^{{-}\frac{1}{2}}\leq 1,$$
 * so
 * $$\left|\frac{q}{m}\right|\leq\left(\frac{G}{K}\right)^{\frac{1}{2}},$$
 * which implies that
 * $$\left|\frac{Q+q}{M+m}\right|\leq\left(\frac{G}{K}\right)^{\frac{1}{2}}.$$
 * Thus, it appears that nature forcefully guards against overcharging. --Lambiam 20:00, 11 March 2021 (UTC)

Why is the outline of heatshield for Perseverance (rover) look so odd?
I am asking outline of heatshield for Perseverance (rover) look so odd:

IMAGE

Odd means it's outline of heatshield look like old retro photo look. Rizosome (talk) 08:43, 7 March 2021 (UTC)
 * What's the provenance of the image? Where does it come from?  -- Jayron 32 19:24, 8 March 2021 (UTC)


 * Additionally, what is it supposed to look like? --OuroborosCobra (talk) 19:31, 8 March 2021 (UTC)


 * It is a still from a video released by NASA, which can be seen here. The still is a frame at 00:20+ into the video. --Lambiam 21:31, 8 March 2021 (UTC)
 * If I read the article correctly, they used some off-the-shelf sports cameras (read GoPros). The graininess of the image could be explained by any number of ways then including the normal graininess one gets when doing image capture off of a video.  -- Jayron 32 15:43, 9 March 2021 (UTC)
 * Yeah, seems to be the case. A few things to note. First, the view of the heat shield is the inside of the heat shield (facing "up" during entry), and not the ablative side. That would explain the aluminum foil looking material and what look like some sort of wiring. Second, as you said, the camera was an off-the-shelf sports camera, like a GoPro. Based on the footage, it looks like it was a camera intended to be used just once during the mission, to film the footage that we saw. Given that, NASA/JPL wasn't likely to spend the money or the mass on a really high quality camera. Why bother if the footage you get from a cheap one is good enough, and the thing only has to last for a single use? --OuroborosCobra (talk) 19:17, 9 March 2021 (UTC)


 * It looks like an almost-in-focus image of a high-contrast object against an out-of-focus, low-contrast background.
 * Here are some images of that heat shield on Earth:
 * Here is a similar photo, but unlike the one linked in the question which is a still from the Rover Descent Camera, this is an image from the B&W Lander Vision System Camera (part of the Terrain-Relative Navigation system), which was hand colored by NASA's Doug Ellison. (That tweet also shows the B&W original.) With a wider angle and being focused on the terrain (in order to do its job), it gives a good view of the Jezero Crater that they were descending into.
 * For a description of the cameras, see The Mars 2020 Engineering Cameras and Microphone on the Perseverance Rover: A Next-Generation Imaging System for Mars Exploration. -- ToE 20:24, 9 March 2021 (UTC)

human / plant symbiosis
Does the symbiosis between humans and plants have a term? Nothing about symbiosis is mentioned in Agriculture, also the symbiosis is older than agriculture. How would one find papers about the subject (as seen from the biology side)? --SCIdude (talk) 09:17, 7 March 2021 (UTC)
 * There is a notion of "service-resource mutualism" in which the symbiont species can exist independently but benefit from the interaction. This is more general than an animal–plant symbiosis, though; cleaner fish and the fish they clean are an example of animal–animal mutualism. Fungi are not plants but can be cultivated in a similar way; a non-vertebrate animal–cultivee example is ant–fungus mutualism. This is more generally referred to as "cultivation mutualism". What type of symbiosis is the human–plant symbiosis older than agriculture you refer to? Which plant species were involved? Some nomadic peoples practice a limited kind of agriculture, planting or sowing in order to harvest on a later return. This practice is probably older than sedentary agriculture, but still is a form of agriculture. --Lambiam 11:08, 7 March 2021 (UTC)
 * It's worth noting that, within agriculture, the interaction between humans and plants takes a specific form: namely, humans nurture and protect particular genetic lineages, rather than individual plants (we only protect individual plants up to the point where we eat them, so at an individual level, the relationship could be viewed as parasitic, but at a genetic level, it could be seen as mutually beneficial). PaleCloudedWhite (talk) 11:30, 7 March 2021 (UTC)
 * Still it's the specific plant species that, in whole, gets the benefits from e.g. transportation, because those lineages can escape control after they are transported. I also don't think fungus cultivation is equivalent because we can live without fungi but not without plants. --SCIdude (talk) 15:20, 7 March 2021 (UTC)
 * Fungi are great nutrient recyclers and have very close relationships with plants, and without fungi I would expect most productive ecosystems to cease functioning as they do now, so in that indirect sense we probably can't live without fungi. PaleCloudedWhite (talk) 15:33, 7 March 2021 (UTC)
 * We certainly couldn't live without Saccharomyces, which is responsible for beer, and beer is necessary for life! -- Jayron 32 19:32, 8 March 2021 (UTC)