Wikipedia:Reference desk/Archives/Science/2020 September 28

= September 28 =

Near-rectilinear halo orbit
What is a Near-rectilinear halo orbit? The article says what it might be used for, but doesn't say anything about what it is. Handschuh-talk to me 00:35, 28 September 2020 (UTC)
 * A halo orbit but more rectilinear. Sagittarian Milky Way (talk) 01:03, 28 September 2020 (UTC)
 * What would we do without you! —Tamfang (talk) 01:29, 29 September 2020 (UTC)


 * The linked ESA video in the article's 'External links' section depicts a Lunar NRHO and also describes it textually thus:
 * "Instead of orbiting around the Moon in a low lunar orbit like Apollo, the Gateway will follow a highly ‘eccentric’ path. At is closest, it will pass 3000 thousand km from the lunar surface and at its furthest, at 70 000 km. The orbit will actually rotate together with the moon, and as seen from the Earth will appear a little like a lunar halo."
 * This means it will look like an elongated oval with the Moon towards one end, and will remain face-on to Earth. {The poster formerly known as 87.81.230.195} 2.219.33.80 (talk) 03:21, 28 September 2020 (UTC)
 * So that sounds like it's not a halo orbit at all? It's directly orbiting the Moon, not a Lagrange point, right? Handschuh-talk to me 03:53, 28 September 2020 (UTC)
 * I guess the term "near-rectilinear" is a variation on the term "almost rectilinear" introduced in the 1984 paper "Almost Rectilinear Halo Orbits" by Kathleen Howell and John V. Breakwell. In that paper they use the term "halo orbit" for any of the "periodic orbits emanating from the general vicinity of any of the 3 collinear Lagrangian libration points", abandoning the notion that the orbit is orbiting an L[1..3] point. This is a still image visualizing the planned orbit as seen from outside the Earth–Moon system. And here is a video visualizing it.  --Lambiam 08:07, 28 September 2020 (UTC)
 * I came across that video before and found it very good at illustrating what the orbit looks like. It sort of vaguely shows how it relates to actual halo orbits, glossing over the mathematics behind it (which I'm happy with), but also glossing over how they can still call the orbit a "halo orbit" when the object is orbiting the barycenter of the system, not the L2 point that they start the explanation from. If the influence of the further primary's gravity (i.e. earth in the example from the movie) is what keeps the orbit "rectilinear", then I guess I can see how they trace the thinking from the concept of the L2 halo orbit through to the NRHO, but it still seems like it's no longer meeting the definition of a halo orbit. Handschuh-talk to me 09:41, 28 September 2020 (UTC)
 * The object goes around the barycentre in an inertial frame, as is true for all halo orbits. But what is its orbit in a rotating frame that keeps the Earth–Moon line stationary? --Lambiam 10:06, 28 September 2020 (UTC)
 * "The object goes around the barycentre in an inertial frame, as is true for all halo orbits." I don't see how this can be true. Take for example the orbit of the James Webb Space Telescope around the Sun-Earth system, shown here. If I imagine the inertial reference frame wrt to the Sun-Earth system, with the sun, earth and L2 point all held stationary, the JWST would just oscillate around the L2 point, never going around the barycenter, which would still be well within the sun. Handschuh-talk to me 08:05, 29 September 2020 (UTC)
 * Wait, I think I'm imagining a rotating frame, not an inertial frame. So if I correct that and imagine a truly inertial reference frame, I guess I can see what you're saying insofar as eventually the JWST goes around the barycenter of the Sun-Earth system (roughly once per year). So to translate my confusion into more exact language: in the rotating sun-earth frame the JWST doesn't circle around the earth, but the ISS does. The JWST is circling around the L2 point, which is what makes it a halo orbit. How is an object in a NRHO of the earth considered to be a halo orbit, when it's not orbiting the L2 point? It seems like it's doing something much more like what the ISS is doing (except its orbit is more or less polar). Handschuh-talk to me 08:25, 29 September 2020 (UTC)
 * The JWST orbits around the L2 of the Sun–Earth system, and in an inertial frame it goes around the barycentre of the Sun–Earth system. As I wrote above, the 1984 paper by Howell and Breakwell uses the term "halo orbit" for any of the "periodic orbits emanating from the general vicinity of any of the 3 collinear Lagrangian libration points". I think this paper may have been responsible for introducing this generalized sense of "halo orbit". --Lambiam 10:26, 29 September 2020 (UTC)

Viruses
In the main page of viruses, there is a picture of a cgi image. Can you please remove the fake, cgi image and replace it with a an actual image of a virus via the electron microscope? Thank you, I hope this is done sooner than later. — Preceding unsigned comment added by 2600:1008:B054:93:C8E9:B631:A283:DDB3 (talk) 20:45, 28 September 2020 (UTC)
 * Do you mean File:SARS-CoV-2 without background.png? Look the thing to do is to discuss it at Talk:Virus. Nothing about it can be done from here. -- Jules (Mrjulesd) 20:51, 28 September 2020 (UTC)


 * There's an electron microscope image on the Coronavirus page. So what's stopping YOU from posting a different illustration on the Virus page, if you think it's so urgent? ←Baseball Bugs What's up, Doc? carrots→ 21:30, 28 September 2020 (UTC)
 * Its semi-protected. -- Jules (Mrjulesd) 22:06, 28 September 2020 (UTC)
 * The reviled image is meant to illustrate the morphology of the COVID-19 virion, and it does a good job at that. There is already an image taken using a transmission electron microscope, which is about the best you can do, but it makes the virions look like pancakes with wattles and leaves you in the dark about crucial aspects of their morphology. --Lambiam 22:15, 28 September 2020 (UTC)


 * It's also incorrect to call the image "fake." You won't find a snapshot image of DNA on the DNA article either (except possibly in the nanotechnology section, but even that's not exactly a traditional image). Rather, you see images that are constructed based on interpreting things like x-ray crystallography that give us fairly high resolution atomic level structures, but not images. In the case of SARS-CoV-2, we have done similar atomic level structures with techniques like Cryogenic electron microscopy, but even then it's usually of individual structure units, such as the spike protein. The CGI image isn't fake; it's a composite of the structures we have measured individually using other techniques that are not suitable for measuring an entire virus structure at once. --OuroborosCobra (talk) 01:07, 29 September 2020 (UTC)
 * All images are "fake"; all models are wrong. The relevant question is, is the image accurate and useful? In an article, there are also editorial questions of emphasis, and which images are most useful in context. As noted, the image is based on scientific data and intended to give an accurate depiction of the virus; it's not something some random person slapped together in ten minutes. --47.146.63.87 (talk) 22:04, 29 September 2020 (UTC)