Timeline for Why do many Mars missions launch now, if the Hohmann transfer orbit is the most propellant-saving one?
Current License: CC BY-SA 4.0
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| Jul 27, 2020 at 14:03 | history | migrated | from astronomy.stackexchange.com (revisions) | ||
| Jul 26, 2020 at 20:32 | comment | added | PM 2Ring | @Mark Yes, that's correct. However, as David points out in a comment on Ross's answer, a simple Hohmann trajectory assumes that all 3 orbits are in the same plane, but since the Mars orbit plane is inclined by 1.850° to the Earth orbit plane, we need to adjust for that. | |
| Jul 26, 2020 at 19:50 | comment | added | Mark | @PM2Ring, the correct alignment is when the two planets are in "time-shifted" conjunction: that is, the position of Earth now is on the opposite side of the Sun from where Mars will be when the spacecraft arrives. | |
| Jul 26, 2020 at 13:33 | comment | added | PM 2Ring | @lwr (cont) Zubrin understands that the major axis of the transfer ellipse touches the 2 planets' orbits, but somehow he has forgotten that Mars has to be at the transfer orbit's aphelion at the time of the arrival of the spacecraft. A wise man once said that there's nothing so simple that it can't be screwed up, but seriously, an error like that coming from someone who's a self-proclaimed expert on traveling to Mars is rather poor, IMHO. | |
| Jul 26, 2020 at 13:32 | comment | added | PM 2Ring | @lwr The quoted statement from Zubrin is a bit weird, and not internally consistent. The terms "conjunction" & "opposition" refer to the angle between 2 celestial bodies, relative to a 3rd body. That 3rd body is normally the Earth, unless otherwise specified. But Zubrin talks about Mars & Earth being in conjunction & opposition simultaneously, which is confusing. He means that Earth & Mars are in opposition relative to the Sun when Mars & the Sun are in conjunction relative to the Earth. | |
| Jul 26, 2020 at 12:32 | vote | accept | CommunityBot | ||
| Jul 26, 2020 at 8:05 | comment | added | David Hammen | @lwr - Or perhaps Zubrin is not at the level he thinks he is at. | |
| Jul 26, 2020 at 6:39 | comment | added | lwr | @PM2Ring That's very interesting. Zubrin, as quoted, is wrong, for precisely the reasons stated in the post. That's a pretty elementary screwup for someone at Zubrin's level, and, having seen him speak in person, I know he has a much better grasp of orbital mechanics than this. Rather than him truly misunderstanding, I think it's much more likely that he tried to simplify the subject for his audience, and went too far. | |
| Jul 26, 2020 at 3:58 | comment | added | par | @notovny Great answer. If it isn't too much trouble, would you mind updating your answer to address what launching now looks like, and what the advantage is in time savings (and if possible, fuel)? I find this very interesting. | |
| Jul 25, 2020 at 20:56 | comment | added | Mark Morgan Lloyd | Upvoted because @novotny tells us what he's used for the pretty pictures. | |
| Jul 25, 2020 at 16:07 | comment | added | PM 2Ring | @Meiki It sounds like Zubrin misunderstood how a Hohmann transfer works. As notovny says, the point of arrival must be opposite the point of departure. That is, the major axis of the transfer ellipse touches the 2 planets' orbits. Calculating Hohmann orbits between Earth & Mars is slightly trickier than shown in the simplified diagram above because you do need to account for the eccentricity of the planets' orbits, especially of Mars, since its eccentricity is 0.0934. See en.wikipedia.org/wiki/Orbit_of_Mars | |
| Jul 25, 2020 at 15:36 | comment | added | Meiki | Did I misunderstand something in this statement? | |
| Jul 25, 2020 at 15:34 | comment | added | Meiki | "As Hohmann discovered, if you want to go easy on the gas, the best time to travel from Earth to Mars occurs when the two planets are in conjunction; at their maximum distance from each other on opposite sides of the sun. This is the easiest way to go, because if you take this path you can travel along an ellipse which is tangent to the Earth’s orbit at one end and tangent to Mars’ orbit at the other, thus minimizing the course change which is required for the spacecraft to depart or rendezvous with each." | |
| Jul 25, 2020 at 15:34 | comment | added | Meiki | Thanks for this great answer! Apparently, I misunderstood something in the book "The case for Mars" by Robert Zubrin. It says following: | |
| Jul 25, 2020 at 12:35 | history | answered | notovny | CC BY-SA 4.0 |