3
$\begingroup$

While playing KSP I came to realize how how counter-intuitive distances in space exploration are. Just because a planet is closer doesn't mean it's easier to get to. For example, I used to think that because a mission like Voyager escaped the solar system, and Voyager flew so far it needed huge amount of energy. After looking at the delta-v map, a mission like that is nothing compared to landing on a much closer Mercury for example.

This makes me wonder. Of all the missions we have flown and will fly, which ones require the biggest delta-v with huge rockets? (I'm guessing Moon landing is #1) In comparison, what are the missions that were optimized through gravity assists, aero-breaking and other ingenuity that even though they would normally need a huge rocket, in the end needed only a moderately sized one?

What I'm really trying to figure out is which missions are the "big ones". For example, when I hear on the news "a probe flew past Pluto", how does that compare with "a return sample mission from Moon" in terms of shear accomplishment and size. Which mission is bigger? And how much bigger? I'm trying to come up with a heuristic to use when thinking about how "big" a mission is:

  • Distance is not helpful (as explained above)
  • Cost is not helpful because some countries/companies are more frugal than others
  • I'm left with delta-v ... or is there some better way to wrap one's head around this?
  • Or maybe I should forget about delta-v (because it doesn't account for smarts like aerobraking and gravity assists) and simply ask for list of "biggest achievements" in space exploration? Maybe we got to a planet that would normally need 40Km/s with a 15Km/s budget?

Bonus: Is there a destination in our own solar system that is harder to get to (because of delta-v) compared to a mission to another solar system?

$\endgroup$
  • 1
    $\begingroup$ I'd say total launch mass is a very good measure. Missions like Apollo or Curiosity being definitely on the "big" side, but still overshadowed by ISS. (it also indirectly relates to cost and delta-V; "small payloads going far" requiring just as big launchers as big but near.) $\endgroup$ – SF. Feb 8 at 16:24
  • $\begingroup$ delta-v is not a good measure. Say I want to land an object twice as big as the LM on the moon and return it. I would need the same amount of delta-v as Apollo, but significantly more fuel and gross launch mass, requiring a rocket bigger then the Saturn V. $\endgroup$ – Polygnome Feb 8 at 18:45
  • $\begingroup$ So there seems to be consensus that the metric I'm looking for is launch mass. Where could I find a list of missions ordered by launch mass? For example, what was a mission with the highest launch mass ever? $\endgroup$ – user3280964 Feb 8 at 18:56
  • 2
    $\begingroup$ @user3280964 This list can be sorted by mass: en.wikipedia.org/wiki/List_of_heaviest_spacecraft - if you exclude ISS and MIR, STS-127 flown by Atlantis was the "biggest" mission, the biggest unmanned missions would be ATV followed by the 1969 soviet proton satellite, and three entries further down hubble, and further 10 down cassini-huygens to saturn. I'm not sure launch ass gives a good metric of "difficulty" of a mission. $\endgroup$ – Polygnome Feb 8 at 19:05
4
$\begingroup$

Going by delta-v, the Moon is pretty close. The reason we needed such a large rocket for the Apollo missions was that we wanted to land a really heavy spacecraft on the Moon. It needed to be that heavy to be able to take off again, and get back to Earth. The CSM+LM weighed about 45 tons at the start of the mission.

In contrast, the Voyagers weighed only 825 kg each at the start of their mission.

Comparing by launch mass makes for a difficult comparison because you can use the same rocket either to deliver a heavy payload to the Moon or a small payload to a Sun escape trajectory.

Missions optimized through gravity assists: pretty much all interplanetary missions beyond Mars and Venus.

Missions optimized through aerobraking: only a few, for Mars, Venus and Earth. Unless you count the Mars and Venus landers (and Huygens) for making soft landings in an atmosphere.

Aerobraking and gravity assists can be compared by the amount of delta-v they imparted (i.e. which the mission got 'for free'). All in all, delta-V is the better standard for comparison, in my opinion.

enter image description here

$\endgroup$
  • $\begingroup$ Did you make this figure yourself? If so, props for a really good visualization of deltaV budgets for traveling from Earth throughout the solar system! Even if it isn't the selected answer I commend the effort. $\endgroup$ – ben Feb 8 at 19:31
  • $\begingroup$ Great figure. Isn't a red arrow missing though from the sun? Aerobraking is possible there I think. $\endgroup$ – zabop Feb 8 at 19:36
  • $\begingroup$ That's not mine, it's from a Reddit user (first link in my answer). $\endgroup$ – Hobbes Feb 8 at 19:40
  • 1
    $\begingroup$ Aerobraking to the Sun (survivably) requires materials we don't have. $\endgroup$ – Hobbes Feb 8 at 19:41
  • $\begingroup$ Another great visualization is this subway map of the solar system. $\endgroup$ – Polygnome Feb 9 at 9:35

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.