The Astronomy Thread

Extra 20%? A round trip to the surface from Mars intercept is just shy of 10000 dV, Deimos is only 2000 dV and Phobos is about 2500 dV. You need 5-6x more fuel for a Mars landing than you do for one of it's satellites.

Of course this isn't KSP so there's no particular benefit to landing on Deimos or Phobos like there is to landing on Ike but it's a far simpler mission in terms of how big a payload you have to move to Mars orbit and back.

Gavinmad said:

Extra 20%? A round trip to the surface from Mars intercept is just shy of 10000 dV, Deimos is only 2000 dV and Phobos is about 2500 dV. You need 5-6x more fuel for a Mars landing than you do for one of it's satellites.

Of course this isn't KSP so there's no particular benefit to landing on Deimos or Phobos like there is to landing on Ike but it's a far simpler mission in terms of how big a payload you have to move to Mars orbit and back.

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(Edit: Found a better source right after posting.)

Delta-V map of the solar system. Not sure where your numbers are from, but this indicates it's about 25% more expensive in dV to land on Mars than either of the moons. Aggregate all the nodes over the total route from origin to destination.

At least 70% of the energy is just getting past low Earth orbit. Gravity is a bitch.

Kiroy said:

So technically after the first few proof of concept missions to mars and getting a small colony going at massive expense, you could then start loading up the moons with gear until the mars colony had a way to create it’s own fuel to go start ferrying it down.

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Might as well just stash stuff in low Mars orbit. The dV cost to get to Mars orbit is not that much higher than the cost of going to Phobos or Deimos orbit, it's landing and taking off again where the massive dV difference comes in. When live people aren't involved the range of viable possibilities to get things down to the surface is much wider. First thing that comes to mind is you could put a payload in a slightly elliptical orbit with it's periapsis just barely dipping into the atmosphere and let it take several weeks to finally slow down enough to de-orbit.

Captain Suave said:

(Edit: Found a better source right after posting.)

Delta-V map of the solar system. Not sure where your numbers are from, but this indicates it's about 25% more expensive in dV to land on Mars than either of the moons. Aggregate all the nodes over the total route from origin to destination.

At least 70% of the energy is just getting past low Earth orbit. Gravity is a bitch.

File:Solar system delta v map.svg - Wikipedia

en.m.wikipedia.org

View attachment 545236

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I was using a map that starts at Earth transfer. Which isn't entirely fair and I should have been starting at LEO, but either way you don't include surface to LEO dV cost.

Gavinmad said:

but either way you don't include surface to LEO dV cost.

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Why? Because that leg is covered by first stage fuel? It's still part of the trip even if you're using separable boosters.

Captain Suave said:

Why? Because that leg is covered by first stage fuel? It's still part of the trip even if you're using separable boosters.

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Because it has nothing to do with the trip to Mars. It could conceivably take 3+ launches to get all the pieces of the mission into orbit but that doesn't add 30k dV to the trip.

Gavinmad said:

Because it has nothing to do with the trip to Mars.

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I guess I don't understand how you get to Mars without first getting off Earth.

Gavinmad said:

It could conceivably take 3+ launches to get all the pieces of the mission into orbit but that doesn't add 30k dV to the trip.

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No, but all three launches would experience the delta v of the surface to orbit leg.

Did the dV cost of getting off of Earth's surface matter for the LEM during the Apollo missions?

Gavinmad said:

Did the dV cost of getting off of Earth's surface matter for the LEM during the Apollo missions?

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It sure mattered for the planning and design of the LEM and the mission as a whole.

If that's the logic, why count anything before where the presumed lander/entry vehicle would separate from the transit bus in Mars orbit?

Captain Suave said:

It sure mattered for the planning and design of the LEM and the mission as a whole.

If that's the logic, why count anything before where the presumed lander/entry vehicle would separate in Mars orbit?

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I meant to refer to the CSM rather than the LEM but this is going nowhere very slowly so I'm gonna stop wasting my time.

Big Phoenix said:

Pretty sure a trip to Deimos/Phobos requires a bit less delta v. Would need a radically different lander to operate in such a low g environment though compared to Mars.

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Realistically you'd have to match the orbital velocity of one of them from a launch of Earth, which is significantly slower than the velocity usually used for Earth-Mars transit missions, which means you'd need to slow down significantly. Since they have negligible gravity, you can't use the "moons" themselves to do that.

It's also theorized they might be effectively loose rubble piles weakly held together

They might be targets in the future, but as Furry mentioned, they aren't really that interesting compared to other objects (or even asteroids) in the solar system.

Cybsled said:

Realistically you'd have to match the orbital velocity of one of them from a launch of Earth, which is significantly slower than the velocity usually used for Earth-Mars transit missions, which means you'd need to slow down significantly. Since they have negligible gravity, you can't use the "moons" themselves to do that.

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A mission to Phobos or Deimos would be nothing like that. There's a dV chart on this page that shows the lower capture cost for Phobos or Deimos compared to Mars, although I prefer the subway map format as it's quite a bit easier to read.

Next Starship test was supposed to happen in late July, it's stuck waiting for regulatory approval.

Big Phoenix said:

Definitely just balls of dirt but would still be a good way to test a lot of your systems out with little extra investment. Even Apollo did a practice landing before 11.

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