7 ms·

Citation please (sounds interesting, but yeah), and does it get exchanged at the speed that this stuff does, or does it take thousands^x years to arrive? Not t

by Groxx 13y ago

Citation please (sounds interesting, but yeah), and does it get exchanged at the speed that this stuff does, or does it take thousands^x years to arrive? Not that I disagree. Just wondering if there are a few orders of magnitude that need to be considered.

raverbashing 13y ago
Well, it's much, much easier to have stuff from Mars to Earth than the other way round, because of the following reasons: 1 - Less gravity on Mars -> smaller escape velocity 2 - Mars is in a higher energy level (gravity wise w.r.t the Sun), so Mars to Earth is "falling down", but you need energy to go from Earth to Mars
- Groxx 13y ago
  1) agreed. there's solid evidence that Mars sends junk to us - not sure about the other way around. 2) does that matter, since they're in orbit, thus zero/microgravity? You still need energy to leave an orbit, whether up or down, I don't see how it makes a difference. There's no friction to speak of to make "down" a direction that things are predisposed to move in. Over ridiculous time scales sure, but ridiculous time scales are nothing like human time scales, so we still have many orders of magnitude difference if we fling stuff at Mars intentionally.
  - dhimes 13y ago
    Yes it matters- the Sun is the main provider of gravity for this consideration.
    - Groxx 13y ago
      orbit. gravity is canceled out by orbital velocity, as far as distance-from-sun is concerned. it costs Mars nothing to stay in orbit, and to go down to a -100m/s orbit or up to a +100m/s orbit costs the same amount of energy: (mass of mars) * (100m/s velocity change) if Mars and the Earth were not orbiting, I would completely agree. drop something from Mars and it'll land on Earth, and the reverse is not true. but they're not - drop something on Mars and it's just in Mars' orbit.
      - mikeash 13y ago
        Delta-v requirements for transfers in space are generally symmetrical. It takes the same amount of delta-v to go from Earth's orbit to Mars's orbit, for example, regardless of direction. The one thing that changes this equation is aerobraking (or, when dealing with stuff whose structural integrity isn't important, lithobraking). Because drag works in one direction, that means that you can take advantage of it when arriving but not departing. For that reason, for example, it takes more delta-v to reach a transfer orbit to Mars from Earth than it takes to reach Earth from a transfer orbit to Mars. Technically that's not true, but when arriving at Earth, a great deal of delta-v can be provided by the atmosphere or the ground.
        dhimes 13y ago
        I see that now. Thank you.
      - dhimes 13y ago
        It's true tat it's been a long time since I played with this stuff, I work out for a circular orbit Total energy of an orbit is -G mp mS /(2 r) mp = mass of planet mS = mass of sun r = distance of planet from sun Even though this is a circular orbit, the basic result should be the same: the greater the distance from the sun the more energy the planet has, per unit mass. I think I understand your basic point, however, which is that whether we speed it up or slow it down we have to do something to the mass, and how much we do depends only on difference of the orbit radii. But note that adding 100 m/s or subtracting 100 m/s gives two different differences in r.
- dekhn 13y ago
  http://en.wikipedia.org/wiki/Interplanetary_Transport_Network http://en.wikipedia.org/wiki/Interplanetary_Transport_Networ...
- vardhanw 13y ago
  About #2, it may be the other way round, anything escaping Earth is easily pulled in towards Mars' orbit by the Sun's gravitational pull. The other way round, it requires more energy to escape.
  - andyjohnson0 13y ago
    "...pulled in towards Mars' orbit..." Mars is further from the Sun then the Earth is.
    - vardhanw 13y ago
      Oops. Took my unconscious mind for granted I guess.
dekhn 13y ago
Looking back for the specific citation, I can't find it. Looking naively to find the source, it seems like it was an estimate (I mean, we know there are meteorites, but the actual frequency and when they landed, and the total amount isn't known exactly). http://arxiv.org/pdf/1205.1059v1.pdf http://arxiv.org/pdf/1205.1059v1.pdf (ignore the part about lithopanspermia. I'm just discussing the facts of the matter, not the crazy speculation part).
- dekhn 13y ago
  OK. Turns out I was using the wrong search term. It's "meteorite flux", and here are several sources. Unfortunately, Google obscured the PDF link, so do: [ mars earth meteorite flux ] In particular, "Assessment of Planetary Protection Requirements for Mars Sample Return Missions" has data.

by Groxx 13y ago

raverbashing 13y ago

Well, it's much, much easier to have stuff from Mars to Earth than the other way round, because of the following reasons: 1 - Less gravity on Mars -> smaller escape velocity 2 - Mars is in a higher energy level (gravity wise w.r.t the Sun), so Mars to Earth is "falling down", but you need energy to go from Earth to Mars

Groxx 13y ago
1) agreed. there's solid evidence that Mars sends junk to us - not sure about the other way around. 2) does that matter, since they're in orbit, thus zero/microgravity? You still need energy to leave an orbit, whether up or down, I don't see how it makes a difference. There's no friction to speak of to make "down" a direction that things are predisposed to move in. Over ridiculous time scales sure, but ridiculous time scales are nothing like human time scales, so we still have many orders of magnitude difference if we fling stuff at Mars intentionally.
- dhimes 13y ago
  Yes it matters- the Sun is the main provider of gravity for this consideration.
  - Groxx 13y ago
    orbit. gravity is canceled out by orbital velocity, as far as distance-from-sun is concerned. it costs Mars nothing to stay in orbit, and to go down to a -100m/s orbit or up to a +100m/s orbit costs the same amount of energy: (mass of mars) * (100m/s velocity change) if Mars and the Earth were not orbiting, I would completely agree. drop something from Mars and it'll land on Earth, and the reverse is not true. but they're not - drop something on Mars and it's just in Mars' orbit.
    - mikeash 13y ago
      Delta-v requirements for transfers in space are generally symmetrical. It takes the same amount of delta-v to go from Earth's orbit to Mars's orbit, for example, regardless of direction. The one thing that changes this equation is aerobraking (or, when dealing with stuff whose structural integrity isn't important, lithobraking). Because drag works in one direction, that means that you can take advantage of it when arriving but not departing. For that reason, for example, it takes more delta-v to reach a transfer orbit to Mars from Earth than it takes to reach Earth from a transfer orbit to Mars. Technically that's not true, but when arriving at Earth, a great deal of delta-v can be provided by the atmosphere or the ground.
      - dhimes 13y ago
        I see that now. Thank you.
    - dhimes 13y ago
      It's true tat it's been a long time since I played with this stuff, I work out for a circular orbit Total energy of an orbit is -G mp mS /(2 r) mp = mass of planet mS = mass of sun r = distance of planet from sun Even though this is a circular orbit, the basic result should be the same: the greater the distance from the sun the more energy the planet has, per unit mass. I think I understand your basic point, however, which is that whether we speed it up or slow it down we have to do something to the mass, and how much we do depends only on difference of the orbit radii. But note that adding 100 m/s or subtracting 100 m/s gives two different differences in r.
dekhn 13y ago
http://en.wikipedia.org/wiki/Interplanetary_Transport_Network http://en.wikipedia.org/wiki/Interplanetary_Transport_Networ...
vardhanw 13y ago
About #2, it may be the other way round, anything escaping Earth is easily pulled in towards Mars' orbit by the Sun's gravitational pull. The other way round, it requires more energy to escape.
- andyjohnson0 13y ago
  "...pulled in towards Mars' orbit..." Mars is further from the Sun then the Earth is.
  - vardhanw 13y ago
    Oops. Took my unconscious mind for granted I guess.

dekhn 13y ago

Looking back for the specific citation, I can't find it. Looking naively to find the source, it seems like it was an estimate (I mean, we know there are meteorites, but the actual frequency and when they landed, and the total amount isn't known exactly). http://arxiv.org/pdf/1205.1059v1.pdf http://arxiv.org/pdf/1205.1059v1.pdf (ignore the part about lithopanspermia. I'm just discussing the facts of the matter, not the crazy speculation part).

dekhn 13y ago
OK. Turns out I was using the wrong search term. It's "meteorite flux", and here are several sources. Unfortunately, Google obscured the PDF link, so do: [ mars earth meteorite flux ] In particular, "Assessment of Planetary Protection Requirements for Mars Sample Return Missions" has data.