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If we placed and maintained a comet at Mars L1 point would we get a coma and tail that would add volatiles to the Martian atmosphere? I've also read comets create a bow shock with the solar wind, slowing it down, so would the comet's coma and tail at L1 provide Mars's atmosphere some protection from the solar wind?

The coma can grow to the size of Jupiter or even bigger than the sun. The hydrogen gas halo can be as large as 1 AU in radius! But by the time a comet gets to within 1.5 AU of the sun, ie the orbit of Mars, the size decreases and the tail increases as the solar wind gets strong enough to blow the gas and dust away from the coma.

The particles given off by the comet slow down and deflect the solar wind. So would this be enough to protect Mars from the solar wind? spacephysics.ucr.edu/index.php?content=solar_wind/sw/swq5.html

Vikki
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Brooks Nelson
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2 Answers2

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TL;DR: It wouldn't work, because you wouldn't be able to keep the comet at L1.

The collinear Lagrangian points (L1, L2, and L3) are unstable; if a body were to be placed at one of these points, any perturbation - no matter how small - will move the body away from the point, at which point gravitational and/or centrifugal forces will move the body further and further away from the point. Think of a ball balanced on the tip of a hill. Anything placed at one of the collinear Lagrangian points, or in an orbit around one of said points, will require constant stationkeeping to keep it in place.

Unless you have a large enough rocket to counteract the inevitable perturbations from things such as the solar wind the comet is meant to protect against as one of its functions, plus enough propellant to keep said rocket fuelled for a long time, it wouldn't matter if a comet at Sun-Mars L1 would block the solar wind, since it wouldn't stay at L1 long enough to make any sort of meaningful difference.

Vikki
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    Good answer! Since Mars' orbit is so elliptical, moving closer and farther from the Sun, the positions of the collinear Lagrangian points in the classical CR3BP (circular, restricted 3-body problem), as such, aren't even well defined to begin with. – uhoh Mar 22 '18 at 00:30
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    I'd put the TL;DR at the top ;-) – uhoh Mar 22 '18 at 00:30
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    @uhoh: Good idea - moved. – Vikki Mar 22 '18 at 00:34
  • It is true that the L1-L2-L3 are instabile, however a Lissajous-orbit around them requires far lesser propellant. This is what also the Webb Space Telescope does. – peterh Mar 22 '18 at 01:02
  • @peterh btw if you can show that quantitatively a larger halo uses less delta-v than a small one, consider posting an answer to Are large orbits around L1's and L2's preferred over small orbits for reasons other than geometry? – uhoh Mar 22 '18 at 01:11
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    A comet is basically a pile of reaction mass and the Sun is power source, so if you could get the comet there, it wouldn't be hard to keep it on station. It would probably be enough just to use sun-shields to control which bit of the coment boiled off fastest. – Steve Linton Mar 22 '18 at 11:25
  • While "...it wouldn't be hard..." might be less than accurate, this is a good point in principle! – uhoh Mar 22 '18 at 13:40
  • @peterh: The James Webb Space Telescope masses 6.5 tonnes. 81P/Wild, a garden-variety comet, masses 23 billion tonnes. You're going to need several billion times as much propellant for stationkeeping a comet than for stationkeeping the JWST. – Vikki Mar 22 '18 at 17:09
  • @SteveLinton: Doing so would be extremely difficult in practice; for one thing, you'd have to be able to rapidly react to large, unpredictable fluctuations in the boiloff rate caused by, for instance, different parts of the comet having different ice/dust/other ratios, different parts of the comet heating up and cooling down faster or slower due to being in contact with different amounts and different compositions of the rest of the comet (such as bumps on the comet, which would heat up and boil off faster than troughs). (1/2) – Vikki Mar 22 '18 at 17:13
  • (2/2) And rapidly moving a many-kilometres-wide sunshield by many kilometres without it overshooting by more kilometres is far, far easier said than done. – Vikki Mar 22 '18 at 17:16
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    @uhoh I am considering, however I can't show it quantitatively now. :-( – peterh Mar 22 '18 at 17:59
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    @Sean Right. This is why the drive should be manufactured by remotely controlled robots, on the surface of the comet. It could be a magnetically propulsed cannon, just what is currently in development in a weapon program of the U.S. Army. It would shot the stony part of the comet to get propulsion, while the gasous part would be transfered into the Mars by the solar wind. These comets have a lot of water ice. As the comet disappears, the Mars will have significantly more water as it has now. Water on the Mars + carbon dioxide in its atmosphere would make it inhabitable with some, maybe – peterh Mar 22 '18 at 18:00
  • @Sean genetically modificated plants. – peterh Mar 22 '18 at 18:04
  • If you use mirrors in orbit around the comet you could use them to direct sunlight on areas that need propulsion from that area in order to maintain station keeping. @Vikki-formerlySean – Brooks Nelson Jun 18 '21 at 20:00
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Since you already have a comet available near Mars, I would propose landing it it on Mars instead. Soft-landing it is only an optional feature and not a hard requirement.

You will get all the volatiles (and non-volatiles as well) on Mars. Quite possible that you'll create a hydrosphere at the same time. You will make quite a strong dust storm, but Mars is used to them and it will settle after a while.

You can as well build an equatorial power line in order to create a planetary magnetic field in order to keep your hard-earned volatiles from being blown in space.

... and you will save more than 90% of your L1 budget while getting incredibly more of these rare substances on Mars.

fraxinus
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