For how long a time is Mars reachable from Earth?

So Earth and Mars have seperate orbit times because they are different distances from The Sun. But what I need to know is how long Mars is theoretically reachable from Earth without The Sun being in the way. By which I mean how much time there is from when Mars is reachable from Earth to when it isn't. Sorry if I'm unclear. Thanks for the help

Update:

Can I change this to, say we are in the future and the travel time is something like a day but ships still travel in a straight line-ish so you needed to get there during the period I previously described. Then what would the window be?

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  • gn
    Lv 4
    1 decade ago
    Favourite answer

    The question is confusing because the people who know physics are going to answer that the "straight line" must in fact be a trajectory and the trajectory cannot in fact be anything but curved. The question makes no sense any other way (but see below). That's particularly true in the post with the Holmann transfer. The other thing physicists would note is that you don't have to go "around" you can go up and over or over and under. You're allowed to think three dimensionally. And in any case you can lob a satellite or space craft at Mars when it is "on the same side" of the sun and the craft can reach Mars when it is "on the other side."

    But your question is more on the order of "If I could travel at the speed of light in something very much like a straight line towards Mars, how much of the time would the sun get in the way?"

    Since light does in fact come from Mars to us, the question is really similar to: "when is Mars observable, so I can get in my 99% speed of light bullet ship and zoom over there in about thirty minutes?" The question is one of "conjunction ingress" and "conjunction egress". In 2008 the conjunction took 27 days. To an Earth bound observer, Mars cannot be seen during that time in observable light, and the truth is, it's better to have even more time so as not to be trying to observe through the Corona, etc.

    2008 Nov 21 Conjunction Ingress. Mars will disappear behind the limb of the Sun.

    2008 Dec 05 Conjunction. Mars is behind the Sun ~2.478 AU.

    2008 Dec 18 Conjunction Egress. Mars will emerge from behind the limb of the Sun.

    Now the question here is if you are bullet-shooting yourself at very near the speed of light towards Mars how do you define how close you can come to the sun without getting vaporized? You seem to have an advanced spaceship, so perhaps your shields will protect you from the Corona's heat. Or perhaps you'd like to add a few days on either side of those dates to keep you farther away from the sun, and give the shields a rest.

    Incidentally though since space is curved and curved even more near the sun, no matter how much you think you're going to travel in a straight line in your bullet ship, you're going to get bent. The same thing happens to light itself. But isn't that sort of true here on Earth? When you're driving on a straight line on an Interstate, you are in face driving an arc that conforms to the Earth's curvature.

    Straight lines are ideas we have in our heads, but we need to "get over it."

    In any case 27 days is I think the number you're looking for, or at least, as good as you're going to get.

    Hope that helps,

    GN

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  • GeoffG
    Lv 7
    1 decade ago

    Fist of all, that's not the way interplanetary travel works. You launch your vehicle into a transfer orbit which is an ellipse which is tangential to the orbits of both Earth and Mars. You time the launch so that the vehicle reaches the orbit of Mars at the same time and place that Mars itself does. This is an elliptical orbit that is entirely between the orbits of Earth and Mars, and doesn't come anywhere near the Sun.

    Your second question seems to be more about finding a straight line through space between the Earth and Mars, which is just not practical for many reasons, most important of which is that it would require enormous energy. However you can _look_ in a straight line from Earth to Mars almost all the time, except for a relatively short period when Mars is on the far side of the Sun. How close you can follow Mars when it passes behind the Sun depends on the quality and location of your telescope. We routinely watch planets pass behind the Sun with the SOHO solar observatory satellite, when Mars is only out of sight for a few days, if at all.

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  • Anonymous
    1 decade ago

    A Holmann transfer to Mars takes about six months. A window for a Holmann transfer opens about every two years. So roughly speaking, a mission to Mars requires six months transit there, a year spent on Mars itself, and another six months return trip. A Holmann transfer, by the way, is a minimum-energy transfer, which is certainly required using chemical fuels only. A nuclear-powered rocket could probably achieve the trip in half the time, but the solar system is still bound by the sun's gravity and our orbits around it. It would still have to wait more than a year until a launch window back to Earth opens.

    Edit -

    I could do the math, but a straight-line trip would take more than a day at 1 g acceleration, probably about a week. That much energy at the disposal of the ship would require fusion power or better, with both high thrust-to-weight and high specific impulse. No window is necessary in that scenario, but also not yet possible with any known technology.

    Edit -

    Alright, I did the math for a 1g trip to mars, accelerating all the way. That is, accelerating the first half, and decelerating the next. I also estimated the closest approach between Earth and Mars as 59.6 million kilometers and the farthest as 401 million kilometers, based on each's perigee and apogee, but it's most likely a narrower gap. Anyway, minimum time: 43.3 hours. Maximum 112.4 hours. No launch window necessary if the acceleration is a constant 1g.

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  • 1 decade ago

    Skipweasel is correct.

    If you wish to send a space probe, the route you choose will cost more at certain times than it will at others, (or it will take longer which amounts to the same thing really since a longer trip affects the cost in terms of fuel lifted up from earth and size of the craft to hold the fuel, etc), but there is no time when you cannot send a probe. Here is an animation of the standard Hohmann transfer orbit that may make it easier to understand...

    http://www.youtube.com/watch?v=uNdB3nmAl_0

    Youtube thumbnail

    This is the cheapest and most efficient way to get there - as others above have mentioned it depends on the existing technology (chemical rocket engines) - if you wanted to use something that could deliver more thrust from a smaller fuel mass, you would need technology which has not really been developed.

    Solar sails are even cheaper but they mean a much slower trip if you are going to Mars. http://www.psfc.mit.edu/library1/catalog/reports/2...

    A plasma rocket engine is in the works, http://www.thefreelibrary.com/NASA,+Montana+Compan...

    Bussard Fusion engines have been examined quite closely - I do not know why they are not considered - except that the cost is not much better than for chemical rockets.

    http://www.askmar.com/Fusion_files/Aneutronic%20Fu...

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  • Dude
    Lv 7
    1 decade ago

    Depends on your line. You could say that Mars is only really hidden by the sun for a couple of days, but you would have to travel very close to the sun. if you blocked off a month's time, you could avoid the sun even more. The question is too vague.

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  • Anonymous
    1 decade ago

    Depending on what route you take, Mars is always reachable. You wouldn't take the "through the middle" route anyway, in most cases.

    There are times when it's more or less efficient to get there, but where you draw the line at what's doable is down to economics, not physics.

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  • 1 decade ago

    a manned mission to mars with return will never be possible the logistics and risks are far too difficult to overcome and the chances of lifting off from the surface are zero, one tiny collision with space debris would instantly destroy the space craft, resupply craft would have to be stationed along the route but it would be impossible to slow down and stop to load supplies or fuel anyway

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