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Hello,
From what I've read online, launch windows to Mars open up only once every two years. Is this because during this interval, Mars is oppositional to Earth on the other side of the Sun? Is this interval limited to only current rocket technology?
Cordially,
EarthWolf
" Man will not always stay on the Earth. "
Konstantin Tsiolkovsky
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Hello,
From what I've read online, launch windows to Mars open up only once every two years. Is this because during this interval, Mars is oppositional to Earth on the other side of the Sun? Is this interval limited to only current rocket technology?
Cordially,
EarthWolf
The launch window opens up every 26 month or just over two years. The best time to launch a manned flight with present technology is when the Earth is just catching up to Mars and the best time to launch a return flight to the Earth is when the Earth is just pulling away from Mars in it orbit. There is another way to go to Mars by going to Venus first, but it take longer to go that way.
But, in answer to your question:
Yes, we are limited by our present technologies as to when we can launch our space craft to Mars.
Depending on what kind of technologies that we develop in the future, this launch widow may not be important. I have raised the idea of having a powered flight to Mars and back with an acceleration of one "G" half way there and a Deceleration of one "G" the other half the way there. If we can build a space ship that can do that rather than having to rely on the slingshot effect of the earth, moon, Venus or Mar, then it would not make any difference. You would be able to pick a different rout depending on where the Earth is in relationship to Mars and which way your going at time you decide to travel. That the difference between a powered craft and one that has to rely on the slingshot effect.
Larry,
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It all really boils down to fuel efficency
The reason why the best times to go to Mars only happen once every two years or so is because those are the times that require the least speed to get there.
As the rocket equation dictates, the amount of fuel you need increases geometricly with the speed change, so if you wanted to go to Mars outside of the two-year launch window, you would require huge amounts of fuel, and so it isn't practical.
The only real solution is to come up with an engine that uses the same fuel more efficently. Right now, no exsisting or near-term engine really improves on matters much. It will take an engine that is much more efficent to make it so you can go to Mars and back year-round.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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Mars goes around the sun once every 687 days. The Earth makes almost two loops in that time and catches up with Mars, then passes it. It takes 26 months (on average) for Mars and Earth to go from one opposition (when they are in a line with each other and sun) to the next.
The best time to launch at Mars is about two months before opposition, arriving at Mars 4 or 5 months after opposition. To return, one launches five months before opposition and reaches the Earth one month after.
The delta-v for a minumum energy (9 month flight) from low Earth orbit is 3.8 km/sec (escape velocity is 3.2 km/sec). To reduce the flight to six months you need to depart at 4.35 km/sec; to reduce the transit time further to 130 days, 5.77 km/sec. If you depart any time other than the optimum, the delta-v increases.
Solar sailing vehicles and vehicles with ion engines could leave earth for Mars just about any time, but that's because their transit times are 1-2 years anyway. If you want to fly to Mars in, say, 1 month, you need a delta v of something like 15 to 20 kilometers per second, which is utterly impossible with our current technology. Furthermore, if you depart slowly (3-5 km/sec delta-v) you can use the Martian atmosphere to slow you into orbit or to land at the other end, but if you head for Mars fast you have to fire your engine to slow down at the other end, because you hit the atmosphere so fast you go through it in very little time and would need an enormous heat shield, generating very high gees of deceleration, to slow down.
-- RobS
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