# We can put a man on the moon, but we can't get a ship to Mars in a couple months...

Here’s another one that’s been bugging me lately:

The fastest man-made object ever was Voyager travelling at 17.4 km/s. The mean distance between Earth and Mars is 78,300,000 km. Now, if I’ve done my calculations correctly, that means Voyager could get to Mars in ~53.08 days. So… why is it that spacecraft to Mars currently (and possible manned missions) take so darn long?

I’ll even accept someone telling me that we just can’t normally make ships as fast as Voyager, but my Encyclopedia tells me that escape velocity from Earth is 11.2 km/s which still makes only 80.92 days to get to Mars (no Jules Verne jokes, please). Isn’t speed in outer space essentially constant? Someone please explain this one.

The problem is that the ship can’t travel the mean distance in a straight line. It has to hit a moving target, which means it has to set up a trajectory that will intersect with Mars at the time it finally reaches the orbit.

If you aim directly at Mars, the planet will be long gone before you get there.

Maybe you don’t relize that Voyager spent longer accelerating than the trip to Mars would be. Also if you would enjoy crashing into Mars at 17 km/s, you can be my guest…but that doesn’t really appeal to me. A trip to Mars would involve accelerating and decelerating within limits humans can tolerate and carrying enough fuel to slow down and maneuver. In short…it’s a lot more complicated than you think.

But why can’t you compute the future of Mars’ orbit? Then you can simply send the ship off in a straight line and Mars and the ship eventually line up. It seems like it would work well that way.

I thought escape velocity meant the velocity it would take at ground level to allow you to get out Earth’s gravity well at any speed. So if you were over escape velocty by 1 m/sec, you’d leave the gravity well at 1 m/sec. But I might be wrong.

On a different note, I think there might be significant velocity loss fighting against the Sun’s gravity.

As to aiming at Mars’ predicted point, I think that works, but the Earth’s motion around the Sun imparts a lot of lateral motion to the Mars vehicle. This means that the vehicle will be sort of sliding sideways going to Mars, which increases the total distance of the flight.

Please correct all the mistakes I’ve made - my physics is pretty rusty.

RealityChuck writes:

This is irrelevant. You can see that this doesn’t matter by simply pretending Mars is “Over there” when you launch your ship - this is a red herring.

Boris B writes:

Correct, essentially. If you have less than escape velocity, you never leave. If you just barely have escape velocity, you just barely escape and take a very long time to ge to Mars.

You could use this to your advantage, since the Earth goes the same direction as Mars, and much faster. It’s like jumping off a merry-go-round and slamming into your brother.

HeadlessCow is the only one with the science clue in this thread so far. We could easily get to Mars in a big hurry, and either keep going forever (like Voyager did) or stop in an even bigger hurry and leave a nice little greasy spot on the red planet. If you a) take the time to slow down and get to Mars in one piece and b) carry all the crap you need to do the same trip again (no Chevron stations on Mars) it either takes a lot longer or you need to use unconventional propulsion such as nuclear bombs.

The title of this thread begs an unasked question…or at least one not developed fully. Namely, why could we go to the moon thirty years ago, but we can’t go to mars now? The answer is simple. Money. Thirty years ago, NASA was one of the government’s top priorities, staffed by competent people, and goal-oriented. Now it seems that the only goal NASA has is to see how little in can accomplish in the most shuttle missions while utilizing maximum personnel to do so.
Don’t misunderstand me, there are some good people in NASA today. But NASA just can’t get things done now like they used to.

Primal
“Life, by it’s very nature, is self gratifying. If it also happens to be good or bad, that is purely coincidence.”

The minimal fuel orbit to another orbiting body like Mars is called a Hohmann transfer orbit, and it is not a straight line. A Hohmann transfer orbit has the spacecraft leaving the earth’s orbit at a tangent, and arriving at Mars orbit at a tangent. Basically, you burn enough energy to convert the spacecraft’s circular orbit into an elliptical orbit with Mars at aphelion and Earth at Perihelion. The details are too complex to relate, but they involve physical limits due to Kepler’s laws and the way orbital bodies behave. The minimum-fuel transfer orbit to mars takes 259 days, and the probe only needs to be accelerated by 3 km/s from Earth, and decelerated 2.7 km/s at Mars. A ‘window’ to launch a spacecraft in such an orbit to Mars comes up every 25 months. At any other time, the fuel required to get there goes up dramatically.

Once we’re on Mars, we have to wait there for ten months before the planets are in the right alignment for another Hohmann transfer orbit back to Earth. If you miss that window, you’re stuck on Mars for another 25 months.

So why don’t we just use more fuel and get there faster? Well, it’s possible, but very, very expensive. You can’t just burn a little more fuel and go a little bit faster. Once you abandon the Hohmann transfer, the fuel requirement skyrockets. We don’t send probes to the outer solar system by burning lots of fuel, we send them there using Hohmann transfer orbits or using gravity assist, and this makes the trip longer instead of shorter. But it’s the only way we’ve got to get a probe going that fast without spending a giant stack of cash.

It’s been a well kept secret, but there WAS a manned mission to MARS back in 1997.
Tragically, however, the engineer responsible for fueling the space shuttle thought that the abbreviation “m” meant METERS when it actually meant MILES… :0

No it isn’t. If the mean distance between the Earth and Mars orbits is x, you can’t calculate the time by using x as the distance to be traveled. You have to add the distance Mars travels during the trip. The original question did not include that factor when determining the time.

No, I am not a space travelling expert, but I do have common sense. It seems to me the real problem is the time it would take. Say for example, the 80 days, thats 160 days for the trip plus time spent on the planet… thats a long time out in space that could easily take its toll on the human body both physically and mentally. Also you need to have enough fuel to take off from Mars, the gravity being very similar to Earths, thats a lot of weight,(even if its just a lander of some design and your ship is in orbit) but the most weight is the food, water, and oxygen needed to sustain several people for 160+ days, and that would make for a pretty big ship. Then there is the technical junk like will they have enough power to support say, cabin heat for that long. Its all about time.

Neobican,

NASA is fully aware of the space needed for such things which is why their current mission plan is to send TWO crafts to Mars for a manned mission: one with the people and one with the supplies for a stay on Mars. One of these two crafts will stay that forming the beginnings of a Martian colony structure. I also know that the people have to stay on Mars for something like 18 months (after travelling for almost a year through space to get there) to wait until Mars is close enough to Earth for the trip back. Apparently, all of this is feasable. We could do it tomorrow if we wanted to, we just don’t have the money to yet.

Thanks to everyone else for explaining the speed thing to me. It makes much more sense now.

I agree with you that the reason we don’t go to Mars now is because, as a nation, we don’t want to. We could, if we decided to make it a priority. It wouldn’t even take much money, really - a tiny fraction of what we spend on social programs or just give away to other countries.

I disagree that NASA is the problem here though. First, their hands are tied by Congress. They HAVE to do the ISS, which is a huge time, money, and resource waste. And NASA’s budget is trimmed every few years; it’s only a fraction, as a % of the total budget, of what it was 30 years ago.

But I think NASA is probably better now than in the past at doing interplanetary missions. The missions they send today are far cheaper than the huge, multi-billion projects of yesteryear, and still do good science; often better than what used to be done due to today’s more advanced technology. I think that bang-for-the-buck-wise, NASA still can get it done.

However, I’m not sure how much longer this will last. As a nation, we do not value space exploration very much. If the colossal NASA budget cut had gone through this year, I think that would have been pretty much all she wrote. You can’t attract and keep the best people in an environment where people are constantly losing jobs to budget cuts, and where the best and the brightest can make lots more \$ in private industry.

## In the end, I think the blame is ours - the American public’s. We elect scientifically illiterate people to run our government, and we don’t value space exploration. Given that, I think we probably are getting more than we deserve from NASA.

peas on earth

On the other hand, there are a bunch of private spacelaunch projects underway which could cut the cost-to-orbit by a factor of ten. We would have had some of these a decade ago if NASA wouldn’t have interfered.

Anyway, once the cost-to-orbit is cut way down, a manned Mars mission becomes a whole lot more reasonable. It also becomes safer, because you can send a more massive ship with better facilities and shielding, and you can send multiple missions to park stuff in orbit and on the ground for the astronauts to use, so before they ever leave Earth’s orbit they’ll know that their habitat is already on the surface and their return fuel is already in orbit.

I think the real problem with getting funding for NASA is that the monetary returns on it are all long-term (i.e. mining asteroids, etc.) and name me one government that values science over money.

RealityChuck writes:

Sorry, RealityChuck. I didn’t notice the ‘Mean Distance’ datum in the original question - you are correct that the mean distance is not all there is to the calculation, in addition to all the delta-v problems mentioned before.

It’s been suggested that if someone,(either the government or even private enterprise?) simply put out a 50 billion dollar prize to anyone who could land a person on Mars and return them safely to Earth, someone would figure out a way to do it for 30 billion and pocket the difference.

The neophyte that I am just wanted to add that this is one of the most instructive and well articulated threads he’s read in quite a while. Many thanks.

I like that idea, Lumpy.

There are probably individuals in the USA who could pay for an entire manned Mars mission.

## I always thought that if I had Bill’s billions, I’d do cool stuff like fund some good (unmanned) science missions to some of the lesser well explored planets. What’s a couple hundred million, when you’ve got billions?

peas on earth

IIRC, it used to be illegal for any of the major NASA contractors to provide materials or parts for a private launch. I don’t have the reference handy but I believe there was a change made last year that allowed NASA and their contractors to cooperate with private entities. There’s also \$1million dollar reward for the first private orbital flight (launch, orbit, and safe return).

I know of at least 2 companies (one in Seattle, one in Texas) that are planning a space launch within the next 10 years. The Texas company is trying to build a launch facility in the Virgin Islands but has been held up by environmental concerns.

Anyway, back to the topic – once companies figure out how to make a profit in space and the government removes the protectionary obstacles, you’ll see a leap forward in space exploration. As an example, can you imagine Intel putting a lab in orbit to build chips in zero-gravity?