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Earlier I had thought that space ships can strike against space rocks while traveling to another planet. But I've read that space is vastly empty, it's highly unlikely that something will come in the path of space ship. It is explained here.

Now, to reach a planet like Mars, it takes around 7-8 months. So I wonder when path is clear, why can't or why don't scientists make a space ship that is significantly faster than current ones? So travel time could be reduced to 1 or 2 months.

Are there greater risks of traveling so fast or it is just matter of time (e.g. we don't have required technology) and we can expect faster space ships in future?

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    $\begingroup$ Keep in mind that the spacecraft will need to brake more (may lead to more fuel consumption) when you reach the target if you are going faster! $\endgroup$
    – AJN
    Jun 8, 2021 at 16:10
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    $\begingroup$ If we had some magic fuel that weighted a lot less than current ones we could simply launch a spacecraft witha ton of that, accelerate to 0.99c, reach Mars in a couple minutes, slow down from 0.99c to a couple km/s and land. Unfortunately to reach these speeds we would need millions of tons of fuel. If we had a way to generate and store antimatter maybe we could make a matter-antimatter engine that would produce tons of thrust with little mass, but, AFAIK, it's 100% fanfinction at this point $\endgroup$
    – GACy20
    Jun 8, 2021 at 16:14
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    $\begingroup$ @gacy20 you don't accelerate to 0.99c for a trip to Mars. Assuming you do not want to subject humans to more than 1g acceleration for prolonged periods, you do not reach even 0.1c on a trip to any neighborhood-clearing planet of our Sun. Not enough distance within which to accelerate. $\endgroup$
    – Oscar Lanzi
    Jun 9, 2021 at 0:05
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    $\begingroup$ @GACy20 weight isn't the issue. Lower weight means less to push against (Newton's 3rd). It's how fast you push it away from you. To reach 0.99c the exhaust velocity must be greater than 0.99c. You would need relativistic fuel. $\endgroup$
    – Aron
    Jun 9, 2021 at 10:56
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    $\begingroup$ @Aron That's not true. Don't forget the fuel is at rest with respect to the spaceship, and then you expel it at the same relative velocity regardless of how fast the spaceship is moving, and you still get some momentum out of it. How could it be otherwise? It's not like there's any absolute frame of reference. Such rockets would be wildly impractical, of course (as if anything travelling at 0.99c wouldn't!). Exhaust velocity is not a speed limit, but the lower it is, the higher mass ratio you need for the same dV. $\endgroup$
    – Luaan
    Jun 10, 2021 at 10:28

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The biggest risk on a flight to Mars is cumulative exposure to radiation, so a 1-2 month flight would actually be much healthier for a crew than a 7-8 month flight. I don't know of any risks that would be increased by a shorter flight.

The limitation to making a faster spacecraft is fuel. In order to go a little bit faster, you need to fire your rocket engine for a little bit longer at the start of the flight (the spacecraft just coasts through space for the 7-8 month duration). To fire the rocket engine longer, you need to take a little more fuel into space, but the mass of that additional fuel itself requires a little more fuel to lift against Earth's gravity, and so on. The relationship between speed and fuel mass for a rocket engine is exponential, and is described by the Tsiolkovsky rocket equation.

Depending on where you're going, you might need to bring along still more fuel to slow down once you reach your destination! At Mars, it's possible to slow down by flying through Mars's atmosphere when you get there, but aerobraking in this way from a higher speed will require still more mass in heat shielding, and thus more fuel to get up to speed when leaving Earth.

Building a launching rocket large enough to get a spacecraft to Mars in 7-8 months is already a huge, expensive project, and it would be probably at least 5 or 10 times more expensive (but definitely not millions of times) to make it a 1-2 month flight.

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    $\begingroup$ 5 or 10 times more expensive?! That isn't how exponential growth works. For a 1-2 month flight you would be looking at millions times more fuel. $\endgroup$
    – Aron
    Jun 9, 2021 at 1:35
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    $\begingroup$ Re "just coasts", not necessarily. You could use something like an ion engine that provides more or less continuous thrust, as with the Dawn mission to Vesta & Ceres: solarsystem.nasa.gov/missions/dawn/overview Of course you are replacing the need for a large mass of chemical fuel with the need for a source of electrical power, so for a manned mission you'd probably need a lightweight nuclear reactor. $\endgroup$
    – jamesqf
    Jun 9, 2021 at 4:16
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    $\begingroup$ @Aron well, for a 1-2 month flight you'd definitely need to look into ion drives. Which, yes, would be much more expensive, but they also do have much higher specific impulse so you would avoid something like a millionfold increase in the fuel needed. — “That's not how exponential growth works” can be a bit of a red herring, because exponential with a sufficiently small basis will still be almost-linear at sufficiently small arguments. The problem is that as the input gets bigger, it will more and more dramatically exceed the linear case. $\endgroup$
    – leftaroundabout
    Jun 9, 2021 at 10:11
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    $\begingroup$ @Aron NASA's trajectory browser unfortunately won't give me a 60-day solution, but for 80 days it gives me a 5.1km/s departure burn and 11.4km/s arrival speed, versus 176 days flight time for 3.6km/s departure burn and 6.4km/s arrival speed. Assuming you do the additional 1.5km/s on a hydrogen engine, and you pay for the braking on arrival using hypergolics, liftoff mass increases by about 6.2 times. That's worst case; aerobraking away some of the arrival speed would probably be more mass efficient. Nothing like millions of times in any case. $\endgroup$
    – Russell Borogove
    Jun 9, 2021 at 16:24
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    $\begingroup$ What I meant was that all of our flights to Mars so far have of course been uncrewed. If the only major advantage of shortening it to 1-2 months is for the health of a crew, then there's a lot of other things to take into consideration. $\endgroup$
    – Darrel Hoffman
    Jun 10, 2021 at 17:09
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The limitations, as we know it:

  1. Fuel. The infamous rocket equation means we get roughly a few times more fuel for every km/s delta-v budget.

  2. #1 is why we avoid braking by rocket engines. When we reach Mars, we need to be almost at the speed of Mars and then brake in its atmosphere.

If we go faster, Mars' tiny atmosphere cannot brake us enough. And even if it does, the heat and acceleration will impose quite a requirement on the payload. More protection means more mass, i.e. more fuel in the first place. And even with all the possible protection, the atmosphere can do only so much and then we risk flying back into space or reaching the surface at rather unpleasant speed.

  1. So we should brake by rocket engines, expending our precious delta-v budget. The rocket equation says we need the fuel for braking and even more fuel to bring the fuel for braking to the place where we need to brake.

  2. Acceleration - humans can survive 1g for quite a while, ~3g for short time (like tens of minutes) and 10g feels and hurts like a car crash. Any scientific payload is hardly 10g-safe. We can probably make it survive 10g, but it will get heavier - see the rocket equation again.

  3. Ah, I forgot - a more powerful rocket will have to withstand its own acceleration, i.e. will be heavier and will carry less fuel per unit of mass. Rocket equation all the way...

So no, unless we make a great leap forward in the rocket science (and it IS a rocket science) we are not going to Mars any faster.

What we can do now is to make the travel e.g. 2 weeks shorter for like double the expense.

So the main risk is quickly going over budget for no apparent gain.

Edit: p.s.

The same factors say we don't save much if we go much slower either. We can probably use a gravity assisted acceleration near the Moon at the price of month or two more and at least 2 more burns.

And yes, engine starts are limited resource as well.

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    $\begingroup$ "human can survive 1g for quite a while" - yes, for decades usually :) $\endgroup$
    – fluffysheap
    Jun 9, 2021 at 6:44
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    $\begingroup$ @fluffysheap My personal experience is the same. $\endgroup$
    – fraxinus
    Jun 9, 2021 at 8:04
  • $\begingroup$ “If we go faster, Mars' tiny atmosphere cannot brake us enough” – that seems quite implausible. Particularly for fast entries, most of the braking has to happen in the very thin outer part of an atmosphere. In fact I'd imagine Mars is better than Earth for this, because the lower gravity causes a more gradual density/height relationship. And you, don't at least in principle, need to scrub all the kinetic energy in a single action – it's possible to e.g. skim the atmosphere once to enter a highly elliptical orbit, then again to make it almost circular, and then finally enter. $\endgroup$
    – leftaroundabout
    Jun 9, 2021 at 10:44
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    $\begingroup$ @leftaroundabout a lot of spacecraft did this at both Mars and Earth, but at least, in the first plunge in the atmosphere, you need to get below the escape velocity. $\endgroup$
    – fraxinus
    Jun 9, 2021 at 11:18
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    $\begingroup$ @Aron I agree that space tethers are a great concept, but to get any useful Δv with acceptable G-force they would need to be many kilometres long. And still strong. IOW, it would have to be a big heavy part of infrastructure, not just a one-time rope. Not nearly as bad as with a space elevator, but still only worthwhile if you can use a tether for hundreds of interplanetary launches. $\endgroup$
    – leftaroundabout
    Jun 9, 2021 at 11:50
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There are no risks, only benefits, but the laws of physics dictate that this won't happen for a loooooong time, even with the first couple generations of nuclear rockets, whenever that happens.

Earth has a minimum escape velocity, irrespective of mass and currently all space-craft just reach it and coast until they get to their destination and then burn the least possible amount of fuel to enter it's orbit. This is called the https://en.wikipedia.org/wiki/Hohmann_transfer_orbit. Currently, we have such puny engines, that even with the most efficient journey the designers of the various space-probes go through the hassle of shaving grams from any component they can.

If we can have a more powerful space-drive, like the currently proposed nuclear ones, with the same Delta V budget, we could get to their destination faster, or we can bring a lot more cargo(which is more important) and you have to get to the level of ludicrously powerful sci-fi level drives(e.g. the Epstein drive in The Expanse), before you can afford to both get their quickly and bring enough stuff with you.

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    $\begingroup$ The author of the question is asking if there are any risks to faster travel that are preventing us from going faster. There are none. The other answers, as well as my own all boil down to to it using far too much fuel, but that's a downside, not a risk. $\endgroup$
    – Eugene
    Jun 18, 2021 at 5:27
  • $\begingroup$ @uhoh What I added was an explanation of the Hohman transfer orbit and how it's transit time is independent of the vehicle's mass and that any plausible drive advancements will logically be used to increase payload mass. As to high Gs, a space-drive that can continuously accelerate at even 1 G is waaaaaaaaay out of our plausible near future tech's possibilities. The answer that seriously gives the downsides of 10G acceleration is completely irrelevant to the question. $\endgroup$
    – Eugene
    Jun 18, 2021 at 6:09

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