We can send vehicles/rovers to space and have them travel at very high speeds for millions of miles to other planets. But vehicles on earth (jets/planes) have significantly shorter range and speed. Why is this? ELI5
In: Physics
How fast can you run 100 meters? How fast can you run 100 meters in waist deep water? There is more resistance when running through water than through the air. There is more resistance moving through air/earth’s atmosphere than moving through the empty vacuum of space.
Not an expert but right off the bat I can tell you friction would be one limiting factor. No friction in the void of space to slow momentum.
Rockets are pretty much perfectly inertial. Once they get up to speed, there are basically no moving parts or anything – they just keep going. The engines don’t even have to run for very long. Maybe a few weeks at most for an electric rocket engine, but usually on the order of minutes. Jets require turbine engines, which are under a lot of stress and have many moving parts at high temperature.
Because there is no air in space.
This is basically it.
Flying through air slows you down through wind resistance.
In space there isn’t really much to slow you down.
In space any little bit of extra engine use adds to your speed.
In the air you need to constantly run your engine just to keep moving at the same speed.
In space once you are in motion, you will stay that way. A spacecraft on some course will keeping going at that speed and course forever unless some outside force acts on it. Spacecraft can basically cost at a constant speed towards their destination without having to burn any fuel.
On earth you are constantly being slowed down by friction. Air resistance means that if you stop your engine for a moment you will slow down. You can do clever tricks with gliding to use the air resistance to extend your range somewhat, but you can’t just keep going forever.
Another problem with air is that it really gets in the way if you are starting to really fast.
As you go faster and faster the friction from the air becomes more and more and you don’t just have to work harder and harder to keep going at that speed, the friction with the air also starts heating up your craft. If you go at speeds many, many times the speed of sound the heat from the friction becomes a real technical problem, which is why spacecraft returning to earth and entering the atmosphere at extremely high speeds get heated up so much that they are in danger of burning up.
Going fast in the air is hard and dangerous.
On the other hand going fast in space is almost required.
Technically if you reach an altitude of 100km above the ground you have reached space. But if you want to to stay there instead of simply falling back down to earth you either have too keep running your engines the entire time or you have to enter into an orbit.
An orbit is basically going so fast sideways that you continue to miss the ground as you fall down. If you are in orbit you won’t have to run any engine or anything you just stay where you are at more or less the same altitude or speed you have (at least for circular orbits).
However orbits require orbital speeds and those are really fast.
To stay in orbit a speed is required that you wouldn’t be able to achieve while in the atmosphere.
The Space Station goes around the earth at what would be something like two dozen times the speed of sound in air near the ground.
Such speeds are very hard to achieve and sustain inside the air and necessary to stay in orbit.
Wave your arms in the air quickly. Do it for about a minute. Now fill a bathtub or go to a pool. Do the same thing for a minute with your arms underwater. Did you feel that you used a lot more energy in the pool or tub?
Why do you think you feel more tired after doing the same thing in the pool? Did the water “resist” your arm movements more?
Air has resistance too and we mostly don’t notice it at the speeds humans can move unaided. However compared to space, air resistance in our atmosphere makes a huge difference to how fast we can make vehicles or objects travel.
Try to run in the sea and then on a road.
In the sea you have water and air slowing you down.
On the road you only have air.
You already know that air exist because when it’s windy you can feel it pushing against you.
In space you have neither air or water so nothin’ stopping you:-)
Air resistance slow object down and the faster it goes the more air resistance increase. So the engine accelerate the Jets, but by doing that more and more of the power of the engine goes to counter the air resistance. At one point the power of the engine will be equal to the air resistance and the Jets won’t be able to go faster. For a land vehicle like a car you have the air resistance and the friction of the ground.
In space you don’t have air resistance or friction. So all the power of the engine goes toward accelerating the spacecraft. If you stop the engine of a Jet, it will slow down, but it doesn’t happen for a spacecraft, nothing slow it down. So if you keep your engine running you will continue to go up in speed indefinitely. Well technically at some point when it reach closer to the speed of light the inertia go up and the engine have an harder and harder time accelerating the spacecraft, but we are far far from this kind of speed, so this effect is almost undectable at the speed of our current spacecraft.
The same principle explain the range. We don’t need our engine to run throughout the whole trip, we can turn them on, burn our fuel and the spacecraft will keep going at the same speed even with the engine turn off. We can’t do that on earth, we need enough fuel to keep the engine running all the time.
Things in space are much farther away, so going faster will get you there sooner.
You can go faster in space because there is much less stuff (like air) in the way. The air and other stuff floating around on Earth cause resistance and friction on moving things. This makes it harder to increase speed and can wear down parts of vehicles.
The atmosphere will absolutely mess you up. The faster you go the more the atmosphere pushes back on you. This makes it significantly harder to get up to the speeds spacecraft get to, and significantly harder maintaining these speeds without the craft breaking apart. Space crafts are a relatively simple affair in this regard. Burn fuel to go fast, burn more fuel to go faster.
There is no air in space. The amount of resistance air gives at very high speed is absurdly huge. What they do with rockets is that they make them gain most of their speed higher up where the air is very thin or there isn’t any. The space shuttle is a good example. If you look at the chart I linked below, you will see that the shuttle gained most of it’s speed way above the altitude where commercial jets fly (that is about 10 km in metric).
[https://www.quora.com/Does-the-space-shuttle-heat-up-when-exiting-the-atmosphere-like-it-does-on-re-entry](https://www.quora.com/Does-the-space-shuttle-heat-up-when-exiting-the-atmosphere-like-it-does-on-re-entry)
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