100 votes
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Could we send a man safely to the Moon in a rocket without knowledge of general relativity?

The trouble with orbital mechanics is that it rapidly gets exceedingly complicated and hard to make intuitive sense of. However I think there is a reasonably straightforward way to show how little ...
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95 votes
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Where does the extra kinetic energy come from in a gravitational slingshot?

Cory, here's a different way of thinking about gravity assists that may help: First is my short answer for readers in a hurry: What is really going on is a giant game of pool, with fast-moving ...
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80 votes
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How long would it take me to travel to a distant star?

Start by considering what is seen by the people watching you from the Earth. Nothing can travel faster than the speed of light, $c$, so the quickest you could get to Kepler 186f would be if you were ...
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71 votes
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If I'm floating in space and I turn on a flashlight, will I accelerate?

Can photons push the source which is emitting them? Yes. If yes, will a more intense flashlight accelerate me more? Yes Does the wavelength of the light matter? No Is this practical for ...
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68 votes

Why don’t astronauts “push” spacecraft?

When an astronaut bumps against the wall of the spacecraft, the spacecraft does gain whatever momentum the astronaut transfers to the wall. However, the astronaut loses momentum-or gains it in the ...
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63 votes

Could we send a man safely to the Moon in a rocket without knowledge of general relativity?

The Jet Propulsion Laboratory has incorporated general relativistic effects in its numerical integration of the planets since the mid to late 1960s. For example, the JPL DE19 ephemeris, released in ...
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  • 39.2k
53 votes

Why don’t astronauts “push” spacecraft?

How do astronauts, especially those inside small spacecraft like the Crew Dragon, not “push” the spacecraft when they bounce and push off walls? You're right that when an astronaut collides with the ...
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  • 4,960
51 votes
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"To come back to Earth...it can be five times the force of gravity" - video editor's mistake?

Reentry speeds are fast. Astonishingly fast. The shuttle reentered at 7.8km/s. Now note the units. That's "per second." That's 28,158km/hr. And you have roughly 100 vertical kilometers ...
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43 votes
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Is it possible to sail "close hauled" with a solar sailship?

A sailboat can make headway against the wind because of the sum of force vectors due to the wind interacting with the sail and, due to the keel interacting with the water. A sailboat without a keel ...
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  • 9,307
41 votes

Where does the extra kinetic energy come from in a gravitational slingshot?

Energy is in fact conserved, even in gravitational slingshots. After the slingshot, the velocity of the spacecraft may indeed change, which means its kinetic energy will also change. If this happens, ...
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  • 2,386
27 votes

Could we send a man safely to the Moon in a rocket without knowledge of general relativity?

A few sanity checks without actually computing anything: First, the error due to neglecting general relativity is so small that it didn't affect prediction of lunar eclipses and wasn't actually ...
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  • 6,319
26 votes

Could we send a man safely to the Moon in a rocket without knowledge of general relativity?

I'll start the ball rolling on this one. My GR knowledge is probably not good enough to make this a truly satisfying answer... The gravitational acceleration for an object moving radially at non-...
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  • 111k
26 votes
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How to travel between two stationary worlds in the least amount of time? (time dilation)

Suppose that A and B are at rest relative to each other (which you have) and in their mutual rest frame are separated by 100 light years. That means that no signal can travel from A to B (or vice-...
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  • 70.2k
20 votes

If I'm floating in space and I turn on a flashlight, will I accelerate?

Can photons push the source which is emitting them? Yes, photons have momentum and momentum must be conserved. The source is pushed in the opposite direction of the photons. If yes, will a more ...
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  • 16k
20 votes

How does a spacecraft enter Earth's atmosphere?

It all comes down to the fact that we are moving too. How can the bird drop down and catch the worm, if the earth and the worm are moving so quickly. It can because the tree, the bird, and the air ...
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  • 3,876
18 votes

Is it possible to sail "close hauled" with a solar sailship?

The question is about solar sails - I thought I would add this (too long for a comment) to clarify some confusion that is apparent in answers and comments. Solar sailing uses photon momentum, not the ...
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  • 111k
14 votes

Why does the speed of the propellant limit the speed of a space ship in open space?

Why can't a space ship accelerate infinitely? Because a space ship needs to carry fuel, and because that fuel needs to be contained in a fuel tank. That need to carry the fuel needed to make the ...
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  • 39.2k
14 votes

Why don’t astronauts “push” spacecraft?

An astronaut pushing against the wall of a spacecraft does cause it to move due to Newton's Third Law and conservation of momentum, as you have noted. However, the movement of the spacecraft is not ...
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  • 4,780
13 votes

Is it possible to sail "close hauled" with a solar sailship?

Yes, sort of, sometimes. As others have indicated, it is not possible for a solar sail to produce a force in the direction of the Sun. This does not, however, mean a solar sail cannot take a ...
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  • 289
13 votes

Parker Solar Probe passing extremely close to the Sun; what relativistic effects will it experience and how large will they be?

I've talked to one of the engineers who is involved with timekeeping on the Parker Solar Probe mission. I have not yet talked with the principle timekeeping lead, so this answer may have to be revised....
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11 votes
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Is interstellar flight possible in near future in a way that would keep our civilization alive?

The link you posted refers to a design which would supposedly make use of faster than light travel. After reading this, I immediately stopped reading, since this is not possible. I don't care if it's ...
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  • 15.7k
9 votes

Where does the extra kinetic energy come from in a gravitational slingshot?

Gravity assists don't change speed in the two body problem. An object approaching a lone gravitating body will enter and leave the vicinity of that body with exactly the same speed. All that a lone ...
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  • 39.2k
8 votes
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Questions after watching the movie Interstellar

1.When they travel to the watery planet, they say that 1 hour on this planet is 7 yrs om earth. How is this possible? Is the planet moving at a speed close to c? Or does strong gravitational field ...
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  • 10.5k
8 votes
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Do stray photons become interstellar daggers to a space traveler near the speed of light?

Yes, but you'll have to go really, really fast. And even then, don't worry about the photons. The relation between velocity $v$ and the observed and "true" wavelength $\lambda_\mathrm{obs}$ and $\...
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  • 10.4k
7 votes

Parker Solar Probe passing extremely close to the Sun; what relativistic effects will it experience and how large will they be?

Even missions to Phobos have to consider relativity, but that's because of the necessary sensitivity of any instrument trying to measure Phobos's gravity. So my cop-out answer is it depends on what ...
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7 votes
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Why can't we prepare the moon for colonization by shipping oxygen to its atmosphere?

The main problem is that it would be a lot of oxygen. Just a ten-foot layer of one atmosphere air (pretending you could somehow get it to stay in a ten-foot layer) over the moon would weigh around $10^...
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  • 16.6k
6 votes

Why does the speed of the propellant limit the speed of a space ship in open space?

The maximum theoretical speed that a spaceship can reach isn't limited by anything (except the speed of light of course). However for a practical spaceship with a finite amount of fuel, the speed of ...
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  • 1,434
6 votes

Does speed in space really matter?

Your assertion that only relative speeds matter is absolutely correct. However, you might want to look at the velocity addition of special relativity for space ships or whatever else travelling at ...
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  • 106k
6 votes

Relativistic Time dilation and travel to exoplanets

You might be interested to look at How long would it take me to travel to a distant star? as this discusses the physics behind the calculation. Acceleration does not cause time dilation, so ...
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