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155 votes
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Do the planets really orbit the Sun?

You are correct, the centre of the Sun is not the Solar System's centre of gravity. A diagram (courtesy Wikimedia Commons), showing how the barycentre of the Solar System has changed over time. ...
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138 votes
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Couldn't I escape Earth's gravity traveling only 1 mph (0.45 m/s)?

The force of gravity decreases with distance. It follows an inverse-square relationship... essential to know when you're grinding out the math, but not essential to a conceptual understanding. The ...
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  • 17k
104 votes

Couldn't I escape Earth's gravity traveling only 1 mph (0.45 m/s)?

Escape velocity reduces as you get further away from the Earth. If you proceed upwards at a constant speed of 1 mph (which as noted will require continuous thrust to counteract gravity), you will ...
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99 votes

Is it possible for a moon to have a higher surface gravity than the planet it is attached to?

Given a pair of objects that are gravitationally bound to each other, they will orbit around their common barycenter (center of mass of the system). The object to be most logically deemed the moon ...
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77 votes
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How can astronauts float in space without being affected by the gravitational force of nearby objects?

Objects in orbit are attracted to each other, it's just their mass is small enough that the force of gravity between them is infinitesimal. Gravitational acceleration is dependent on mass and distance....
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76 votes
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Is the zero gravity experienced in ISS the "artificial" kind?

Gravity is everywhere. There is never any actual true "zero gravity" in the universe. But if you're in freefall - meaning following gravity's pull rather than resisting it, or being blocked ...
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  • 1,836
74 votes

Is it possible for a moon to have a higher surface gravity than the planet it is attached to?

Gravity isn't just about mass, but about distance, too. Our moon has a surface gravity of about 1/6th of Earth, because it is small and less dense than the Earth is. Surface gravity of a body is ...
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70 votes

Have we attempted to experimentally confirm gravitational time dilation?

You don't need a space probe. Or an aircraft. Or even a car. NIST has measured the predicted general relativity time dilation due to a change in altitude on Earth of one foot!
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  • 57.4k
68 votes

What is the context of this seemingly "zero-gravity" photo on Earth?

The biggest give away is the size of this chamber: its too big for any of the known NASA's KC-135 or ZG's 727-200. That leaves us one other candidate: their Russian counterpart IL-76 MDK The interior, ...
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  • 1,249
66 votes

If human space travel is limited by the G force vulnerability, is there a way to counter G forces?

The problem isn't so much that humans cannot sustain high G forces for any extended length of time: The problem is that rockets cannot. If a rocket could sustain 1 g acceleration for a bit over a day, ...
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64 votes
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Why does Voyager 1 lose speed after the sudden gain in speed from gravity assist?

It's the gravitational attraction of the Sun. Voyager is moving away from the Sun and is pulled back by its gravity. Since Voyager is not moving directly away from the Sun, it's trajectory also curves....
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61 votes

Couldn't I escape Earth's gravity traveling only 1 mph (0.45 m/s)?

To sum up the answers: the escape velocity is the velocity that, at a given distance, is sufficient to escape the gravitational field so that no additional energy (= acceleration) is needed. That is, ...
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61 votes

How much stronger does gravity have to be for space travel to be impossible?

There's no "bright line" at which space travel would become impossible; a slightly stronger gravitational pull would require bigger and more expensive rockets. Linear increases in gravity require ...
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58 votes
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Did the Mars rovers actually confirm the gravity of Mars?

The "gravity of Mars" is not a number but rather a complex field. The most recent is remarkably detailed, made up to spherical harmonics degree and order 120, described by 29,512 coefficients: These ...
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57 votes
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Does it require less energy to reach the Sun from Pluto's orbit than from Earth's orbit?

Yes. 1st scenario: A spacecraft orbiting the Sun at Earth distance vs. Pluto distance, shedding its orbital velocity The orbital velocity decreases with distance, according to the following formula, ...
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54 votes
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What causes microgravity (i.e. non-zero gravity) in orbit?

The reason the Space Station is called a micro-g environment rather than a zero g environment is because the Space Station is rotating, because it's in low Earth orbit, and because it's big (for a ...
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54 votes

Do the planets really orbit the Sun?

Using the approximation $$\Delta \mathrm{center}=\frac{m_p}{m_\odot}\cdot\frac{\mathrm{dist}_p}{r_\odot}$$ and data from List of gravitationally rounded objects of the Solar System - Wikipedia: <...
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53 votes
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If a spaceship ran out of fuel somewhere in space between Earth and Mars, does it slowly drift off to the Sun?

What you're missing is some combination of the following: objects launched from Earth orbit are still in orbit around the Sun, objects in orbit don't need fuel to stay in orbit. All the planets ...
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  • 1,744
53 votes

Does Virgin Galactic experience real weightlessness?

This is a point worth emphasizing: When you dive off a high dive, or go on a free fall ride at an amusement park, or fly on Virgin Galactic, you are experiencing weightlessness in exactly the same way ...
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  • 6,903
52 votes

Have we attempted to experimentally confirm gravitational time dilation?

In addition to specific probes like the one mentioned by called2voyage, the effect is significant enough that it affects everyday operations. For example, the GPS constellation needs regular clock ...
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  • 1,482
51 votes

Could colonizing Moon dangerously affect its gravity?

No. The moon isn't that big but it isn't exactly small either. The moon's mass is 73,500,000,000,000,000,000,000kg, that's 73 sextillion, 500 quintillion kilograms. If we moved the whole of mount ...
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  • 18.7k
50 votes
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How does a space probe maintain its trajectory while passing through the extreme gravitational field of the gas giants of our solar system?

The trajectory was not only "unhindered" - it was enhanced! Knowing mass of the planet you can calculate very precisely how the trajectory of a probe flying by will be affected. You modify the ...
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  • 52.4k
46 votes

If human space travel is limited by the G force vulnerability, is there a way to counter G forces?

Ignoring the major point that human tolerance of G forces is not the limiting factor on space travel, plenty of thought has been made on how to counteract G forces, not least by 60s sci-fi writers. ...
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  • 6,067
44 votes

Escaping moons conflict with what I understand of gravity

A very good question! The reason is essentially to do with tides. And a slightly over-simplified summary is: If the moon orbits more slowly than the rotation of the parent body (as our Moon does, 12 ...
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43 votes
Accepted

Could weights make walking on the Moon feel like walking on Earth?

It's complicated! Keep in mind the distinction between weight and mass. On the moon, weight is 1/6 what it is on Earth, but mass is the same. When you hold your arm straight out, you have to exert ...
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41 votes

Unix Epoch in International Space Station

POSIX time doesn't include leap seconds, and is not implemented the same way in every UNIX, so it routinely gets inconsistent for several seconds every couple of years. It is not a high-precision ...
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  • 5,187
40 votes

Have we attempted to experimentally confirm gravitational time dilation?

Yes, time dilation was experimentally confirmed by Gravity Probe A, launched by NASA on June 18, 1976. The clock rates of two masers (one on the probe and one on Earth) were compared, and it was ...
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  • 23.5k
39 votes

Is there a self-rounding celestial body from which an Olympian could jump into space?

No. Saturn's moon Mimas is the smallest body in the solar system known to be rounded through self-gravitation, and it still has a surface escape velocity of 159 m/s, far above the speed achievable by ...
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  • 4,271
38 votes
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Can you take a bath on Mars?

Short answer, No different from Earth in floating. Buoyancy in water or any fluid is based on the weight of water displaced. Floating is based on the weight of the item displacing water. This is ...
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  • 2,211
35 votes

Couldn't I escape Earth's gravity traveling only 1 mph (0.45 m/s)?

You are confusing velocity and acceleration. If you were to jump standing on the surface of the Earth you might experience 8 m/s which is 17 mph velocity upward, but the acceleration of ...
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  • 4,361

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