110
votes
Accepted
Why is the period of a geostationary satellite not exactly 1440 minutes?
A day, i.e. 24 hours, isn't how long the Earth takes to rotate. It's the time between the Sun being in the same place in the sky, but that isn't the same as the time it take the Earth to rotate 360° ...
- 360k
97
votes
Accepted
Why is Microgravity called "Microgravity"?
Microgravity is used because zero gravity is inaccurate. The ISS, at 400 km, experiences an average atmospheric density of 4 nanograms per cubic meter. It's frontal area varies from 700-2300 square ...
- 37k
68
votes
Why don't we put satellites into an orbit where there is (almost) no time dilation/contraction compared to Earth's surface?
The higher the satellite's orbit, the more of the Earth it can see (and hence the fewer satellites you need to ensure complete coverage of the Earth). The particular orbits chosen for navigation ...
- 10.1k
60
votes
Accepted
Cause of weightlessness
The statement in the image is shockingly wrong. Given that the Earth is visible behind the astronaut, the reduction of the gravitational force due to distance must be fairly small; on the ISS for ...
- 2,405
59
votes
Why doesn't the Moon disrupt the orbits of geostationary satellites?
The moon absolutely disrupts the orbit of a geostationary satellite. Just as the moon can cause tides on the Earth's surface, the moon's gravitational pull on a satellite will cause it to begin to ...
- 1,671
51
votes
Accepted
Why aren't satellites disintegrated even though they orbit earth within earth's Roche Limits?
The Roche limit denotes how close a body held together by its own gravity can come. Since gravity tends to be the only thing holding moon-sized objects together, you won't find natural moons closer ...
- 9,038
49
votes
Why aren't satellites disintegrated even though they orbit earth within earth's Roche Limits?
The Roche limit applies only to bodies which are held together purely by internal gravitational attraction. Compact objects such as artificial satellites are held together by the much stronger inter-...
- 8,522
47
votes
Why don't we put satellites into an orbit where there is (almost) no time dilation/contraction compared to Earth's surface?
You have to differentiate between corrections that are sources of error and corrections that are well understood and can be accounted for.
Time dilation is a well understood effect, and if you wanted ...
- 7,560
27
votes
Can there be other Positions of Geostationary Satellites?
Lets consider what "Geostationary" actually means:
A geostationary orbit is a particular type of geosynchronous orbit, the distinction being that while an object in geosynchronous orbit returns to ...
- 2,234
23
votes
Accepted
Are there any satellite orbit earth perpendicular to the sun and what is the name of this kind of orbit?
There are satellites that constantly observe the sun. Among them are the STEREO pair of satellites. These satellites don't orbit Earth, but orbit the Sun–one a little faster than Earth, one a little ...
- 24.5k
21
votes
Why aren't satellites disintegrated even though they orbit earth within earth's Roche Limits?
The Roche limit is a limit on objects being held together by their own gravity. Satellites are held together by much stronger forces. Different parts of the satellite are ultimately connected by ...
- 104k
18
votes
Why is Microgravity called "Microgravity"?
Microgravity doesn't mean that the gravitational field is negligible, just that the system in question does not feel its effects due to being in free-fall. The interior of a freely-falling elevator ...
- 70.5k
16
votes
Are there any satellite orbit earth perpendicular to the sun and what is the name of this kind of orbit?
This is called a “sun synchronous” orbit.
If the Earth were a perfect sphere, orbits around it would have a fixed orientation in space. As the Earth goes around the Sun, an orbit would have the ...
- 14.6k
16
votes
Why doesn't the Moon disrupt the orbits of geostationary satellites?
The moon will always "perturb" the orbit to some extent, but to "disrupt" an orbit is to kick the satellite out of orbit. And this will only happen if the orbit is very wide, ...
- 5,691
15
votes
Accepted
Kepler's 3rd law: ratios don't fit data
that equality should be a proportional to sign. In particular, in SI, the squared period has units of seconds squared, and the semi-major radius of of the orbit cubed is in meters cubed, so they can'...
- 41.8k
15
votes
Accepted
Why can't the Chinese rocket Long March 5B keep revolving around the Earth without re-entering?
You are describing something called orbital decay, which is well-understood. Here are the basics, now including more detail:
With increasing height, the earth's atmosphere gradually fades out and ...
- 96.7k
14
votes
Can there be other Positions of Geostationary Satellites?
Strictly speaking, the only geostationary orbits are circular equatorial orbits at a very specific altitude above the surface of the Earth.
Satellites orbit on a plane that contains the center of ...
- 41.7k
12
votes
Are satellites doing free fall?
May be Newton's cannonball-thought-experiment
can convince your intuition
that there is no principal difference
between a body falling down to the earth
and a satellite circulating the earth.
...
- 40.4k
12
votes
How does a satellite maintain circular orbit?
There is something which I do not like about the answers, and it has to do with the fact that you have a really good intuition here and the other answers are giving some specific exceptions to that ...
- 38.6k
11
votes
Why is Microgravity called "Microgravity"?
You are not the only person who has problems with the term microgravity. I doubt there is any record of how or why the term was coined.
My guess is that NASA introduced the term microgravity because ...
- 57.6k
10
votes
How does a satellite maintain circular orbit?
Yes you are completely right! The astronaut could apply a force onto the satellite and make it break its previous circular orbit! Suppose in fact the following scenario: the astronaut exits the ...
- 4,565
9
votes
Accepted
Can a radially launched projectile orbit a planet?
No. Assuming the planet is not rotating (so the launch velocity is truly radial), it will go up and then come straight back down - unless its velocity is greater than the escape velocity from the ...
- 70.5k
9
votes
Why don't we put satellites into an orbit where there is (almost) no time dilation/contraction compared to Earth's surface?
It's probably worth adding that the altitude you're talking about is in the inner Van Allen radiation belt, which extends from about 1,000 to 12,000 km above the surface. Any satellites at that ...
- 199
9
votes
Is the mass of Satellite + Earth system less than their individual masses?
The fall in P.E is not compensated by increase in K.E.
Actually it is, unless the satellite is acted on by some external force. If we just let the satellite free fall from infinity, then it has an ...
- 57.6k
8
votes
Accepted
Can there be an orbit beyond the Geostationary orbit?
Note that the moon, 385000km away, is way beyond geostationary orbit (42164km from the Earth's centre), but still "functions" as a natural satellite, orbiting the Earth regularly every 28 days or so.
...
- 745
8
votes
Why does Kepler's Law not apply to geostationary satellites?
Kepler's law does work for Geostationary orbits. The reason is that the $T^2=kr^3$ is valid for k for only one particular system. Using the first data you calculated k for the earth-sun system(where k ...
- 2,958
8
votes
Accepted
What is the trajectory of a satellite the speed of which in circular orbital is reduced instantaneously?
The orbit of the satellite would be elliptical, as it must honour 2 elementary conservation laws:
Law of conservation of total energy
Law of conservation of angular momentum ( measure of rotational ...
- 1,912
8
votes
Accepted
Satellite image anomaly
I don't know about satellite photography, but ground-mapping cameras on aircraft often re-construct color images from sequential red, green, and blue images. The re-construction algorithm takes into ...
- 15.7k
8
votes
Cause of weightlessness
You are correct that the caption is wrong. The cause of weightlessness is that gravitational and inertial forces cancel, exactly according to the equivalence principle.
- 25.9k
8
votes
Cause of weightlessness
Whatever written in the book is wrong.
But let us assume it is correct then had it been the case then the moon would have flown away as well as the ISS along with the astronaut.
- 1,424
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