75
votes
Accepted
Why is the asteroid belt shaped like a triangle?
It's not. The image doesn't show the main asteroid belt. It shows the Jovian Trojans (in green) , and the Hilda Asteroids (in red).
The Hildas are a dynamical group of a few thousand known asteroids ...
- 4,823
59
votes
Accepted
What is the **actual** average distance of the Moon from Earth?
Mean distance averaged over time for any Keplerian orbit is $a(1+\frac{1}{2}e^2)$, where $a$ is the semi-major axis and $e$ is the eccentricity. Using your NASA fact sheet, I get about 384,979 km for ...
- 16.7k
53
votes
Accepted
Is Earth's Surface "In orbit"?
1. Is material on Earth's surface not in free fall around Earth's center?
No. Material on the Earth's surface -- or inside it -- is not in orbit, and so is not in free fall. You can temporarily put ...
- 18.4k
44
votes
Accepted
Are there any stars that orbit perpendicular to the Milky Way's galactic plane?
The Sun and most of the other stars are in the bulging disk of the Milky Way galaxy, but about 1% of the galaxy's stellar mass is in the galactic halo. The halo also includes 50 globular clusters and ...
- 16.7k
43
votes
What is the shape of orbit assuming gravity does not depend on distance?
Circular orbits are always possible for any central force law, but noncircular orbits would resemble rosettes. Here's a specific example for the case where the force is constant with distance:
By ...
- 4,885
41
votes
Is Io a magic energy machine?
The energy driving Io's volcanism is derived from its orbital energy and (to a lesser extent by the orbital energy of the other moons and the energy of Jupiter's rotation). Similar mechanisms power ...
- 3,455
41
votes
Can a planet with no atmosphere be orbited at extremely low altitudes?
Yes, but.
Firstly the 400km orbits of the ISS are already extremely low, in comparison to the 6400km radius of the Earth. See https://what-if.xkcd.com/58/ for pictures of orbits. So if you rephrase ...
- 129k
39
votes
Accepted
Would an object shot from earth fall into the sun?
Assume that a spacecraft is instantaneously accelerated at the Earth's surface (disregarding the atmosphere for simplicity). We'll consider this from the Sun's reference frame; in other words, the Sun ...
- 506
37
votes
If a satellite is put into the same orbit of the Sun as Earth, how does it avoid hitting Earth?
An orbit is defined by an instantaneous position and velocity. An object in the same position as Earth but with a different velocity does not have the "exact same orbit" as Earth. The STEREO ...
- 2,632
35
votes
Why is the Moon's orbit so complicated?
The moon is so big that the processes that circularize and reduce the equatorial inclination would take much longer. The moon is big because of how it formed: a huge collision in the early solar ...
- 129k
34
votes
Is it possible to have satellites (natural or not) orbit the same celestial object in different directions ? (clockwise, counterclockwise)
It is absolutely possible. Moons that formed with their planet will be in prograde orbits, but moons that are captured bodies (such as the outer satellites of Jupiter, Saturn, Uranus, and Neptune (...
- 4,823
32
votes
When a protoplanet falls toward the Sun, even from a billion miles, how could it ever be flung out of the Solar System?
Your initial analysis is correct for a two body system.
In a 3 or more body system then yes, the total energy must be conserved, but how that is partitioned between the bodies can be varied by their ...
- 163k
28
votes
If suddenly "knocked" or perturbed from its orbit, would gravity eventually return the Earth to its original orbit?
There's a few parts to this question so there's more than one answer.
Earth gets knocked a little bit out of its orbit all the time by gravitational influence of other planets in our solar system. ...
- 24.2k
25
votes
Accepted
Does the Inverse Square Law mean that the apparent diameter of an object of same mass has the same gravitational effect?
Exactly the same tides, yes.
The Sun is the same angular size as the Moon but about 400 times further away.
If the Sun were as dense as the Moon then its gravitational pull would be 400 times that of ...
- 1,840
24
votes
Why does the closest approach of star S2 to Sgr A* not appear to be near the focus of its elliptical orbit?
The orbital elements are on wikipedia:
$$e=0.884\ a=0.125'',\ i=134^\circ,\, \Omega=228^\circ$$
(At an assumed distance of 8kpc, $0.125'' = 1000au$)
It is the inclination that means that the black ...
- 129k
24
votes
What is the **actual** average distance of the Moon from Earth?
Lunar theory is complicated. ;) The Moon's orbit around the Earth is only approximately a constant Keplerian ellipse. As I mentioned here, the Moon's orbit around the Earth-Moon barycentre has an ...
- 16.7k
24
votes
Is it coincidence that the earth's rotation and revolution are in the same direction?
Not a coincidence, most other planets rotate the same way. It is a consequence of how planets were formed from a disc of gas and dust that was orbiting around the sun as it formed.
The sun formed ...
- 129k
23
votes
Is Io a magic energy machine?
And this seems to go on forever.
Not strictly true.
Io seems to remain in orbit perpetually. Have we discovered magic?
No and no.
Io's orbital speed is $v = 17.33 \mathrm{km/s}$. Its mass is $m =...
- 18.4k
23
votes
Accepted
Why does the Parker Solar Probe slow down as the distance from the Sun increases?
Why does the “parker solar probe” lose speed as the distance from the sun increases?
Because energy and angular momentum are individually conserved quantities in the two body problem. Except for ...
- 36.3k
22
votes
Accepted
Does anything orbit the Sun faster than Mercury?
The recently discovered asteroids 2019 LF6 and 2020 AV2, each taking 151 days to orbit the Sun, have the shortest periods currently listed in the JPL Small Body Database.
Vulcanoids are difficult to ...
- 18.8k
21
votes
Are there any stars that orbit perpendicular to the Milky Way's galactic plane?
Most stars in the galaxy are in the disc, but there is also a population in the galactic halo, these are in orbits at essentially random inclinations to the disc. There will be some that orbit ...
- 129k
21
votes
Under which conditions could a planet's massive moon's orbit get closer to the planet?
Yes, it is possible for a moon of a planet to move closer to the planet. If a moon moves close enough to the planet, the moon will eventually reach its Roche limit and shatter or else collide with ...
- 8,037
20
votes
Accepted
Can the paper narrowing Solar System's barycentre to within 100m help find Planet Nine?
Short answer
Short answer is no, it can't prove or disprove the existence of Planet 9. Even if there is a significant difference between the barycenter of the solar system with and without Planet 9 ...
- 950
20
votes
Accepted
Is S2 still the fastest known star in the galaxy?
S0-2 (I assume you mean the star orbiting the Milky Way central black hole) actually reaches speeds of 7650 km/s at periapsis.
However, if you read further down the wikipedia article you will see that ...
- 163k
20
votes
Accepted
Why is the L1 point (Lagrange) almost 1 million miles from Earth? Shouldn't it be closer to us?
If you divide 333,000 by 10,000, you get 33.3, meaning the Sun should be yanking on an object placed at L1 with more than thirty times the force as the Earth is....
That's not how the Lagrange points ...
- 36.3k
20
votes
What are the orbital velocities of the other planets? For objects in a 'Low-Earth-Orbit' around planets other than Earth, e.g.?
The formula for orbital velocity is $\sqrt{GM/r}$ and for a "low" orbit you would mean orbit at, or close to the surface, ie with a radius equal to the radius of the planet. This makes ...
- 129k
20
votes
Accepted
Orbit of the moon so that there are no eclipses
Answer: yes, a no-eclipse orbit is possible
The plane of the Earth-Sun orbit (the ecliptic) and the plane of the Earth-Moon orbit must intersect each other because they both contain at least one point ...
- 1,151
19
votes
Is it possible to have satellites (natural or not) orbit the same celestial object in different directions ? (clockwise, counterclockwise)
Partial and supplemental answer.
As an example1 from Wikipedia's List of natural satellites; Mooons by primary:
(Jupiter's) 84 known irregular moons are organized into two categories: prograde and ...
- 30.7k
17
votes
Accepted
Can a bullet be fired on the moon and sent it into orbit?
Almost. (EDIT: actually yes, see below)
The speed of an object in orbit depends on the radius of the orbit, and the mass of the body being orbited.
The lowest orbit possible is where the bullet is ...
- 18.4k
17
votes
Would an object shot from earth fall into the sun?
The launch you described is similar to that of the Parker Solar Probe launched August 2018 at 12km/s in a direction opposite Earth's orbital velocity, so it fell toward (rather than into) the Sun, in ...
- 406
Only top scored, non community-wiki answers of a minimum length are eligible