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98 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 ...
Anthony X's user avatar
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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 ...
Starfish Prime's user avatar
52 votes

Can a moon orbit its planet faster than its planet rotates?

What immediately springs to mind is the Martian moon Phobos, orbiting the planet in 7 hours 39 minutes. That's a fair bit quicker than the 24 hour 37 minute sidereal period of Mars. From the surface ...
SE - stop firing the good guys's user avatar
45 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 ...
Martin Kochanski's user avatar
45 votes
Accepted

Are all satellites of all planets in the same plane?

Most large regular satellites orbit in the equatorial plane of their planet. Every planet spins, and thus has an equatorial bulge caused by centrifugal forces that over time aligns the satellites' ...
HigherIdeals's user avatar
28 votes

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

Yes, it is possible. As James K observed in a comment, the surface gravity of Uranus is slightly less than that of Earth, but its mass is 14 times larger. If Earth were orbiting Uranus, it would be a ...
Mark Foskey's user avatar
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26 votes

If colonists burrowed far enough under the ice on Ganymede or Europa, would the ice provide adequate protection for them from Jupiter's radiation?

Yes, it absolutely would! The radiation on Europa is about 5.4 Sv (540 rem) of radiation per day. Looking at this guide, and assuming you want to meet OSHA standards of 5 rem per year, you would need ...
PearsonArtPhoto's user avatar
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17 votes
Accepted

Many moons in same orbit

You can have small moons in the two stable (L4 and L5) Lagrangian points of a main moon, like the Saturn moon Dione's companions Helene and Polydeuces, or Tethys' Telesto and Calypso. If you want ...
SE - stop firing the good guys's user avatar
17 votes

Why is there never enough room on satellites to hold all the equipment needed?

Why is there never enough room on satellites to hold all the equipment needed? Welcome to Space Exploration SE! Because of the time involved from planning until completion of goals at the end of the ...
uhoh's user avatar
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16 votes
Accepted

Delta V required to land and then ascend from the surface of every celestial body

Most search results I get for "solar system delta-v map" includes this information. I quite like this one: To read out the ascent cost from these maps, you take the single number between ...
SE - stop firing the good guys's user avatar
16 votes

Why is there never enough room on satellites to hold all the equipment needed?

Perhaps there is an assumption that new photos of Earth taken from space would be inspiring, enlightening, and meaningful in some way. That was certainly true of the first photos that were taken, but ...
Steve Pemberton's user avatar
14 votes

Why is there never enough room on satellites to hold all the equipment needed?

You seem to have overlooked DSCOVR, which pretty much just stares at our planet. https://epic.gsfc.nasa.gov/
Organic Marble's user avatar
13 votes

Why is there never enough room on satellites to hold all the equipment needed?

We have a large number of sources taking photos of our planet. This includes: low-altitude Earth observation satellites (Landsat, SPOT, Maxar). These take high-resolution photos of a small area at a ...
Hobbes's user avatar
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12 votes

Can a moon orbit its planet faster than its planet rotates?

Are there any known examples of this situation? Yes! In addition to Phobos mentioned in this answer and from Astronomy SE: How did “oddball” Valetudo, Jupiter's new prograde moon, end up in a wider ...
uhoh's user avatar
  • 149k
11 votes

Bombing the Moon: how much debris would there be in stable orbit?

The empirical answer is that there is absolutely no risk of debris reaching a permanently stable orbit. If so, then there would already be a lot of such because of the millions of impacts that the ...
Everyday Astronaut's user avatar
11 votes

Escaping moons conflict with what I understand of gravity

Orbits beneath synchronous orbits have a higher angular velocity than their planets rotation, orbits above have a slower angular velocity. Drag (atmospheric or tidal) would try to match the angular ...
David-H-K's user avatar
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11 votes

Would a Foucault pendulum work on the Moon and on the Galilean moons?

tl;dr As long as the pendulum can oscillate reliably, the Foucault Pendulum rotation is independent of the strength of gravity. If your question is about a normal pendulum and you didn't actually mean ...
uhoh's user avatar
  • 149k
10 votes
Accepted

Smallest known asteroid with moon

This may not be complete or up to date, but sorting the List of minor planets with moons for smallest primary shows 2015 TD144 as having a 0.1 km diameter primary and 0.1 km diameter secondary. That ...
uhoh's user avatar
  • 149k
10 votes
Accepted

How close do gravitational 2-body hierarchy levels get?

The zone of influence of a larger body over a smaller body is known as the hill sphere. Within this sphere the gravitational attraction of the larger body will dominate its satellite and satellites ...
Slarty's user avatar
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9 votes

Did the spacecrafts Galileo or Juno use the Galilean moons for a gravity assist before entering Jovian orbit?

https://history.nasa.gov/sp4231.pdf Galileo definitely did gravity assist at Io. See the link (3.7 Mb pdf), pages 202-203. quote: Two of the first events of Arrival Day were the Orbiter’s flybys of ...
Heopps's user avatar
  • 9,041
9 votes

Is it possible to create an analytical ephemeris from raw position and velocity of a Body?

It's possible, because that's partly how VSOP was made, but the amount of effort involved is immense. What you are describing is at the very least a doctoral thesis, if not several of them. Why not ...
Ryan C's user avatar
  • 7,852
8 votes

Why are most of Jupiter's moons retrograde?

According to the Wikipedia article on irregular moons, retrograde orbits further out from a planet are more stable than prograde: Retrograde satellites can be found further from the planet than ...
Russell Borogove's user avatar
8 votes
Accepted

Why is Enceladus the "most interesting place for astrobiology" in the Solar System other than Earth?

According to the recent discoveries Europa can have plume activity too. Hubble's observations in ultraviolet and re-analysis of Galileo magnetometer data show the possibility of the plumes at Europa. ...
Heopps's user avatar
  • 9,041
8 votes

Can a moon orbit its planet faster than its planet rotates?

Though it's a stretch, GNSS satellites in MEO like GPS, Galileo, GLONASS, etc... are examples of such "moons" orbiting planet Earth, since they're positioned at an altitude lower than ...
Will's user avatar
  • 180
8 votes

Are all satellites of all planets in the same plane?

Are all satellites of all planets in the same plane? Arguably... No, not even for the satellites around a given planet, not even if their orbits were nearly coplanar. Depending on how you look at it, ...
uhoh's user avatar
  • 149k
7 votes

How big a nuke would be needed to break Phobos out of orbit?

Simply to lift get Phobos out of Mars orbit you would need to increase its orbital velocity by a factor of $\sqrt{2}$ (this is generally true for any object in circular orbit). Phobos orbits at about ...
Steve Linton's user avatar
  • 19.6k
7 votes

Why is there never enough room on satellites to hold all the equipment needed?

In fact, there is always enough room on a satellite for everything it needs, because if it couldn't fit something necessary, it wouldn't be able to go into space. (At least, not without failing very ...
Nij's user avatar
  • 301
6 votes
Accepted

Which moon is best? (for gravity assists)

Using a selection from Wikipedia's List of natural satellites (moons with listed masses) I, with some simplifying assumptions, calculated both the deflection, $\delta$, and $\Delta V$ for each moon. ...
BrendanLuke15's user avatar
5 votes

Where does Io's sulfur come from?

It's erroneous to think of those volcanos on Io as sulphur volcanos. This is an old idea. Io's volcanos are instead somewhat similar to volcanos on the Earth, where the primary product is mafic rock (...
David Hammen's user avatar
  • 73.8k

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