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99

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 will be the one of lesser mass because it will be further from the barycenter than its companion. For example, Pluto has a gravitationally bound companion named ...


74

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 inversely proportional to the square of its radius, holding mass constant. That means that if you compressed the moon such that it was $\frac{1}{\sqrt{6}}$th of its ...


59

The instability in orbits of our artificial satellites come from a few basic causes: Atmospheric drag and solar wind effects The Earth isn't a perfect uniform sphere but is slightly lumpy, which means its gravitational field isn't uniform Other massive objects in the solar system perturb their orbits with their gravity So let's consider them one by one. ...


54

No, because Mars can't have eclipses. Strictly speaking, Mars has only transits. The difference is that Mars's moons are smaller than the Sun as viewed from Mars, thus they don't block out the entire sun. Eclipses are defined as only occurring if the entire sun is blocked, or at least the vast majority. Phobos blocks out only about 60% of the sun at most. ...


51

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 of the planet, Phobos and Deimos will therefore appear to cross the sky in opposite directions. Other solar system examples include the small inner Jupiter moons ...


43

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 degrees per day while the Earth rotates about 360 degrees per day) then the moon will gradually orbit further and further away. If the moon orbits faster than ...


42

Essentially, this is a result of observational bias. A natural satellite will only orbit a parent for extended time periods precisely because the orbit it is in is stable †. The plain truth of the matter is that we are simply injecting satellites into unstable orbits. If you were to move natural satellites into the same orbits, they'd be unstable too. ...


35

Yes, it is. Given two spherical, uniform, bodies one with mass $m_1$ and radius $r_1$ and the other with mass $m_2$ and radius $r_2$, then the surface acceleration due to gravity will be equal when $$r_2 = \sqrt{\frac{m_2}{m_1}} r_1$$ For the Moon to have the same surface gravity as the Earth, we can plug in suitable numbers, and you end up with a radius ...


28

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 very large moon, but it would still be considered a moon, and thus a moon with a higher surface gravity than its planet. The reason this is possible is that ...


23

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 to only allow 1 part in 40,000 of the base radiation to make it through. The website linked indicates you want a mass of about 375 pounds/square foot to only ...


17

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 more than three moons, you can organize them in a Klemperer rosette, although this type of configuration is not stable. Another option is to have a huge number of ...


14

This is the 1978 image of the Pluto system that led to the discovery of Charon. This is a negative, so the big black blob in the middle is Pluto and Charon. Charon? It's the little bump on the upper right of that blob. You can barely make out Charon. Additional satellites? No. This is a 1990 image of Pluto and Charon taken by the Hubble: This was before ...


14

Your guess is correct. To quote NASA's page, The mosaics each consist of multiple narrow-angle camera (NAC) images with data from the wide-angle camera used to fill in areas where NAC data was not available.


13

The most general answer is this: The Solar System is 4.6 billion years old, anything that happens "quickly" has happened a very long time ago. For example, it is thought that in the early solar system the orbits of Neptune and Uranus where switched, with Neptune being further in than Uranus, but the gentle tug of Jupiter and Saturn eventually pushed them ...


12

It is not being overlooked. There are studies which are being carried out about the feasibility of life on Titan based on methane. It is true that Titan has all the necessary conditions for life: It is not in thermodynamic equilibrium It has more than enough of carbon based molecules It has a fluid environment (ethane/methane) for chemical reactions to ...


12

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 orbit? Why are most of Jupiter's moons retrograde?


11

If my understanding in correct, the orbit is perfectly circular if the dimensionless orbital parameter of eccentricity (e) is zero. However, I'm not sure how the inclination works with this, and that comes down to the question "circular in what plane?" Anyway, I present to you: Asteroid 113474 (2002 ST57). http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=113474 ...


11

Depends on what you mean by a small stone and if it has to be in a long-term stable orbit around Mars. According to Mark Adler et al. in the Use Of MRO Optical Navigation Camera To Prepare For Mars Sample Return, JPL 2012 (PDF), the Mars Reconnaissance Orbiter's Optical Navigation Camera (ONC) could be used to detect small Mars' satellites: Unknown small ...


11

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 to talk about the Foucault Pendulum phenomenon, then a normal pendulum will still work even in very weak gravity in a reasonable way, as long as the length of ...


10

would you be able to "fall off" Phobos ? Right now, no. In a few million years, yes. Phobos is slowly spiraling in towards Mars. In a few million years it will be close enough to Mars that tidal forces from Mars will tear Phobos apart. The very rocks that loosely comprise Phobos will fall off of Phobos. Phobos is 27 × 22 × 18 km. Your "edge of Phobos" is ...


10

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 Moon has been subject to in the past. However, for human missions, the question should not only be about permanently stable orbits but also about orbits that ...


10

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 velocity to the planets rotation. So below a synchronous orbit objects get slower, above it they would speed up (and slow down the rotation of the body they are ...


9

In order to leave a gravity well, you need to exceed the escape velocity. The escape velocity depends on the mass of the body and your current distance from its center. The escape velocity on the surface of Phobos is approximately 11.4 m/s or 41 km/h. It likely varies depending on where you are on Phobos because the moon is quite irregular-shaped. You would ...


9

"Plan" is a vague word... NASA has an orbiter mission in the works: Europa Clipper as a predicate mission prior to any attempt to land. A Europa Lander is a "concept under study" - that is, they are making plans to do the project, but haven't budgeted for it, nor have then given the actual project the proverbial "green light." NASA does have a page about a ...


8

It is theoretically possible, but such a satellite would probably not be in a stable orbit. Such a system is not known in the Solar system and due to gravitational perturbations it would not last long. Popular Science Astro


8

They took it seriously enough to publish a rebuttal to it 3 years later, as is documented in Wikipedia. As is well known today, there was an error in the mathematics. Specifically, Wikipedia quotes: Singer's critique was justified when earlier studies were discovered to have used an overestimated value of 5 cm/yr for the rate of altitude loss, which ...


8

The short answer is that they DON'T need a correction to stay in orbit. But, as Russell, Rikki, Ross, and others pointed out, they need correction for the "stationary" part of a "stationary geo-synchronous orbit" because they need to stay where our terrestrial dishes are pointing. If they correct small errors they only require small amounts of energy (fuel)....


8

The common word, as was mentioned in the comments, is the use of the word System. I have seen this in reference to the Jovian, Saturnian, and Plutonian systems, referring to the planet and collection of moons. If one is referring to just the moons, without the planet, usually the word Satellite is injected (Saturnian Satellite System). It seems this isn't ...


8

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. Cassini took lots of photos of water plumes at Enceladus. But the Galileo probe had a problem with a failed main antenna, that dramatically reduced its ...


8

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 geostationary orbit but are still high enough for atmospheric drag to be so low that they can maintain that orbit without needing additional sources of propulsion/...


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