Gravitational anomalies slightly distorting the orbits of some Lunar Orbiters led to the discovery of mass concentrations (dubbed mascons) beneath the lunar surface caused by large impacting bodies at some remote time in the past. These anomalies are significant enough to cause a lunar orbit to change significantly over the course of several days.
Joining an earlier subsatellite PFS-1, released by Apollo 15 astronauts eight months earlier, PFS-2 was to measure charged particles and magnetic fields all around the Moon as the Moon orbited Earth. The orbit of PFS-2 rapidly changed shape and distance from the Moon. In 2-1/2 weeks the satellite was swooping to within a hair-raising 6 miles (10 km) of the lunar surface at closest approach. As the orbit kept changing, PFS-2 backed off again, until it seemed to be a safe 30 miles away. But not for long: inexorably, the subsatellite's orbit carried it back toward the Moon. And on May 29, 1972—only 35 days and 425 orbits after its release—PFS-2 crashed.
Be careful of the orbit chosen for a low-orbiting lunar satellite. "What counts is an orbit's inclination," that is, the tilt of its plane to the Moon's equatorial plane. "There are actually a number of 'frozen orbits' where a spacecraft can stay in a low lunar orbit indefinitely. They occur at four inclinations: 27º, 50º, 76º, and 86º"—the last one being nearly over the lunar poles. The orbit of the relatively long-lived Apollo 15 subsatellite PFS-1 had an inclination of 28º, which turned out to be close to the inclination of one of the frozen orbits—but poor PFS-2 was cursed with an inclination of only 11º.
edit: added additional information about frozen orbits.
A 'natural' moon formed early in the life of a solar system could be much further from its host, with much more symmetrical gravity and many stable orbits ready to host sub-moons. A moon is also in a much better place from which to capture a passing body.
It is on average further away from Neptune than Mercury is from the Sun! It takes more than 26 years to orbit Neptune, it still hasn't completed a full orbit since we have discovered it.
Also, I think there could be "binary-moons", if a binary asteroid gets captured as an irregular moon.
Asteroids with natural satellites exist and have been found.
There could also be very big trojans moons/planets in other star systems.
Submoons will probably clump together much easier in the formation phase of any system and a moon will never capture an asteroid before it's planet does.
As the author pointed out, the question remains how stable it is. The almost nonexistent cases (at least in our Solar System) shows how rare it is. Maybe only distance is a factor. Huge Solar Systems with twice the distance between objects will also have more submoons.
>The almost nonexistent cases (at least in our Solar System) shows how rare it is.
It might not exist in our solar system, our solar system isn't a model from which all others operate from. Submoons may be rare, but not because they don't exist in our solar system.
Sure, "submoons are rare because they're rare in our solar system" isn't a proof. (It's a hypothesis.) But the observation is evidence. In the absence of other evidence, it defines the status quo.
There are still artificial satellites around the Moon but the problem as stated in the paper is "long-lived" submoons, on stable orbits, which the Moon can't have.
I believe the publicly accepted term for these is “moonmoon”.
> The <x> that is held by <y> is also a <y><x>, so if you go to a food truck, the stuff you buy is truck food.
The same goes for the Sun. Our sun is either proper name 'Sun', or if your prefer Luna above, Sol. Any other star with stuff orbiting around it is just another sun (noun).
I have also seen Selene or Selena being used from time to time, more often as a adjective (selenic) but also as a noun every now and then.
No you don't. Not in english at least. The moon or sun is understood in context and with the article "the". "The moon" and "the sun" without qualifications is understood to mean earth's moon or earth's sun. General references like "a sun" or "a moon" connote non-specific and non-identifying moon or sun. If you want to specifically identify other moons or other suns, you qualify it like - the moons of jupiter or sun of X solar system.
As an example, the recent SpaceX announcement.
"The proposed trip—a trip around the moon for two passengers lasting about a week—is almost the same as a scheme hatched by the tourism company Space Adventures, whose president told Quartz last year it had two clients booked for the mission."
In Romance languages, there is a separate Latin term, closely related to the word "soil," which survives in French and Portuguese. Maybe sol/solo will be borrowed back into the Romance languages where it died out.
地 can also mean soil, but it’s a different word from 地球 meaning the planet earth. I don’t think it’s commonly used by itself to mean soil either, but I could be wrong.
Soil, or ground (on any planet) is a different word. There’s not much room for confusion.
That these are essentially generic nouns can show up more clearly in astronomical texts and of course in literature (science fiction).
No. The difference between “star” and “not star,” within the context of a solar system, is well defined. (Brown dwarfs muddy the line in general.)
The difference between “planet” and “not planet” is a little more arbitrary, with the line between “moon” and “non-moon satellite” being more arbitrary still.
The all-encompassing term you’re looking for is “satellite.”
There are several spacecraft orbiting other planets (both functional and inactive) but none orbiting any of the moons of other planets, if that is what you mean.
arXiv is an archive of preprints. This is a preprint. It hasn't been published (yet). In some scientific fields, it's customary to share preprints publicly before they are published. Sometimes they aren't published (i.e. by a peer-review journal) at all.