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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 ...


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?


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/...


5

The current best ephemerides available is the JPL DE440 as described in Park et al. 2021 (paywalled) and available from the JPL ftp site. This is an incremental improvement over the previous DE430 ephemeris (detailed in the freely available IPN Progress Report 42-196). The main changes are that there is a longer baseline of spacecraft position data to fit to,...


4

It looks like this falls into the "complicated but plausible, and being investigated" category - this is from a recent overview paper, "Pulsar timing and its applications", RN Manchester, 2017. Journal of Physics; arXiv. Another interesting application of precision pulsar timing is to navigation of spacecraft that are distant from the ...


3

Here's how you can calculate this for any spinning body. First, you need its stationary orbital radius $$r_{stationary} = \sqrt[^3]{\frac{\mu}{\omega}}$$ $\mu$ is the body's mass parameter, a shorthand for its mass times the gravitational constant. $\omega$ is it's rotational angular velocity. If you have it's rotational period, you can obtain it as $\omega ...


3

This is equivalent to asking whether it's possible for a moon to orbit lower than a geosynchronous orbit, and there are of course satellites in orbit around the Earth that are lower. It's also equivalent to whether it's possible for a planet to rotate slower than its moon orbits, and clearly there's no lower limit on how slowly a planet can rotate (at least ...


3

What are all the activities related to defending planets against impacts from things like asteroids or comets? Are they only things like "identification", "tracking", and "deflection" or are there others? Mitigation. Suppose the Chelyabinsk meteor had been first observed a month prior to impact, and tracked well enough such ...


2

There are a few different ways you can get this information, depending on exactly what you want. Horizons is a good way to just get the relative positions of the planets. You can query it, download a set, and get the data you need via that tool pretty easily. If you have a limited range of time scales for this, that is probably your best bet. If you want a ...


1

Any planet or moon that has an atmosphere that is sufficiently dense at some point to support turbulent mixing will have a density knee above which the atmosphere fails to be dense to support turbulent mixing. This density knee is the object's turbopause. The Earth's atmosphere has a turbopause, as do the atmospheres of Venus, Mars, the giant planets, and ...


1

If a planet has a large moon that causes significant tides on the planet, then the Moon will spiral inward if it rotates faster than the planet and will then eventually collide with the planet. This is because the tidal bulges caused by the moon are going to be pulled back relative to the moon by the planet's slower angular rotation. Th force of gravity ...


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