Deimos escaping Mars

[Klapaucius]

I was reading an article in the New York Times today talking about how Deimos is on track to escape Mars's orbit.  Since all I know about orbital mechanics, I know from KSP, I am lacking in many departments. In the real world, how is this possible without an external agent?  Is there some way in which Phobos's influence as they pass closer to each other changes its orbital path every so slightly so that over the course of loads of years the effect is cumulative?

[Fraktal]

This seems to be a natural thing. The Moon is also on an escape course from Earth orbit, but will take billions of years to actually do it.

[Bill Phil]

Tidal acceleration - Deimos is above synchronous orbit so it gets pulled along by Mars’ slight bulge. Phobos is below synchronous orbit so it’s undergoing tidal deceleration - it’ll fall into Mars over time and pass the Roche limit.

In a point mass model this is indeed impossible, but astronomical bodies are not point masses.

Something similar is happening to every moon in the system - Titan was recently discovered to experience a much larger tidal acceleration than previously believed.

[mikegarrison]

The most easily explained ways for something to be captured or to escape a stable orbit are "three-body problems". But tidal forces will also change orbits. You can think of them as exchanging rotational energy for kinetic energy.

Edited July 25, 2020 by mikegarrison

[K^2]

3 hours ago, Fraktal said:

This seems to be a natural thing. The Moon is also on an escape course from Earth orbit, but will take billions of years to actually do it.

Ok, this was me just using Google search as calculator to throw some numbers together, but I got mutual tidal lock between Earth and Moon at a little over 600,000km. That's well within Earth's Hill sphere. If my math is not wrong, Moon will never get ejected from Earth's orbit by the tidal interaction, but it will end up getting parked almost twice as far away as it currently is. Eventually.

Somebody should double-check me on this, though. I could very easily have made a mistake.

[mikegarrison]

21 minutes ago, K^2 said:

Ok, this was me just using Google search as calculator to throw some numbers together, but I got mutual tidal lock between Earth and Moon at a little over 600,000km. That's well within Earth's Hill sphere. If my math is not wrong, Moon will never get ejected from Earth's orbit by the tidal interaction, but it will end up getting parked almost twice as far away as it currently is. Eventually.

Somebody should double-check me on this, though. I could very easily have made a mistake.

Someday the sun will become a red giant and it won't matter either way.

[K^2]

2 hours ago, mikegarrison said:

Someday the sun will become a red giant and it won't matter either way.

I just hope Moon has time to recede enough for barycenter to be outside Earth so that I can point at the Moon and say, "That's no moon." (Although, I do that already, because I prefer the tug-of-war definition.)

[kerbiloid]

Once the Moon leaves the Earth, we will get a whole near-Earth planet, passing every turn much closer than most dangerous asteroids do.

The Moon had started the life on the Earth, the Moon will finish its remains.

Edited July 26, 2020 by kerbiloid

[mikegarrison]

1 hour ago, kerbiloid said:

Once the Moon leaves the Earth, we will get a whole near-Earth planet, passing every turn much closer than most dangerous asteroids do.

Nah. Once the moon leaves the Earth it will pass through a black hole and go wandering around the galaxy. This will happen in the far future year of 1999.

[K^2]

Earth and Moon are close enough in masses and current orbit of Earth-Moon system is stable enough that were Moon to escape Earth's gravity, we'd have a good chance to end up with Janus/Epimetheus tag orbit that would be stable for a good amount of time.

[Hannu2]

On 7/26/2020 at 3:45 AM, K^2 said:

Ok, this was me just using Google search as calculator to throw some numbers together, but I got mutual tidal lock between Earth and Moon at a little over 600,000km. That's well within Earth's Hill sphere. If my math is not wrong, Moon will never get ejected from Earth's orbit by the tidal interaction, but it will end up getting parked almost twice as far away as it currently is. Eventually.

Somebody should double-check me on this, though. I could very easily have made a mistake.

That is about the same order of magnitude I remember. Orbital period (and length of the day) is then about 50 (current) days if I remember correctly. After that system begin to loose energy due to tidal interactions of Sun and other planets and in very very far future Earth and Moon will be combined. However, Sun goes through a transition to red giant and white dwarf and may affect very radically to Earth-Moon system before such things will occur.

15 hours ago, K^2 said:

Earth and Moon are close enough in masses and current orbit of Earth-Moon system is stable enough that were Moon to escape Earth's gravity, we'd have a good chance to end up with Janus/Epimetheus tag orbit that would be stable for a good amount of time.

As far as I know tidal interactions can not lead this. It would need another massive body to kick Moon out of Earth's sphere of influence.

 

[kerbiloid]

Sun?

[Hannu2]

4 minutes ago, kerbiloid said:

Sun?

 

Tidal force depends on gradient of gravitational acceleration. Sun's tidal force on Earth is about half of Moon's. Other planet's influence is very small and have to be taken into account only in very accurate predictions.

[K^2]

7 hours ago, Hannu2 said:

As far as I know tidal interactions can not lead this. It would need another massive body to kick Moon out of Earth's sphere of influence.

Or a more direct kick to Earth itself, greatly increasing its angular momentum. Yeah, either way, the total angular momentum isn't there intrinsic to Earth-Moon system itself, so it'd have to come from outside. That part of a reply was more of an "what if?" What if something provided a gentle push to slowly raise Moon out of Earth's SoI? It doesn't look to me like we'd have collision risk for a good while after that. Instead, Earth and Moon will occupy the same orbit around the Sun, with moon going to slightly higher or slightly lower orbit during each approach, resulting in a planetary game of tag. Like I said, we see it with Janus/Epimetheus. Though, masses of these two are more similar, and they are moons. So maybe perturbations on Earth/Moon dynamic will be stronger.

It'd be kind of an amazing setup if it can be stable, though. To have another planetary body that's within almost as easy of a reach as Moon is now for a few decades out of a few centuries, and then being harder to get to than Mars for the rest of that time span, because it's effectively on the same orbit on the other side of the Sun.

The fact that we don't see these arrangements more often does suggest that it won't be that stable, of course. But a few million years is an instant when we're talking about billions of years over which star systems evolve, and it's still a huge amount of time on civilization scale. There's probably a planet out there with population that's looking at the former moon coming in for another close approach going, "Now! Now is our chance to go there and plant a flag on it!"

Edited July 27, 2020 by K^2

[RCgothic]

On 7/27/2020 at 6:26 PM, K^2 said:

Or a more direct kick to Earth itself, greatly increasing its angular momentum. Yeah, either way, the total angular momentum isn't there intrinsic to Earth-Moon system itself, so it'd have to come from outside. That part of a reply was more of an "what if?" What if something provided a gentle push to slowly raise Moon out of Earth's SoI? It doesn't look to me like we'd have collision risk for a good while after that. Instead, Earth and Moon will occupy the same orbit around the Sun, with moon going to slightly higher or slightly lower orbit during each approach, resulting in a planetary game of tag. Like I said, we see it with Janus/Epimetheus. Though, masses of these two are more similar, and they are moons. So maybe perturbations on Earth/Moon dynamic will be stronger.

It'd be kind of an amazing setup if it can be stable, though. To have another planetary body that's within almost as easy of a reach as Moon is now for a few decades out of a few centuries, and then being harder to get to than Mars for the rest of that time span, because it's effectively on the same orbit on the other side of the Sun.

The fact that we don't see these arrangements more often does suggest that it won't be that stable, of course. But a few million years is an instant when we're talking about billions of years over which star systems evolve, and it's still a huge amount of time on civilization scale. There's probably a planet out there with population that's looking at the former moon coming in for another close approach going, "Now! Now is our chance to go there and plant a flag on it!"

How much C3 does it take to inject into an elliptical solar orbit with a period of half a year, bounding case?

I'm away from my orbital parameters spreadsheet.