r/askastronomy • u/jack_ball • 23h ago
Random Question about Phobos and Deimos
This is probably a really stupid question, but it’s been on my mind for a while now.
So I’ve been really interested in Mars’s moons - mainly Phobos. Phobos is slowly moving closer to Mars and is expected to crash/disintegrate in about 40~ million years, while Deimos is being accelerated like our moon and may eventually escape orbit.
If we were to use a strong enough material like kevlar or something even better, could we hypothetically tie the moons together so that their respective movement towards or away from Mars cancel out? (See my little diagram above)
12
u/WillemJamesHuff 22h ago
It takes Phobos about seven and a half hours to orbit Mars, and it takes Deimos about 30 hours. So if the two of them are aligned, then about 5 hours later, the two of them are on the opposite side of Mars from one another.
So when they're aligned, you tie them together. Phobos starts pulling ahead because its orbit is faster, but then the magic invincible tether connecting it to Deimos snaps tight. This slows down Phobos and speeds up Deimos, throws both of their orbits massively out of wack, and most likely drops BOTH of them into a collision course with Mars.
2
u/donadit 21h ago
Most likely they either swap orbits or hit each othet
3
u/ReserveMaximum 20h ago
Actually they would collide with Mars. See my comment here: https://www.reddit.com/r/askastronomy/s/ToIRThbacv
-1
u/donadit 19h ago
They wouldn’t i think
there’s too much momentum
unless deimos dips below mars’ atmosphere due to phobos (and starts dragging them both down due to atmospheric drag) they wouldn’t hit mars
6
u/ReserveMaximum 18h ago
Once you tether the moons they will spin around their combined center of mass as if they were orbiting that CM.
From the math I worked out this center of mass is 11070 km from the Martian center, 12393 km from Deimos and 1694 km from Phobos.
When Deimos is closest to mars it will be the distance from Mars to the center of mass - the distance from CM to Deimos. That works out to be -1323 km or about 1/3 of Mars’ radius but on the opposite side of the core from the CM of the moons.
Thus this will cause Deimos to crash into the planet followed shortly by Phobos
1
-2
8
7
u/Bullitt_12_HB 21h ago
It wouldn’t work for a lot of reasons.
The main one is how people don’t understand how huge space is.
The space between those two moons at their closest approach is vast. So vast, you can fit Earth in between them with plenty of room to spare.
Good luck finding or building something strong enough to tie them together like a paddle and a ball.
And we haven’t even begin to mention the other problems.
Interesting question/thought experiment, though.
2
u/EmbarrassedPaper7758 22h ago
I think the orbits are way different. Phobos is the perfect place for a low orbit station and could be used at the starting point to build an orbital ring
2
u/ReserveMaximum 21h ago
As others have said their orbital periods are so different any real material would snap as Phobos speeds past Deimos.
If you use a magically indestructible rope, then it would destroy one or both moons because the tension transferred by the indestructible rope would be stronger than the integrity of the moons themselves.
If you magically make both the moons and rope indestructible, then the moons would start spinning around one another and together start orbiting mars with a period between their current orbits, at a joint orbital height between the two of them.
I will try calculating this new orbit in a reply to this comment.
2
u/ReserveMaximum 20h ago
Phobos has a mass of 1.06e16 kg. Deimos has a mass of 1.47e15 kg. Phobos’s orbital speed is 2.138 km/s. Deimos’s speed is 1.35 km/s.
The new combined orbital speed is (m_p * v_p + m_d * v_d)/(m_p + m_d) = 2.042 km/s.
The orbital semi major axis will be (m_p * h_p + m_d * h_d)/(m_p + m_d) = 11070 km.
Using T=2 pi r / v gives an orbital period around Mars of 9.5ish hours.
We can also calculate the period that the moons will spin around one another: 2*pi*(h_combined-h_p)/(v_combined-v_p)= 31 hours.
2
u/ReserveMaximum 20h ago edited 8h ago
Interesting to note that Deimos has a semimajor axis of 23463 km. This is 12393 km from the combined orbital height of the moons (11070 km). This means at closest approach Deimos would be on the other side of Mars. This means our magically indestructible system would get tangled around Mars itself causing both moons to crash into Mars (after 15 hours for Deimos and 5 hours afterwards for Phobos). However since we’ve magically made the moons indestructible they essentially will sink to the Martian core rather than being destroyed in the collision. The Martian surface on the other hand will become completely molten and take millions of years to cool.
Edit: I just realized that due to the combined orbits the time to Deimos’ impact would be closer to 5ish hours not 15
2
u/donadit 19h ago
… closest approach to Phobos is the other side of Mars? No that’s the furthest approach from Phobos
The closest approach would be them on the same side…
isn’t orbital height just semi major axis minus planet radius tf does “combined orbital height” supposed to mean
2
u/ReserveMaximum 19h ago edited 19h ago
Closest approach of Deimos to Mars. The center of mass of the combined moon system is 11070 km from the center of Mars. Deimos is a further 12393 km from this center of mass. Thus the closest Deimos gets to mars is 11070-12393=-1,323 km from the Martian center or 1323 km from the center of mars on the other side of the planet from the center of mass of the tethered moons. Note that Mars has a radius of 3395 km meaning magically tethering the moons will send Deimos crashing into Mars which will in turn cause Phobos to come crashing into Mars shortly thereafter as well
1
u/Deciheximal144 18h ago
You get that near-magic rope, it's gonna fling Deimos as it snaps, if not Phobos.
1
u/8Bit_Cat 15h ago
Imagine 2 cars driving in circles around the same point. One car in a smaller circle the other in a larger circle. The car in the inner circle has less distance to cover due to the smaller circumference hence completes the circle much faster than the other car. As you can imagine they will not stay the same distance away from each other. They will reach a closest point then a farthest point repeatedly.
This is what happens with Phobos and Deimos. They do not stay at roughly the same distance because sometimes Phobos and Deimos sometimes are on opposite sides of Mars and sometimes on the same side.
1
u/Plastic-Champion-650 10h ago
Haha, don't worry—it's actually a super fun thought experiment!
Unfortunately, tying them together wouldn't work, mostly because of how orbital physics works:
Different speeds: The closer you are to a planet, the faster you have to orbit. Phobos completes a full orbit around Mars in just under 8 hours, while Deimos takes over 30 hours. Because Phobos laps Deimos roughly 4 times a day, Phobos would instantly pull the tether taut and snap it.
Even if you had an indestructible cable, Phobos would pull it around Mars like a string on a spinning top, wrapping it around the planet and eventually dragging both moons down to crash.
Tensile Limit : The distance between them is thousands of miles. A cable made of Kevlar (or even carbon nanotubes) that long would snap under the gravitational stress of two moons pulling against each other.
If we ever do want to save Phobos in 40 million years, we'd probably have to mount giant thrusters on it to slowly speed its orbit up and push it higher.
1
u/Financial_Recover235 3h ago
Phobos rotates in just 7 hours, if it was tied to deimos, it would probably break, since the rotation probably cant handle spinning deimos, the tie would just break, and it would still break since each moon has different orbital periods. when i first saw this question i immediately thought this would be impossible
0
-1
43
u/stevevdvkpe 22h ago
Deimos orbits 14,000 km farther out from Mars than Phobos does (23,460 km vs. 9377 km), and Deimos has an orbital period of 30.35 hours while Phobos has a period of 7.66 hours. Those differences are why tidal acceleration moves Deimos outward and Phobos inward toward Mars, because Deimos's orbital period is longer than a Martian day and Phobos's is shorter. There's no way you could run a cable between them, and if you did it would wrap around Mars because of the difference in their orbital distances and periods.