# What would increasing the gravity of a moon do to its planet?

If I terraformed a moon to have enough artificial gravity/atmosphere to support life, would they end up just crashing into each other? Is there a way to not have this happen, e.g., increasing distance between the two?

• Welcome to worldbuilding. You are asking at least two different questions, and one of them is overly broad. We prefer to answer 1 question per post, and the question shall not require writing an entire book or more to be answered. You can find more in the help center and taking the tour. Please rework your question to fit our standards. Oh, we also have a be nice policy, which applies also to oneself. – L.Dutch - Reinstate Monica Apr 28 '19 at 15:35
• Thanks for paring down the body of the query! I would suggest editing the title to reflect this. Something like "What gravitational effects would terraforming a moon have on the relationship with its planet?" We like focused questions like this! – elemtilas Apr 28 '19 at 15:51
• You kinda need to explain how ypur terraforming works in order to get useful answers. E.g. think of the following: spaceships in Star Trek have 1G gravity aboard, yet they don't have any effect on other bodies next to them - thus their gravity systems must be extremely localized. – dot_Sp0T Apr 28 '19 at 16:30
• The technology required to do any of the things you describe is essentially equivalent to magic from our point of view. We have no way to realistically answer this. As we understand what's possible with physics, I would say no to all of it. – StephenG Apr 28 '19 at 16:34

This is a partial answer, addressing the atmosphere part of the question only.

The Moon weighs 7.342 × 1022 kg. Earth's atmosphere weighs 5.1480 × 1018 kg. Earth's biosphere (i.e. from 60 km above to 5 km below the surface) weighs between 1 and 4 × 1015 kg. The surface of the Moon is 0.074 that of Earth. So the Moon's biosphere plus atmosphere would approximately weigh:

(Earth's biosphere + Earth's atmosphere) × 0.074 = (4 × 1015 + 5.1480 × 1018) × 0.074 kg = 3.81248 × 1017 kg

which is

biosphere and atmosphere of the Moon divided by the weight of the Moon = (3.81248 × 1017) / (7.342 × 1022) = 0.0000051927 % of the weight of the Moon

which is too small a change from the current weight to make much of a difference on Earth.

As we understand physics today, the only way to increase the gravity of a moon is to increase the mass significantly. Smashing that much stuff into a moon is just not practical, and if it were there would be debris hitting the planet.

The Roche limit probably won't become a problem, but tides could become devastating.

If it were possible to add mass "gently" then there should also be the technology to turn the planet-and-moon pair into a binary-planet-pair. But that's so far beyond science as we know it that speculation about the side effects is pointless.

• Thanks for the information about the Roche limit! It's for a speculative fiction idea, so I know there's going to be a lot of hand-wavium and shrugs, but this still gives me some pieces of the puzzle. Thanks! – Rhap Apr 28 '19 at 16:54
• We can use a truncated cone that is spinning to take advantage of centrifugal force and combine it with natural gravity to produce a higher net downward force relative to the inner surface. – 0something0 Apr 28 '19 at 17:41
• @0something0, I think building something like that is not a realistic option with present-day science. People argue about the viability of beanstalks, this will be several orders of magnitude harder. – o.m. Apr 28 '19 at 18:00

With the density of the moon being 3.34 g/cm3, if you replaced ~80% of the core with osmium, 22.59 g/cm3, it would have the same mass as the Earth, with the diameter of the moon. May need to decrease the mass a little to account for the difference of distance or the radii.

The 2 bodies orbits would have to be altered to share a common barycenter. However, the pull of the sun on the binary planets may cause one of the two to either be ejected or crash into one another.