# Spacecraft pulled by the moon

The idea is- as the moon is passing over, the pull of Earth's gravity on immediate area surrounding the spacecraft somehow temporarily cancelled out (still working on the how) and the craft falls toward the moon. After clearing Earth's atmosphere the craft's thrusters (or other means of propulsion) direct the vessel away from the moon to avoid crashing into it.

Assuming a way to nullify Earth's pull exists (asking a lot, I know), how light would the spacecraft need to be? What likely velocity would the 'moonfall' (for lack of a better phrase) have for say, a two-person craft?

• If you can nullify gravity, why not nullify gravity on both objects? Presumably, if you can't do it partially, you can use something like PWM to generate controlled thrust. – timuzhti Oct 1 '15 at 2:07
• How: cavorite. – JDługosz Oct 1 '15 at 2:08
• I believe that Larry Niven used the idea of selective gravitational attraction in the gravity planer (the "Kzinti drive"). Everything old is new again... – Bob Jarvis Oct 29 '16 at 11:52

This isn't going to work. Using the formula for gravitational attraction: $$F = G\frac{m_1m_2}{r^2}$$ and plugging in $m_1=7.34767309×10^{22} kg$ for the mass of the moon and $r=363,104 km$ for the distance of the moon (at its closest), we get $$a = 3.71945379 × 10^{-5} \frac{m}{s^2}$$

Meaning that after about 7.5 hours of acceleration using only the moon's pull, you'd finally be able to get up to the blazing fast speed of... 1 meter per second. That's a slow walking pace. And that is if the moon were to be pulling consistently in the same direction, so you'd first have to solve the (much) harder problem of getting the moon to stay still.

Yeah... not gonna work.

• Also, in case you're not aware, the mass of the object doesn't matter - it will accelerate at the same rate whether it's a pea or Mount Everest. – Rob Watts Sep 30 '15 at 22:58
• Actually, it'll work fine. Assume that the full moon is directly overhead at local midnight. If you launch from the equator at midnight, equatorial velocity (463 m/sec) will get you to the moon's orbit in about 10 days. Since the moon took 7 days to reach that point, there's some catchup involved, but if you launched at about 9 PM 3 days before the full moon, that should about do it. – WhatRoughBeast Oct 1 '15 at 5:37

How: cavorite. That's so 19th century though.

But a variation of the idea was used in "hard" SF by Wil McCarthy. He wrote of a "gravar", a gravity laser. Essentially a tractor beam that can make the craft attracted strongly to any body, specifically including the moon.

You still need to put energy in to accelerate: it just gives you a "rope" to pull on.

• As I noted above - Larry Niven, Kzinti gravity planer. – Bob Jarvis Oct 29 '16 at 11:54