[evan_au]

towing rockets from space

Trying to tow something when there is a big difference in velocity is problematic.
- The original post imagines a spaceship in orbit, traveling 11km/second
- Which tries to tow a "stationary" space probe on the ground, which is moving perhaps 1km/second due to the Earth's rotation
- This requires a towline which is incredibly strong and light, and yet incredibly resilient to withstand a sudden change in speed of 10km/second
- We are talking about something much more "stretchy" than spider silk, here!   

towing rockets from space

There is one area where towing objects in space has been actively investigated: diverting near-Earth asteroids.

You can use a "big bang" approach, and try to smash something into the asteroid
- But that risks turning a dinosaur-killing 10km asteroid (which devastated life over the whole Earth)
- into 10 to 1000 smaller asteroid pieces, each of which could devastate life on 10% of the Earth. Not a real "win".
See: https://en.wikipedia.org/wiki/Double_Asteroid_Redirection_Test

A gentler solution has advantages (if you have enough advance warning to prepare).
- It does mean that you need to rendezvous with the asteroid/comet, ie match orbits, so there is a low relative velocity.
- Then you can try towing, pushing or just "being there" to exert a gentle force on the entire asteroid.
- If the asteroid/comet is a loose pile of rocks (as some of them seem to be), push won't work well, and towing won't work unless you enclose the comet in a net.
- But the "gravity tractor" idea has the space probe exert a gentle gravitational force on the whole asteroid, which will divert its course (if you do it for long enough)

So you don't have large differences in speed, and just exert a slow, steady force over many years.
See: https://en.wikipedia.org/wiki/Asteroid_impact_avoidance#Asteroid_gravity_tractor

Providing enough fuel to exert this steady force is is a problem in itself, which is why some of the asteroid-diversion proposals suggest using solar power.
See: https://en.wikipedia.org/wiki/Asteroid_impact_avoidance#Focused_solar_energy