Dynamics of binary asteroids

Thread view:

Enable EMail Alerts

Start New Thread

Thread rating:

Patrick Underwood - 21 Oct 2003 17:16 GMT

Today the news is that the rediscovered Hermes asteroid may be a
binary of roughly equal masses.

Makes me wonder if there is some way to exploit the energy of this
rotating system, tether perhaps, to fling the two masses
apart--putting one of the masses into a more advantageous orbit (for
whatever purpose), letting the other one go as "exhaust."

I'm not an engineer so bear with me... If you tether two masses and
winch the tether in, I suppose the total energy of the system doesn't
change, but the angular velocity goes up, right? If you then cut the
tether, will the masses merely fly out to their previous orbits, or
will they fly apart?

Tether the two masses, winch them closer, and increase the energy with
tangentially firing rockets of some sort? Maybe using propellants
mined from the asteroids themselves. Then cut the tether at the right
moment. Does this give an energy advantage over just sticking the
engines on a single asteroid and making it a big conventional rocket?

Patrick

Reply to this Message

Henry Spencer - 23 Oct 2003 02:55 GMT

>I'm not an engineer so bear with me... If you tether two masses and
>winch the tether in, I suppose the total energy of the system doesn't
>change, but the angular velocity goes up, right?

Not quite correct. Angular momentum -- mass times radius times (linear)
velocity -- is conserved. Energy is not: you're putting energy into the
system with your winch, doing work against centrifugal force.

>If you then cut the
>tether, will the masses merely fly out to their previous orbits, or
>will they fly apart?

Hmm, depending on details, the winching could add enough kinetic energy to
the system to exceed the gravitational binding energy. (Escape velocity
rises as the radius shrinks, but conservation of angular momentum makes
the linear velocity rise too, and faster.) Otherwise they'll end up in
elliptical orbits.

>...Then cut the tether at the right
>moment. Does this give an energy advantage over just sticking the
>engines on a single asteroid and making it a big conventional rocket?

As above, you may not need engines. If you do, the energy advantage is
probably small. If you don't, it's considerable, because the winching
will probably be much more energy-efficient. And unlike a rocket, it
doesn't throw mass away. (Well, if you don't count the other half of
the asteroid, which probably doesn't end up anywhere useful.)

Signature

MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. | henry@spsystems.net

Reply to this Message

Space Forum / Space Flight / October 2003

Dynamics of binary asteroids