Post by James Nicoll Post by Lynn McGuire Post by James Nicoll Post by Dan Tilque
I miss-used the word fuel there. I meant propellant. Yes, they use tiny
amounts for spacecraft, which have a mass less than a metric ton. (Dawn,
which uses an ion thruster, has a mass of only 747 kg.) This rock is
well over a billion metric tons, possibly as much as 2 billion, so the
propellant and power amounts are going to have to scale up.
For example, say the delta vee to put the rock into an Earth crossing orbit
is 1 km/s and the rocket's exhaust velocity is 30 km/s (which is what ISTR
Dawn's is). The rocket equation says the mass ratio will be 1.03, so about
33 to 66 million tonnes of reaction mass will be needed.
Each tonne has an Ek of 4.5x10^11 Joules so in total the operation takes
between 1.5x10^16 to 3x10^16 J. Dawn generates about 10,000 Joules/second
at one AU so a Dawn-sized solar array would only take 48,000 years to huck
enough reaction mass.
Don't forget that changing the tumble of the asteroid needs to be taken
into account. This is the "propellant" that the authors are using to
change the path of the asteroid. The path change of the asteroid is
actually quite minor in the book.
I am looking at the online free sample and I don't see any propulsive
techniques used on the asteroid aside from two years of electric thrust
and a flyby of Earth. Where do they discuss your method? Which btw
I cannot work out how it's supposed to work.
Launch of the asteroid rocket:
Reuters note on electric thrusters:
"Electric rockets look futuristic, like an advanced space propulsion
system should. As the xenon gas fuel is stripped of its outer electron
and accelerated toward the rocket’s exhaust by carefully designed
electric and magnetic fields, the entire engine emits a brilliant blue
glow. There is none of the fire and smoke that would be seen from some
conventional chemical rocket engines. Chemical rockets are a brute force
approach to moving things around in space and the only realistic way to
get off the surface of a planet, deep in a gravity well, and into space.
But once you are in space, highly efficient electric rockets are an
Chemical rockets produce all the thrust they’re going to produce in
their first few minutes of use by providing spectacular acceleration—the
kind an astronaut can feel as he is pushed back into his seat while the
rocket begins to speed up. Electric rockets produce a continuous, very
small thrust that might not even be felt by a person. But it is a
continuous thrust, and, given enough time, an electric rocket can
accelerate a spacecraft to much higher speeds using only a fraction of
the fuel required by a chemical rocket. Such was the case with the
electric thrusters bound to the surface of the Sutter’s Mill asteroid.
The gentle push began as soon as the thrusters were turned on. Sutter’s
Mill, which would weigh just over two billion tons on Earth, didn’t have
any weight in space. It still had mass, so it still required a
significant total force in order to alter its motion so that it would go
where the mission planners from Asteroid Ores wanted it to go. The
entire operation was similar to a swimmer pushing a barge off its
original course. A single swimmer couldn’t make any abrupt changes to
the course of a multiton ship. But if that swimmer could swim sideways
into the barge for a very long time, then the barge would drift slowly
onto a different path.
The electric thrusters were designed to operate continuously for the
entire two years it would take to nudge the massive rock from its
current course to one that would make it accessible for Earth-based
miners to exploit, including an Earth flyby in just another eleven
months. With each day of operation, the asteroid would be on a slightly
different course on its billion-year journey around the Sun. To bring it
where its operators wanted it to be, two full years of thrusting would
"The electric propulsion system’s small but relentless push on asteroid
Sutter’s Mill was slowly altering its trajectory. Had the high-voltage
power supply that fed the thrusters not shorted out, there is no doubt
they would have placed the rock on a path that would take it safely into
a lunar orbit, making it accessible for Earth’s resource-hungry
population to mine. But the power supply did fail, and the beautiful
blue glow of the thrusters winked out, stopping before the asteroid was
placed in the desired orbit, leaving it on a path that no one had
planned or even yet knew. But soon the smart people back on Earth would
know where it was heading—and they would be terrified."
"“Isn’t the rock spinning? All the studies we’ve done show that it’ll be
risky to try and rendezvous with a rotating mountain in space,” asked
the Lockheed-Martin CEO.
“They attached two small spacecraft to the asteroid, one on each side,
each containing a long cable—a tether. Think of an ice skater when she
pulls her arms and a leg in in order to speed up. She’s conserving
angular momentum. As she reduces her rotational inertia by pulling her
arms and leg in, her rotation speed must increase to maintain constant
angular momentum. Now do it in reverse with a spinning rock and extend
five-kilometer-long tethers instead of arms and legs. The rock stops
spinning. Cut the tethers and you’re ready to go.”"
"Electrically powered spacecraft propulsion"
"An electrically-powered spacecraft propulsion system uses electrical
energy to change the velocity of a spacecraft. Most of these kinds of
spacecraft propulsion systems work by electrically expelling propellant
(reaction mass) at high speed, but electrodynamic tethers work by
interacting with a planet's magnetic field."