On 4/5/2018 10:15 AM, Peter Trei wrote:
> On Wednesday, April 4, 2018 at 9:53:31 PM UTC-4, Lynn McGuire wrote:
>> On 4/4/2018 5:57 PM, James Nicoll wrote:
>>> In article <firstname.lastname@example.org>,
>>> Lynn McGuire <***@gmail.com> wrote:
>>>> On 4/4/2018 9:45 AM, James Nicoll wrote:
>>>>> In article <email@example.com>,
>>>>> Dan Tilque <***@frontier.com> wrote:
>>>>>> 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
>> excellent alternative.
>> 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
>> be required."
>> "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."
> Here's the problem. The asteroid is described as being roughly cylindrical ,
> a mile long, and a quarter of a mile in diameter. It weighs 1-2 billion tons.
> That's big, and a lot of mass.
> For the plot to work (I haven't read it, so I could be off here).
> * It has to be undiscovered.
> * It has to be able to be undetectably rerouted to pass near Earth, and
> be captured into Lunar orbit.
> * This all has to be done in a reasonable time frame (a few years, max), else
> it is not a good investment.
> I question whether these constraints can plausibly be met.
> It's estimated that over 90% of NEOs (Near Earth Objects) larger than 1 km
> have already been found: https://cneos.jpl.nasa.gov/stats/
> This rock is not only larger, but we're some distance (about 10 years) into
> the future. The chances of an NEO of this size being undiscovered then are
> quite low.
> The energy requirements for getting this much weight to change orbit
> significantly are staggering.
> If we pick a rock which is *almost* in the right orbit already, we can greatly
> reduce the energy requirement, but that runs into the problem alluded to above;
> its an NEO, with a near-Earth encounter in the near future. The chances that it
> hasn't been spotted, and no one is checking on it, are very, very low.
> A rock that's not an NEO with a soonish near-Earth encounter would require far
> more delta-v and energy to shift its orbit. Even ion thrusters are only 65-80%
> efficient - where is the power coming from?
> The plan is to put it into lunar orbit. We have to get it into just the right
> orbit to be captured, which is pretty constraining. This means it has to
> approach the Earth-Moon system with a pretty low velocity, since we can't
> do any kind of high-acceleration maneuver to put it in orbit. Perhaps something
> clever could be done with a 3-body maneuver to reduce the energy needed, but
> that would require great finesse.
> Finally, as others note, its tumbline, and the use of weights on cables cannot
> reduce tumbling to zero. Some space probes us this technique after launch, but
> they still need thrusters to remove remnant spin.
> All these objections can be handwaved for the sake of a good story, but at some
> point you have to stop calling it 'hard' SF.
Another problem is how to the protagonists know about this asteroid but
no one else does? As I understand it the search for NEOs is a world
wide effort. When astronomers find something they think might be a NEO,
they call upon others to observe and confirm, with the results being
shared very widely. So this one has to somehow be found by the
protagonists _without_ astronomers who are specifically looking for
bodies like it with more observatories and almost certainly better
equipment also finding it.
If it is an already known NEO, then the protagonists would have to reach
it without being observed (all the space powers routinely monitor for
launches, NORAD has a division specifically for tracking anything in
Earth orbit down to the size of an individual bolt so good luck with
that and how are they going to communicate with Earth without everyone
else detecting the ship's radio signals?) and then alter its orbit
quickly enough and greatly enough that the routine followups don't see
it not quite where its supposed to be (which would trigger a lot more
attention) AND without it being rediscovered.
Inquiring minds want to know while minds with a self-preservation
instinct are running screaming.