On 10/3/10 1:16 PM, brooksn wrote:

> > Not if you're still talking about capturing an asteroid into Earth
> > orbit. We're talking about a 300-ton asteroid that ranges from 0.7 to
> > 3.6 AU from the Sun. You're not going to circularize that and match
> > Earth orbit just by parking a spacecraft next to it.
>
> But there's no need to circularize it first, as long as it comes on a
> close approach to Earth, adjust it a little so it hits the Earth's upper
> atmosphere. Perhaps add a heat shield so you aerobrake better and reduce
> losses.

Yikes! I didn't realize you were talking about aerobraking it.

Even if you could get that through politically, I'm still a bit
skeptical about it though. Usually aerobraking is done in several
passes. But in this case, any given NEA is only going to pass close by
the Earth every century or so -- even with nudging, I doubt you could
finish the job in less than decades.

Not trying to be a nay-sayer here, just being realistic. I do think NEA
mining will be important at some point, but it will be very much a
long-term enterprise (and I suspect we'll be sending the mined materials
back rather than trying to capture the whole asteroid).

Best,
- Joe

I don't really understand why it would take decades and multiple passes to nudge a tiny (5 meter) asteroid from an already passing very near to Earth course, to an aerobraking course with the upper atmosphere. But I could be missing something.

Brooks

--- In spacesettlers@yahoogroups.com, Joe Strout wrote:

On 10/3/10 10:08 PM, brooksn wrote:

> I don't really understand why it would take decades and multiple passes
> to nudge a tiny (5 meter) asteroid from an already passing very near to
> Earth course, to an aerobraking course with the upper atmosphere. But I
> could be missing something.

My assumption here is that (as in other aerobraking maneuvers) it takes
more than one pass to effect that much change in the orbit. But in this
case, each pass will be very long.

But I could be missing something too -- this is tricky stuff, and we
really need someone experienced in orbital mechanics to crank out some
numbers.

Best,
- Joe

Agreed, that would be nice, hopefully someone will pop up on here since the yellow pages and NASA don't really have 800 numbers to call. :)

Brooks

--- In spacesettlers@yahoogroups.com, Joe Strout wrote:

I presented this before, but as a secondry arguement. I appreciate the feedback on the rock size, and I agree (at least to the extent that I was too lazy to do the math myself).

Bottom line is how could one guarantee that an asteroid would not break apart with aerobraking? I suspect more often than not you apply a little pressure and a lot of heat (from air friction) to something that has been in equilibrium in space for millions of years. Anyone that has used a pressure washer on brick knows that if you don't pay attention it is quite easy to break off brick and/or mortar if you get too close. While the stresses (water vs. very very thin air) aremany orders of magnitude less, to me the same idea applies. Plus if a little air works itself into cracks then expands you will loose material. How much, who knows?

Another consideration is LEO debris. Even if you don't loose that much material, some of it will burn up; however, some of it will continue back to orbit. The KE may be reduced to a point where it is in LEO.

Hate to be a naysayer, but I don't think aerobraking is viable (Maybe it is just monday morning). Now controlled mass-shedding may be more effective than even rockets energy-wise to alter the orbit around the sun to earth orbit (likely a combination of both).

--- On Sun, 10/3/10, Joe Strout wrote:

From: Joe Strout
Subject: Re: [spacesettlers] Kalpana One design (was Re: Rotating space colonies)
To: spacesettlers@yahoogroups.com
Date: Sunday, October 3, 2010, 11:18 PM

On 10/3/10 10:08 PM, brooksn wrote:

> I don't really understand why it would take decades and multiple passes
> to nudge a tiny (5 meter) asteroid from an already passing very near to
> Earth course, to an aerobraking course with the upper atmosphere. But I
> could be missing something.

My assumption here is that (as in other aerobraking maneuvers) it takes
more than one pass to effect that much change in the orbit. But in this
case, each pass will be very long.

But I could be missing something too -- this is tricky stuff, and we
really need someone experienced in orbital mechanics to crank out some
numbers.

Best,
- Joe

Well we would need to study the nature of the asteroid and determine if it will break up just by shooting at the upper atmosphere. Another option is to strap a heat shield to the front of the asteroid to reduce the loss. Another option is to mold the front of the asteroid to maximize drag and reduce uneven heating. I'm not sure on how much we would actually be adding to the LEO debris problem, its possible that most pieces would disintegrate after braking off. So someone would have to study the potential, the other side of the coin is we are already having impacts, atmosphere skipping, and near misses all the time now, so can we be said to be really adding to an already existing phenomenon?

Possibly on the mass shedding, but similar to the direct rocket approach, another issue is the solidity of the body. Many if not most asteroids are rubble piles, which is why the gravitor tractor is so advantageous. But shielding and break up will be difficult with rubble piles as well so more work to do.

Brooks

--- In spacesettlers@yahoogroups.com, Christopher Pratt wrote:

Chris,
I'll grant you the math and the physics...it sounds about right to me. You are using an 'average' velocity for the NEA. we can target slower rocks to begin with until our ion tech can produce larger specific impulses. As to the time line, even if the 'average' NEA takes a couple decades to bring it into orbit, there's nothing to stop us from beginning a continuous pipeline of rocks being deccelerated to orbit. As the slower rocks begin to show up for processing the faster and later captures will be slowly alligning themselves to be processed as they arrive. We're not going to maintain and expand a spacefaring society by targeting asteroids as we need them, but by ensuring a steady supply. Meanwhile, we can be developing lunaside operations, building infrastructure, etc.
Victor

From: csmyth3025
Sent: Saturday, October 02, 2010 10:18 AM
To: spacesettlers@yahoogroups.com
Subject: [spacesettlers] Kalpana One design (was Re: Rotating space colonies)

--- In spacesettlers@yahoogroups.com, "csmyth3025"
>
> Let's do a "back-of -the-envelope" reality check on the Brooks'
> proposals listed below (edited for brevity). Wikipedia is the source
of
> all the basic information used in the forthcoming calculations except
> for the density of meteorites ( (www.meteorites.com.au
> ).
>
> 1) The orbital velocity of the ISS is about 7.7 km/s at an average
> altitude of about 350 km.
>
> 2) Meteoroids (small NEA's that enter the Earth's atmosphere) travel
at
> various orbital speeds in the vicinity of the Earth ranging up to 42
> km/s. They also approach Earth in its orbit around the Sun at various
> angles along the plane of the elliptic - from "overtaking" Earth from
> behind to coming "head-on". The orbital speed of the Earth around the
> Sun is about 29 km/s. Thus, any given small NEA could have a velocity
> relative to the Earth ranging from 13 km/s ("overtaking" - 42 km/s
minus
> 29 km/s)) to 71 km/s ("head-on - 42 km/s plus 29 km/s). The average of
> these velocities is 42 km/s. For the sake of argument (and to simplify
> our calculation), lets say that we only want to try to capture the
> slower NEA's with a velocity relative to the Earth of about 20 km/s.
>
> 3) The density of stony/iron NEA's averages about 4.5 gm/cm^3. The
> density of nickel/iron NEA's ranges from 7gm/cm^3 to 8 gm/cm^3. We'll
> call them about 7.5 gm/cm^3. If we want to use asteroids for
> construction material (initially) we would prefer the nickel/iron
NEA's.
> We may not have the luxury of being that picky, though, so lets say we
> capture a mixture of both - with an average density of about 6 gm/cm^3
> (6000 kg/m^3).
>
> 4) We'll assume that we're only going to pick the NEA's that are about
> 4m. Again, for the sake of simplifying the calculations, lets assume
our
> NEA's are cubes. A 4 meter NEA would then be 64 m^3. This is about the
> volume occupied by my (rather small) living room.
>
> ....to be continued - I have to go to work.

Continuing on.....If we assume our NEA is a perfect 4 m sphere, its
volume will be about 33.5 m^3. We'll be conservative and say that the
NEA's we're going to capture are about 30 m^3. At an average density of
6,000 kg/m^3 our NEA will have a mass of 180,000 kg (180 metric tons).

5) In order to put our NEA in an orbit around the Earth close to the
orbit of the ISS, we'll need to slow it down from about 20 km/s to about
7.7 km/s (the orbital velocity of the ISS). This is a change of 12.3
km/s, or 12,300 m/s. If we multiply the mass of our NEA (180,000 kg)
times the change in velocity we need to make (12,300 m/s) we get
2,214,000,000 kg*m/s (for the scientifically minded, this is called a
NEWTON*s and is a measure of momentum - in this case it's a measure of
the change in the momentum of our NEA that we need to make in order to
slow it down enough to orbit the Earth somewhere in the general vicinity
of the ISS).

6) In rocketry they use the term "specific impulse" (Isp). This is a
measure of the change in momentum that a particular rocket engine (using
a given fuel) can produce per unit of propellant used. In this case
we're using kg for units of propellant. The Isp of a rocket engine/fuel
type is usually given in "seconds". Because this term is normally
applied to lifting things from the ground to orbit, the units "seconds"
works or mathematically for the formulas that the "rocket scientists"
use. In our case we need to convert this term to one we can use to
figure out how much fuel we'll need to move our NEA. As it turns out,
one Isp (second)=9.8 N*s/kg (fuel used).

7) The Space Shuttle's main engines (burning LOX and LH2) produce a
specific impulse of about 455 seconds. This is about as good as it gets
for a chemical rocket engine. Ion engines have higher Isp's, but the
only ones we have today produce very low overall thrust. If we plug our
numbers for the Shuttle's main engines into the above formula we have
455*Isp (seconds)=455*9.8 N*s/kg (fuel used). In the case of the Shuttle
main engines, one kg of fuel will give us 4459 N*s change in momentum of
our NEA.

8) Since the change in our NEA's momentum has to be 2.214*10^9 N*s and
our best chemical rocket engine will give us 4.459*10^3 N*s per kg of
fuel we have: kg (fuel)=(2.214*10^9 N*s)/(4.459*10^3 N*s)=about 496,500
kg of fuel. For comparison, the Shuttle external (fuel) tank holds about
735,600 kg of fuel (LOX and LH2) - the specifications for this tank,
according to Wikipedia, are as follows:

SLWT Specifications [3]
07-2> (Super Light Weight Tank)

* Length: 153.8 ft (46.9 m) * Diameter: 27.6 ft (8.4 m) *
Empty Weight: 58,500 lb (26,500 kg) * Gross Liftoff Weight:
1,680,000 lb (760,000 kg)

9) If we use current technology, we'll have to find some way to loft
nearly 500,000 kg of fuel (plus a rocket engine complete with plumbing
and pumps) into LEO (7.7 km/s) and then accelerate it another 12.3 km/s
to catch up to our 30 m^3 NEA. Then we'll have to use our fuel to
remotely slow down our NEA so that it settles into an orbit somewhere in
the vicinity of the ISS. This is a pretty tall order.

10) A problem that hasn't yet been mentioned - but is critical - is that
we can't see 4 m NEA's until they're on top of us or are past us. We'll
have to vastly improve our tracking and observation capabilities if we
hope to capture one of these things.

11) We could greatly reduce our fuel requirements by using ion drives.
The trade-off is that - with the ion engines we have (or are on the
drawing boards today) - it will literally take many decades to produce
the change in orbital velocity needed for a 180 metric ton NEA. Also,
ion drives need a power source - solar panels or a nuclear reactor - to
produce the electricity needed to accelerate the ions to high
velocities.
>
> Chris
>
> --- In spacesettlers@yahoogroups.com, "brooksn" bhn1700@ wrote:
> >
> > --- In spacesettlers@yahoogroups.com, Matt Gallimore happygallimore@
> wrote:
> > >
> > > From: brooksn bhn1700@
> > >
> > > "But we aren't taking large asteroids from the asteroid belt, we
are
> taking 5 to
> > > 10 meter asteroids that are passing by Earth, in fact they pass
> within the
> > > moon's orbit all the time. The amount of fuel needed to divert
that
> small an
> > > asteroid just enough to enter the upper atmosphere for
aerobraking,
> strap on a
> > > heat shield and you preserve more of the asteroid, is not very
> costly
> > > energetically. And then adjust the new elliptical orbit until you
> meet up with
> > > the ISS. "
> > >
> > > Agree that this can be done, but I have liability concerns
bringing
> objects
> > > larger than 1 to 2 meters into the LEO.
> >
> > http://en.wikipedia.org/wiki/Meteoroid
> > "Beech & Steel writing in Quarterly Journal of the Royal
Astronomical
> Society proposed a new definition where a meteoroid is between 100
m
> and 10 m across."
> >
> > "The biggest asteroid to hit Earth on any given day is likely to be
> about 40 centimeters, in a given year about 4 meters, and in a given
> century about 20 meters."...
> >
> > >... "You have to travel to the location, mine it, then strap it to
a
> rocket (or
> > > expensive mass driver) and then launch it. An asteroid can weight
> 100's of tons
> > > and can be captured easily by adjusting an already Earth crossing
> orbit. I doubt
> > > 100's of tons can be sent up from the moon for the same cost."
> > > .....
> > > "But the purpose of your base was to launch material into orbit,
you
> need fuel
> > > for that and volatiles in particular. Regolith or asteroid scrap,
> radiation
> > > shielding will be the easy part. What should be the number one
> concern is cost,
> > > energetically and monetarily and I don't see how a new lunar base
> beats an ISS
> > > small NEA capture. "...
> > >
> > > 1. Production scheduling
> > > If we are doing orbital processing, then asteroid capture is done
on
> a schedule
> > > of years to minimized fuel use adjusting dv....
> >
> >... Why? We can agree that start up costs favor ISS, but if 4 m
bodies
> are raining down on Earth once a year, how many are coming between
Earth
> and the moon? The Earth is ~6,300 km in radius, from Earth to the moon
> is ~375,000 km, if we get one 4 m a year hitting Earth I can only
> imagine we are getting over a dozen a year (being very conservative)
> between us and the moon. 12 a year to pick from sounds like a pretty
> generous production schedule. Sure you'll have to plan ahead for these
> encounters and the first one will be a couple of years out (depending
on