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#60 by sheltonjr on 15 Apr, 2013 21:08
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Though there's the question of economies of scale. I would guess a 1,000 ton asteroid would cost about 20% more than a 500 ton asteroid - subject to a suitable launcher.
The Keck study includes a lot of margin for error.
The mass of the asteroid will be difficult to determine before hand but they are shooting for 500t. But they have allocated enough fuel to retrieve a asteroid that masses 1300t. Though it will take longer.
From the Keck study page 14:QuoteThe return time would range from 2 to 6 years depending on the actual mass of the NEA. The concept system could return asteroids with masses in the range 250,000 kg to 1,300,000 kg, to account for uncertainties in size and density.
For de-tumble, they estimate it to take 300 Kg for the job and also added a lot of margine.
From the Keck study page 24:QuoteThe propellant required to de-tumble the asteroid was estimated to be about 300 kg. A margin of 50% is added to this along with an estimated 200 kg of propellant to control the spacecraft before and after capture for a total requirement of 650 kg. Adding addition margin brings the total estimated RCS propellant load to 900 kg.
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#61 by deltaV on 15 Apr, 2013 21:27
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So - it might make sense to use a Falcon Heavy and scale up the mission. The launch savings might exceed the marginal cost of an a larger retreival spacecraft.
Good point. The Keck study proposes spiraling out from LEO so the crappy BEO performance of Falcon Heavy would be irrelevant. If managed carefully having almost three times the payload to play with could come in handy, especially if our asteroid searches find candidates that are just a bit too big / far away. If managed poorly on the other hand I could see the bigger launch vehicle encouraging greater costs. Maybe it would be best to plan for an Atlas 551 class vehicle but keep the Falcon Heavy option in reserve? -
#62 by Solman on 16 Apr, 2013 00:25
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...and the one with the /smallest/ aphelion has an aphelion of 4AU, i.e. well past the orbit of Mars:
http://neo.jpl.nasa.gov/cgi-bin/neo_elem?max_rows=0;fmt=full;action=Display%20Table;type=NEC;show=1&sort=ad&sdir=ASC
(sort by "Q")
Of course comets come into the inner solar system, it's just that they don't stay here long and the delta-v to them is very large.
But there are icy asteroids in the asteroid belt.
Even so, a very-short-period comet might be a better candidate for capture since it'd spend some of its orbit down near 1 AU (or 1.5 AU, if we wanted to capture it around Mars...), where you'd get a little Oberth effect (from being deeper in gravity well of the Sun) and more solar energy for the electric propulsion.
Thanks for posting that. Using water vapor from the comet in solar powered resistojets means high delta V is not as much of a problem.
It also means that you can take better advantage of the Oberth effect if using Mars capture-cool idea.
Payoff is doing ISRU from the start while developing a practical mining system. -
#63 by A_M_Swallow on 16 Apr, 2013 02:22
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...and the one with the /smallest/ aphelion has an aphelion of 4AU, i.e. well past the orbit of Mars:
http://neo.jpl.nasa.gov/cgi-bin/neo_elem?max_rows=0;fmt=full;action=Display%20Table;type=NEC;show=1&sort=ad&sdir=ASC
(sort by "Q")
Of course comets come into the inner solar system, it's just that they don't stay here long and the delta-v to them is very large.
But there are icy asteroids in the asteroid belt.
Even so, a very-short-period comet might be a better candidate for capture since it'd spend some of its orbit down near 1 AU (or 1.5 AU, if we wanted to capture it around Mars...), where you'd get a little Oberth effect (from being deeper in gravity well of the Sun) and more solar energy for the electric propulsion.
Thanks for posting that. Using water vapor from the comet in solar powered resistojets means high delta V is not as much of a problem.
It also means that you can take better advantage of the Oberth effect if using Mars capture-cool idea.
Payoff is doing ISRU from the start while developing a practical mining system.
The tug does not have the hardware to purify the water so a mixture will be going through the resistojets. At best the thrust is not smooth and there is a risk that the jets will block. -
#64 by Solman on 16 Apr, 2013 02:38
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A M Swallow wrote:Quote
The tug does not have the hardware to purify the water so a mixture will be going through the resistojets. At best the thrust is not smooth and there is a risk that the jets will block.
The tug doesn't have a resistojet either
I think that may be a valid consideration but inclusion of purifying equip. seems practical. The extra mass of the purification sys. should be easily made up for by the lower power supply and engine mass of the resistojet as compared to the SEP system.
That's assuming that maintaining the resistojet nozzle at a very high temp. wouldn't solve the problem -
#65 by Solman on 16 Apr, 2013 02:49
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In looking at the table Chris provided of short period comets I noticed IIRC that several pass inside the orbit of Venus. If the target comet could be nudged to a close pass by Venus then a thrust at that point coupled with a gravity assist perhaps, could really lower aphelion. This might really lower the delta V the resistojet has to provide.
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#66 by sdsds on 16 Apr, 2013 04:45
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If the target comet could be nudged to a close pass by Venus then a thrust at that point coupled with a gravity assist perhaps, could really lower aphelion.
What are the chances an object passing the orbit of Venus comes within the sphere of influence of that planet? Let's be generous and assume the trajectory of the object is coplanar with the orbit of Venus. The diameter of the planet's sphere of influence is 1.2x10^6 km. The orbit of Venus is 2*pi*108x10^6 km long. So I think your chances are no better than 1 in about 565 on each crossing.
It could be a long wait. -
#67 by alexterrell on 16 Apr, 2013 10:38
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A M Swallow wrote:
QuoteThe tug does not have the hardware to purify the water so a mixture will be going through the resistojets. At best the thrust is not smooth and there is a risk that the jets will block.
The tug doesn't have a resistojet either
I think that may be a valid consideration but inclusion of purifying equip. seems practical. The extra mass of the purification sys. should be easily made up for by the lower power supply and engine mass of the resistojet as compared to the SEP system.
That's assuming that maintaining the resistojet nozzle at a very high temp. wouldn't solve the problem
Long term, using various forms of mass driver for comet retrieval will make sense. Right now, the technology for getting solid, dirty ice into a mass driver is about TRL1.
Personally I'd heat up the comet matter and capture the vapours at just above the triple point of water. Then pressurise back to a liquid for pumpng to the resito-jet. -
#68 by alexterrell on 16 Apr, 2013 10:44
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At this point, it would be useful to be able to vary the specific impulse - e.g. to lower it to give more thrust for less power, and use more fuel.So - it might make sense to use a Falcon Heavy and scale up the mission. The launch savings might exceed the marginal cost of an a larger retreival spacecraft.
Good point. The Keck study proposes spiraling out from LEO so the crappy BEO performance of Falcon Heavy would be irrelevant. If managed carefully having almost three times the payload to play with could come in handy, especially if our asteroid searches find candidates that are just a bit too big / far away. If managed poorly on the other hand I could see the bigger launch vehicle encouraging greater costs. Maybe it would be best to plan for an Atlas 551 class vehicle but keep the Falcon Heavy option in reserve?
That way, if the target is lighter than expected, increase the Isp to complete the mission in the alloted time, and return with fuel on board - ready for the next mission. If heavier, lower the Isp to complete the mission on time, and return empty to HEO - ideally meeting a rocket with a full container of propellant.
What's needed is a VAriable, Specific Implulse Magneto Reactor. Anyone heard of such a device?
And if such a device - call it VASIMR - isn't being used for this mission, do we pronounce it dead?
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#69 by Solman on 16 Apr, 2013 12:53
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Which is why I said that the target asteroid would be "nudged" toward a Venus encounter. A retrieval system has to be able to alter the velocity of the target significantly so perhaps the wait wouldn't be that long.If the target comet could be nudged to a close pass by Venus then a thrust at that point coupled with a gravity assist perhaps, could really lower aphelion.
What are the chances an object passing the orbit of Venus comes within the sphere of influence of that planet? Let's be generous and assume the trajectory of the object is coplanar with the orbit of Venus. The diameter of the planet's sphere of influence is 1.2x10^6 km. The orbit of Venus is 2*pi*108x10^6 km long. So I think your chances are no better than 1 in about 565 on each crossing.
It could be a long wait. -
#70 by Solman on 16 Apr, 2013 13:02
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Couldn't see the point of cooling the vapor to condense it before heating it to exhaust it as vapor at first but it would be good to store it for use at the proper time to optimize thrust timing.A M Swallow wrote:
QuoteThe tug does not have the hardware to purify the water so a mixture will be going through the resistojets. At best the thrust is not smooth and there is a risk that the jets will block.
The tug doesn't have a resistojet either
I think that may be a valid consideration but inclusion of purifying equip. seems practical. The extra mass of the purification sys. should be easily made up for by the lower power supply and engine mass of the resistojet as compared to the SEP system.
That's assuming that maintaining the resistojet nozzle at a very high temp. wouldn't solve the problem
Long term, using various forms of mass driver for comet retrieval will make sense. Right now, the technology for getting solid, dirty ice into a mass driver is about TRL1.
Personally I'd heat up the comet matter and capture the vapours at just above the triple point of water. Then pressurise back to a liquid for pumpng to the resito-jet.
Mass drivers require mining and processing and the mass driver itself and its power supply. Seems overly complex and massive to me. -
#71 by MP99 on 16 Apr, 2013 13:51
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So - it might make sense to use a Falcon Heavy and scale up the mission. The launch savings might exceed the marginal cost of an a larger retreival spacecraft.
Good point. The Keck study proposes spiraling out from LEO so the crappy BEO performance of Falcon Heavy would be irrelevant. If managed carefully having almost three times the payload to play with could come in handy, especially if our asteroid searches find candidates that are just a bit too big / far away.
The Keck study wasn't trying to get this done in the shortest time, but I believe it is under time pressure now.
Using FH to get the mission immediately to escape would decrease mission time by a year or two, and increase capability by allowing about the same sat mass without consuming some prop spiraling out of LEO.
A 50t mission, OTOH, would increase costs.
cheers, Martin -
#72 by yg1968 on 17 Apr, 2013 00:17
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Mike Gazarik, NASA Space Technology Mission Directorate AA, held a media telecon today. Here is the zipped mp3 file of that teleconference:
http://www.gamefront.com/files/23200340/Space+Technology+FY+14+Budget.zip
It discusses the technology for the asteroid mission among other things.
See above. -
#73 by Hop_David on 17 Apr, 2013 16:51
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I also wonder whether L2 is the right place for this. It needs to have an orbit that will passively dispose of itself into the Moon if Congress stops paying the bills to go back to it, ISTM L2's potential energy means it could eventually wander into an Earth orbit. (Someone correct me on that if it's wrong.)
I don't know about the long-term orbitology associated with L2, but have also wondered about EML4/5. As points of stable equilibrium, wouldn't they be worry-free places to park asteroids?
Orbitalogically speaking, all of L1-3 are all unstable. The Keck paper envisions parking the asteroid in lunar orbit rather than at L2 explicitly because an eventual collision with the moon rather than with the earth can be guaranteed.
I'd wondered about L4 and L5 too. I'll bet it's more difficult to arrange a low-delta-V capture there than in high lunar orbit.
In an ideal circular 3 body system, L4 and L5 are very stable. However the earth moon neighborhood has the sun as an influence. The sun can destabilize rocks parked at EML4 and 5.
An ordinary 2 body orbit at EML2's altitude would travel about .9 km/s. But since EML is moving at the same angular velocity as the moon, it's moving 1.2 km/s. If nudged loose from the moon's Hill Sphere, an object from EML2 would sail to a 1.8 million kilometer apogee, beyond earth's Hill Sphere.
An orbit's specific energy is 1/2 v^2 - Gm/r.
Specific energy for EML4 and 5 is about -.52 megajoules per kilogram.
Specific energy for EML2 is about -.18 megajoules per kilogram.
Specific energy for a parabolic orbit is zero. An incoming rock would be hyperbolic wrt earth and so have a positive specific energy.
Once at EML2, a small deceleration wrt moon would drop the rock deeper into the moon's hill sphere where it'd stay put for awhile without station keeping.
So EML2 is my favorite at the moment. My BOTEs for parking a rock at EML2: Catching an Asteroid.
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#74 by Hop_David on 17 Apr, 2013 18:41
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In looking at the table Chris provided of short period comets I noticed IIRC that several pass inside the orbit of Venus. If the target comet could be nudged to a close pass by Venus then a thrust at that point coupled with a gravity assist perhaps, could really lower aphelion. This might really lower the delta V the resistojet has to provide.
Best case for this scheme is perihelion at Venus and aphelion at earth. The rock would have a Vinfinity of 2.5 km/s.
I believe they're hoping to shed hyperbolic excess V by doing a lunar swing by. But 2.5 km/s is too much to shed in this manner.
Using a comet's water for reaction mass assumes a mining and transportation infrastructure to get water to the rocket engines. A far more ambitious project than what the Keck study calls for.
If the comet has a perihelion as close to the sun as Venus, I would guess out gassing would make for a chaotic, violent environment on the comet's surface -- not good for the mining and transportation infrastructure nor the rocket engines.
It's possible for short period comets to have their aphelions lowered by planet fly bys. There may already be some accessible dead comets whose insulating mantle protects an icey core. Who knows? Personally I'd give even odds there are one or two dead comets within easy reach.
But it's a pretty sure bet that there are water rich carbonaceous rocks within reach of Keck style missions. -
#75 by HappyMartian on 21 Apr, 2013 03:56
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....
Once at EML2, a small deceleration wrt moon would drop the rock deeper into the moon's hill sphere where it'd stay put for awhile without station keeping.
So EML2 is my favorite at the moment. My BOTEs for parking a rock at EML2: Catching an Asteroid.....
But it's a pretty sure bet that there are water rich carbonaceous rocks within reach of Keck style missions.
Yep, there are lots of meteoroids and asteroids that companies could eventually capture and move to a stable high Lunar orbit, not an unstable EML2 orbit.
As the mass continuously increases of those captured meteoroids, asteroids, and their eventual Deep Space Habitat space station needed for ISRU, so would the annual amount of propellant required to maintain them in an unstable EML2 orbit.
However, a stable high Lunar orbits means that for at least a century no propellant is needed to maintain the orbit of the Deep Space Habitat space station and the increasing mass of captured meteoroids and asteroids.
The Keck Asteroid Retrieval Feasibility Study
At: http://kiss.caltech.edu/study/asteroid/asteroid_final_report.pdf
has the following:
Page 6 "Placing a 500-t asteroid in high lunar orbit would provide a unique, meaningful, and affordable destination for astronaut crews in the next decade."
"These three factors combined suggest the immediate vicinity of the Moon as a reasonable choice. Whatever the final destination the mission must clearly define the end-of-mission conditions and asteroid maintenance and disposal effort (e.g., lunar surface). For the purposes of the trajectory design described later, we assumed a high lunar orbit as the destination for the returned asteroid."
An "affordable destination" is a stable high Lunar orbit in the "immediate vicinity of the Moon as a reasonable choice" which means, not an unstable and distant EML2 orbit.
An asteroid in an unstable EML2 orbit is part of the President's politically unsupported and vague anti-Moon policy with "increasingly demanding targets" that would add significant flight time and risk for Orion missions to captured and hauled asteroids.
Note:
"So I believe it’s more important to ramp up our capabilities to reach -- and operate at -- a series of increasingly demanding targets, while advancing our technological capabilities with each step forward."
From: REMARKS BY THE PRESIDENT ON SPACE EXPLORATION IN THE 21ST CENTURY April 15, 2010 John F. Kennedy Space Center Merritt Island, Florida
At: http://www.nasa.gov/news/media/trans/obama_ksc_trans.html
Do we want Orion to fly politically devised higher risk and more "demanding" captured asteroid missions to an unstable and distant EML2 orbit or do we want lower risk and quicker missions to a captured asteroid in a stable high Lunar orbit? -
#76 by KelvinZero on 21 Apr, 2013 04:09
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Im confused.
Both the recent Keck report and the recent nasaspaceflight article seem to be discussing using a High Lunar Orbit.
(which btw is very cool, if just for the view) -
#77 by sdsds on 21 Apr, 2013 06:08
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The advantage of the high lunar orbit is that the asteroid stays firmly bound within the Moon's sphere of influence. No small perturbation would lead to the asteroid leaving the vicinity of the Moon, thus it can't come crashing into the Earth.
(The Obama speech in 2010 was not about bringing asteroids into the Earth's vicinity.) -
#78 by Robert Thompson on 21 Apr, 2013 08:13
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The advantage of the high lunar orbit is that the asteroid stays firmly bound within the Moon's sphere of influence. No small perturbation would lead to the asteroid leaving the vicinity of the Moon, thus it can't come crashing into the Earth.
How stable is HLO wrt mascons?
How much station keeping compared to EML2? -
#79 by Proponent on 21 Apr, 2013 08:35
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In the Keck study, the asteroid did not leave HLO over a twenty-year period with no station keeping at all. Of course, some station keeping might be still desirable if rendezvous with an Orion is planned.
I'm pretty sure, by the way, that in HLO the dominant perturbations come from the sun and the earth, not from mascons. The further you are from a body, the less important are its deviations from spherical symmetry.
Staying at lunar L2 requires on the order of 10 m/s per year (see the attached presentation), depending on the strategy chosen and the precision needed
