Recycling Orbital Debris

[Northstar1989]

The Deep Impact spacecraft, along with Rosetta, has successfully rendezvoused with comets, each carrying a bunch of scientific sensors and sub-spacecrafts (an impactor and a lander, respectively). I believe replacing these payloads with ion thrusters along with RTG/solar panels and some propellant wouldn't be much of a challenge, but I may have missed some other considerations.

Your best bet of actually capturing a comet would be to send up equipment to harvest the ice and electrolyze it into hydrogen and oxygen. You could then burn most of the oxygen in conventional rocket engines with the hydrogen (I say *most* because most real-life rocket engines tend to burn oxygen and hydrogen in a no more than 7:1 fuel mass-ratio, usually only 6:1. An 8:1 ratio tends to produce too much heat and slowly melts many combustion chambers...) and save the rest of the oxygen for other purposes (such as for life-support on the ISS, once the comet were captured).

Of course, you'd have to have to burn a significant fraction of the comet's mass to move it to Earth orbit, and working with nothing but a handful of solar panels to electrolyze the ice (as opposed to a nuclear reactor, or beamed microwave power- both of which we could make use of for this purpose today if we really wanted), it would take a LONG time to capture the comet with the trickle of fuel you'd have available. On the order of 10-12 years, I imagine... But, it COULD be done with current technology- we already know how to melt ice and electroylze water in space...

...Which is Zubrin's Mars Direct plan, if I recall correctly. I personally find the plan intriguing.

I never heard of the ISS having a Sabatier reactor installed within it before. Interesting fact, I must say.

The Sabatier Reactor has been in place on the ISS for over 3 years now. No problems with it yet that I've heard of...

Mar Direct is more than an intriguing plan- it's probably the only one we could actually carry out with current funding levels. The main problem is NASA is only developing some of the necessary technologies (like specialized units to create dry ice from the Martian atmosphere to concentrate CO2 for a Sabatier Reactor- something they're actually currently working on... I'll try to remember to find the article) at a snail's pace...

Regards,

Northstar

Edited July 16, 2014 by Northstar1989

[Northstar1989]

I should add that Mars Direct actually survived in NASA as the "Design Reference Mission", a considerably more NASA-typical plan full of fail-safe's and redundancies that greatly add to mission mass and expense.

But it has undergone a number of upgrades and refinements, now having reached "Design Reference Architecture 5.0" status (a bloated version of Mars Direct was the 1.0 version, more or less), and retained at least SOME reliance on ISRU- though, strangely, the 5.0 plan involves shipping methane from Earth to burn with Oxygen produced from the Martian atmosphere, rather than shipping hydrogen from Earth and using it to make the methane- like the earlier 1.0, 2.0, and 3.0 plans did... Which makes little sense considering the 3.0 and the preferred version of the 5.0 plan include a nuclear reactor on the transfer vehicle, and are already cryogenically cooling a large mass of LH2 with power from that reactor, for use with BNTR engines (powered by said reactor) developed in collaboration with Russia in 1991/1992- an updated version of the 1970's NERVA program...

Regards,

Northstar

Edited May 24, 2014 by Northstar1989

[shynung]

Your best bet of actually capturing a comet would be to send up equipment to harvest the ice and electrolyze it into hydrogen and oxygen. You could then burn most of the oxygen in conventional rocket engines with the hydrogen (I say *most* because most real-life rocket engines tend to burn oxygen and hydrogen in a no more than 7:1 fuel mass-ratio, usually only 6:1. An 8:1 ratio tends to produce too much heat and slowly melts many combustion chambers...) and save the rest of the oxygen for other purposes (such as for life-support on the ISS, once the comet were captured).

Of course, you'd have to have to burn a significant fraction of the comet's mass to move it to Earth orbit, and working with nothing but a handful of solar panels to electrolyze the ice (as opposed to a nuclear reactor, or beamed microwave power- both of which we could make use of for this purpose today if we really wanted), it would take a LONG time to capture the comet with the trickle of fuel you'd have available. On the order of 10-12 years, I imagine... But, it COULD be done with current technology- we already know how to melt ice and electroylze water in space...

I don't think using the comet's mass as propellant is a good idea if one were to capture it into Earth orbit. The best LH2/LOX engines can manage about 450 seconds of I_SP, while ion drives have I_SP in the thousand seconds range.

However, if the capturing spacecraft had nuclear reactors in the first place, using the liquid water (or H₂/O₂) as NTR propellant could yield a greater I_SP than burning it outright.

Though, I have to admit, no matter what engine we use to capture it, it would probably still take decades to successfully capture a comet to Earth orbit. The possible gains (tens, maybe hundreds tons of fuel not needing launch costs), though, are almost irresistible.

By the way, I have read NASA's DRM 5.0 (DL'ed a copy of it weeks ago). On the ISRU section, they have 3 options to consider: Martian O₂/Earth CH₄ (which you mentioned), Martian O₂/CH₄ from Earth H₂, or Martian O₂/CH₄ from Martian soil H₂O. Plan #1 requires the most payload delivered to the surface, while #3 requires the most power generation capacity. Plan #2 is somewhere in the middle of the 3.

Edited May 24, 2014 by shynung

[Northstar1989]

I don't think using the comet's mass as propellant is a good idea if one were to capture it into Earth orbit. The best LH2/LOX engines can manage about 450 seconds of I_SP, while ion drives have I_SP in the thousand seconds range.

However, if the capturing spacecraft had nuclear reactors in the first place, using the liquid water (or H₂/O₂) as NTR propellant could yield a greater I_SP than burning it outright.

Though, I have to admit, no matter what engine we use to capture it, it would probably still take decades to successfully capture a comet to Earth orbit. The possible gains (tens, maybe hundreds tons of fuel not needing launch costs), though, are almost irresistible.

By the way, I have read NASA's DRM 5.0 (DL'ed a copy of it weeks ago). On the ISRU section, they have 3 options to consider: Martian O₂/Earth CH₄ (which you mentioned), Martian O₂/CH₄ from Earth H₂, or Martian O₂/CH₄ from Martian soil H₂O. Plan #1 requires the most payload delivered to the surface, while #3 requires the most power generation capacity. Plan #2 is somewhere in the middle of the 3.

The more I read about all the things we could do if we gave our spacecraft/missions more power/electricity, the more I think we really need to actually start making use of Microwave Phased Beam Power:

http://en.wikipedia.org/wiki/Beam-powered_propulsion

http://www.cnet.com/news/rocket-scientist-aims-to-relaunch-propulsion-technology/

The fact is, we've had the technology for 9 years to efficiently create the powerful microwave beams necessary for this technology (after the breakthroughs in gyrotrons circa 2005). And the necessary receivers have been around for over 50 years according to Wikipedia:

In 1964 William C. Brown demonstrated a miniature helicopter equipped with a combination antenna and rectifier device called a rectenna. The rectenna converted microwave power into electricity, allowing the helicopter to fly.[15]

So, why generate all the energy for a rocket on the ground? Why not beam a rocket all the necessary energy- ideally to power Thermal Turbojets run off beamed microwave power (rather then nuclear reactors) in the launch vehicle, which then switch over to internal fuel (anything from LH2 to NH3 or H2O would work) as the launch vehicle enters the upper atmosphere.

Such beamed-power systems could easily get a rocket all the way to LEO and through an ejection burn to Mars for VERY LITTLE fuel mass (the lack of a need for heavy combustion chambers, combined with the very high ISP of the system- comparable to nuclear thermal rockets, but without the heavy reactors) without any danger of radiation from nuclear reactors. And, the transmission losses would be low enough that you could probably use a large solar farm in Earth orbit (launched with microwaved beamed power from Earth- at a fraction of the cost of current chemical launches) to beam all the power you could need to Mars for a return mission...

It would also open up the door to powering cities with microwaved power beamed from solar farms in space, a la SimCity 3000. With the MUCH lower launch costs to orbit beamed microwave power would open up (especially with combined with a Space-X style reusable launch vehicle, though SSTO's could also easily work off the technology with a lower payload fraction... Launch costs of $10-20 a kg or less could be expected...), this might actually become economic for major cities... (greater potential power-density than nuclear power, with none of the radiation issues...)

Regards,

Northstar

P.S. I know $10-20 a launch seems crazy optimistic, but these are the actual conservative cost estimates in reports on the concept. Using the NTR-like ISP's and TWR's better than chemical rockets, you would be able to build 100% reusable launch vehicles with almost no moving parts, no combustion chambers, and in fact nothing to maintain but fuel tanks, guidance systems, a relatively simple antenna/rectifier in the thermal receiver/ heat exchanger, and decouplers for a Space-X (reusable Falcon-9) style launch profile...

Edited May 25, 2014 by Northstar1989

[Kruleworld]

If it was an actual net, it would have been shredded away long before the accumulated payload becomes useful.

No, I was thinking of something like a metal box or cylinder, with some sort of mechanism to either capture nearby debris (robotic arm with a scoop?) or trap incoming debris that impacts them (the box had better be really durable), and store them in a container.

that wouldn't work very well either? a box of gel would be the way to capture small objects (like screws) that can impact it safely (as it slows them down)