With a huge robust ship you could go get an ice-teroid. With effectively unlimited reaction mass, finely balanced trajectories become unnecessary and launch windows a thing of the past.

A pulsed nuclear launch would provoke a political furore, but the military advantage of being first to have a robust ship and substantial reaction mass is incalculable. It would be tremendously enabling from both military and industrial perspectives. In particular it would enable preventing anyone else from acquiring the capability.

So why aren't the superpowers in a frantic scramble to acquire an ice asteroid?

Stuff I mistakenly thought was obvious:

  • Pulsed nuke is only for getting a big heavy ship into orbit and going to get the ice.
  • Once you have the ice, heat from a reactor produces steam for a steam thrusters.
  • You don't carry all of it on one ship, if you did that you would have to use a ridiculous amount to move. You park stockpiles orbiting planets and LaGrange points.
  • You freeze it around ships as ablative armour, radiation shielding and thermal mass, and spin the ship fairly fast to get both gravity and temperature distribution buffered by the ice.
  • For sudden high acceleration you could use steam to throw a lump of ice. This would also make a good weapon when others try to become space powers.
  • I don't know about further out but between Earth and the sun, UV radiation with platinum catalysts can split water into hydrogen and oxygen. With ready supplies of fresh oxygen you could simply dump excess CO2 until there is industrial demand to reclaim it.
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    $\begingroup$ Which superpowers? There's only one such these days, and it isn't much interested in racing. Indeed, it seems that most politicians are only interested in space-related stuff as a way of bringing pork to their districts. Then too, do we know that there actually are such things as 'ice-teroids' any closer than the Oort Cloud? Rosetta data seems to suggest that most of that comet is dust, not volatiles. $\endgroup$
    – jamesqf
    Feb 10, 2015 at 6:25
  • $\begingroup$ I was under the impression that quite a lot of the debris in the solar system was water or methane ice. If this is not in fact the case, or not known with certainty to be the case, then that would explain the apparent lack of interest. $\endgroup$
    – Peter Wone
    Feb 10, 2015 at 6:44
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    $\begingroup$ The Outer Space Treaty is somewhat relevant. $\endgroup$
    – user8406
    Feb 10, 2015 at 7:05
  • $\begingroup$ Directly related: Feasibility of Project Orion. $\endgroup$ Feb 10, 2015 at 19:06
  • $\begingroup$ @andy256 - part of my point is that treaties are only binding when a balance of power exists, and being the first space power would tilt the playing field so much you could abandon even the pretence of diplomacy. $\endgroup$
    – Peter Wone
    Feb 10, 2015 at 22:33

2 Answers 2


Let's start from the misconceptions in your question:

  • "Unlimited reaction mass" - there's no unlimited reaction mass in pulsed nuclear propulsion - you can go as far as your bomb supply lasts, but no further. Specifically, you cannot manufacture nuclear devices on board.
  • "Military advantage of getting an ice-teroid" - there's no advantage to be gained by capturing water off-world, military or commercial. The only things that can be done with ice asteroids are exploration/in-situ extraction of water for life support, or hydrogen/oxygen for propulsion. If you want to blast someone with an asteroid, you can do it better and cheaper with nukes. You cannot use the Orion ship to fight terrorists/insurgents/whatever, either.
  • "Preventing others from developing the capability" - exactly the opposite is true. Once the blueprints are out there, it's easier to steal them than to develop anew.
  • "Superpowers" - as noted in the comments, an old-fashioned and slightly incorrect term.

Otherwise, the question has two parts:

  • why nobody seems to be in a race for a very heavy launcher,
  • and why Project Orion is mothballed.

There are nice answers to the first part elsewhere on the site, and you have been pointed to Test Ban and Outer Space Treaties. I'm only going to summarize the two main objections:

  • a very heavy launcher is uneconomical due to extremely high fixed costs of R&D and infrastructure maintenance and very unfrequent launches (there are no commercial or military payloads that have to be launched to LEO);

  • there exist high specific impulse propulsion technologies not requiring the huge dead weight of the pusher plate (like nuclear thermal rockets).

For a high thrust launch solution, we're still bound to think about Environmental Impact Assessments, and while chemical rockets are not fluffy cute animals due to ozone depletion and (for solids) large plumes of not really healthy particulate matter, they are better than injecting radioactive fallout and unburnt plutonium into the stratosphere for (un)timely transport to millions of civvies around the world by jet streams.

An obligatory reference to Winchell Chung's site: Project Rho.

  • $\begingroup$ Several misinterpretations in your answer. In itself pulsed nuclear propulsion wouldn't provide unlimited reaction mass. However if it could bring back ice-asteroids then we would indeed have a lot of reaction mass at our disposal. You also seem to think the only military use of ice asteroids would be kinetic kill weapons. There are possible military uses for hydrogen/oxygen propellent. $\endgroup$
    – HopDavid
    Feb 10, 2015 at 18:19
  • $\begingroup$ @HopDavid Like...? It's not as if water is rare. $\endgroup$
    – cpast
    Feb 16, 2015 at 0:06
  • $\begingroup$ @cpast There's lots of water at the bottom of an 11.2 km/s gravity well and beneath an ~100 kilometer layer of air. You haven't heard to Tsiolkovsky's rocket equation? Delta V? $\endgroup$
    – HopDavid
    Feb 16, 2015 at 23:13
  • $\begingroup$ I really would not call solid-core nuclear thermal rockets "high specific impulse", especially given their poor thrust-to-weight. They're at best "mediocre" compared to chemical's "abysmal". The performance difference between nuclear pulse propulsion and NTR is like night and day. $\endgroup$
    – ikrase
    Nov 9, 2019 at 21:08

Bringing propellent rich asteroids to the earth moon neighborhood is an interesting idea. There are a number of entities pursuing this.

However most proposals suggest solar powered ion rocket engines for asteroid retrieval rather than a vehicle driven by a series of nuclear blasts against a pusher plate.

The U. S. is thinking of developing a solar powered asteroid retrieval vehicle. It would be part of the Asteroid Redirect Mission (A.R.M.). The A.R.M. vehicle is expected to be modeled after the vehicle described in the Keck Report.

However A.R.M. is not being sold as an enabler for using asteroid resources. The mission is promoted as an intermediate step to Mars (a foolish sales pitch, in my opinion).

Critics will say politicians embrace A.R.M. because it's doable by SLS and they want to justify the SLS pork barrel program that keeps folks employed in certain congressional districts. They further contend that SLS missions to an asteroid in lunar orbit do little to get us closer to Mars. I believe the critics are correct. But what the politicians as well the critics miss is the potential usefulness of the A.R.M. SEP vehicle.

Planetary Resources has made a nice video giving a layman's explanation of Tsiolkovsky's rocket equation and delta V budgets:

Aseroids aren't the only possible source of extra-terrestrial propellent. Paul Spudis is an advocate of using lunar water as a propellent source. There may be large deposits of ice at the lunar poles. Another advocate of lunar propellent is Bill Stone of Shackleton Energy Company.

Extraterrestrial propellant, whether asteroidal or lunar in origin, would require substantial mining and processing infra-structure. This is an extremely expensive barrier. It may be a large enough hurdle that we will never make use of extraterrestrial water.

There are those who argue captured rocks offer no military advantage. It's true kinetic kill weapons would be horrendously expensive. Nuclear bombs could wipe out a city for a small fraction of the cost. But those using this very obvious argument are stuck in 1970's science fiction. There are other possible military uses for space resources than throwing rocks.

What extraterrestrial propellent might offer is routine access to orbital assets in cislunar space. This includes surveillance satellites, GPS, communication satellites and weather sats. Easy access to these would indeed be of incalculable military value.

As suggested I will elaborate on what I mean by routine access. Presently a ship from earth's surface to LEO has a 9 km/s delta V budget. Returning to earth's surface entails enduring the extreme conditions of an 8 km/s re-entry through earth's atmosphere. So far economical reusable vehicles haven't been realized.

In contrast vehicles between earth orbits would have 3 to 4 km/s delta V budgets. Most wouldn't have to endure the high temperatures and dynamic pressures of re-entry. Those ships with LEO destinations might use aerobraking for circularizing at LEO but the aerobraking would be much less extreme than an 8 km/s re-entry.

With the present paradigm the routine is launch, use and discard. With cheaper transportation between orbits it would be possible to upgrade and maintain existing assets rather than letting them die and add to the population of orbital debris. It would also be possible to salvage dead sats.

  • $\begingroup$ A small correction: the fiction dates back to the 1980's, not the 1970s. Still don't understand the military value of cislunar space, this section could be expanded. $\endgroup$ Feb 10, 2015 at 19:16
  • $\begingroup$ @DeerHunter You don't understand the military value of surveillance satellites, GPS, communication satellites and weather sats? $\endgroup$
    – HopDavid
    Feb 10, 2015 at 19:25
  • $\begingroup$ What have they got to benefit from cryopropellants from asteroids? $\endgroup$ Feb 10, 2015 at 19:28
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    $\begingroup$ Routine access. At this time we launch a sat, wait till it dies and then possibly launch a new sat. With routine access it would be possible to maintain and upgrade sats. It would also be possible to salvage parts from dead sats like the parabolic high gain antennas. $\endgroup$
    – HopDavid
    Feb 10, 2015 at 19:30
  • 1
    $\begingroup$ Ion engine travel from low earth orbit to GEO would take months, a slow spiral through the Van Allen belts. The delta V budget for these slow spirals higher than a Hohmann ellipse -- generally around the difference in departure and destination orbit speeds. For LEO to GEO about 4.6 km/s going up as well as 4.6 km/s for getting back to LEO. And how about the tanker that delivers xenon propellent? 9 km/s going up and then 8 km/s re-entry going down. $\endgroup$
    – HopDavid
    Feb 10, 2015 at 19:48

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