A Closer Look at Medusa

by Paul Gilster on July 23, 2012

I see that ‘Zarmina’ is back in the news. The informal designation refers to Gliese 581 g, an exoplanet candidate announced by the Lick-Carnegie team in an effort led by Steven Vogt (UC-Santa Cruz). First you see it, then you don’t — Gl 581 g has been controversial from the start, and is now the subject of a new analysis describing a 32-day orbit, a super-Earth in the habitable zone. More on the analysis later in the week, because my purpose today is to keep digging into the options for getting to a place like this once we’re sure it really does exist.

Gl 581 is just over 20 light years from the Sun in the constellation Libra, a red dwarf whose planetary system is one of the nearest yet detected. Among the options for propulsion in a future interstellar probe is Medusa, the brain-child of Los Alamos physicist (now retired) Johndale Solem. As examined here on Friday, Medusa is a nuclear-pulse system, like Orion in that it relies on the explosion of a series of atomic bombs to propel the vehicle. Where Medusa changes the equation is in its use of a gigantic sail, or ‘spinnaker,’ which replaces Orion’s massive pusher plate. The gossamer sail is connected to the payload by high-tensile strength cables.

Image: The Medusa hybrid design, combining nuclear pulse propulsion with a sail. (A) the payload capsule, (B) the winch mechanism, (C) the main tether cable, (D) riser tethers, and (E) the parachute mechanism. Credit: George William Herbert/Wikimedia Commons.

Because I’m just finishing up Kelvin Long’s Deep Space Propulsion (subtitled ‘A Roadmap to Interstellar Flight’), I was curious to see what Long had to say about Medusa, a concept that has received less attention than other nuclear methods like Orion or Daedalus’ inertial confinement fusion (ICF) design. Long wonders whether Medusa might be useful as a deceleration mechanism for a star probe:

As a probe approaches a target destination, the sail is unfurled rearwards with detonations causing a force in the negative thrust direction to the direction of motion. This may only require a small quantity of units to produce this result. Alternatively, ICF capsules could be ignited by laser beams rearwards of the vehicle, giving rise to the same effect, but on a more moderate level.

The immediate objection to this is the complexity of the Medusa mechanism, but Long anticipates this:

One of the biggest problems for the Medusa sail being used in a deceleration mode however is the reliability of it deploying in deep space after decades of being stored away. If the sail doesn’t deploy, then the probe will essentially be a flyby probe with limited observing time of the target star system and potentially left with a lot of unused units, which would have to be destroyed safely in a controlled way by the vehicle main computing system.

Make no mistake about it, Medusa is a complex system (see Friday’s post for a diagram of the propulsion cycle). Solem’s initial Los Alamos report refers to a canopy with a radius of 500 meters. The plan is to spin-deploy it and its 104 tethers, each made of high-strength polyethylene (aligned polyethylene) material, though Solem notes that superior materials will be available by the time the kind of manned missions he envisions come into play. Remember that he was thinking in terms of an interplanetary mission with Mars as the primary destination. Solem’s sketch, at the end of the Los Alamos report, gives you a feel for the idea, but translate this webwork of lines into fully 10000 tethers and ponder the issues such a contraption raises.

Image: Medusa and its tethers, as sketched by Johndale Solem. Credit: Los Alamos National Laboratory.

The problem of deployment and storage is enormous, and I can see why Long would wonder about re-deploying such a system after a lengthy interstellar transit, which is why he also considers it in terms of a first-stage booster, to be ejected after use. On the other hand, Solem’s calculations in the Los Alamos report and elsewhere showed that the canopy should be able to withstand the ignition of nuclear devices that would propel it. The tethers are another matter — as noted by ‘Eniac’ in the comments to Friday’s post, the tethers have to absorb the blast energy and are obviously crucial to the design. Like Robert Forward, Solem liked to think big, and in this case a gigantic canopy with extremely long tethers was expected to minimize radiation dangers for the crew and allow the tethers to survive the acceleration phase of the mission.

How fast could Medusa fly? The answer depends on the number of propulsion units detonated, with each detonation pulse adding to the velocity. Long also points out that the larger the canopy area, and the closer the canopy is to the detonation point, the higher the velocity. Using the values given in Solem’s 1993 paper in the Journal of the British Interplanetary Society, Long comes up with a specific impulse of 4100 kilometers per second, with an exhaust velocity in the region of 40 km/s.

Like Orion, Medusa runs into the problem of putting nuclear materials into space, one that Solem was all too familiar with. In the conclusion of the Los Alamos report, he has this to say:

We are currently prohibited by treaty from: (1) deploying weapons of mass destruction in space and (2) testing nuclear weapons in space. MEDUSA violates neither the letter nor the spirit of either prohibition, but it does use nuclear explosives. The radioactive debris from MEDUSA’s exhaust is so finely dispersed that it will be nearly undetectable. I assert that MEDUSA’S net environmental impact is less than NERVA; you have to do something with the spent reactor. I see no reason why nuclear explosive propulsion for interplanetary missions cannot be made politically acceptable. Perhaps we can be more creative and consider an international mission in which the nuclear explosives were jointly supplied by the superpowers. What a wonderful approach to nuclear disarmament and the enhancement of science for the benefit of all humanity!

Orion proponents make the same case, that their design would allow us to use up our nuclear stockpile for peaceful purposes while offering large vehicles for deep space exploration. It’s a utopian vision, and in both cases, it’s hard to see it happening anytime soon. Nuclear-pulse propulsion may make sense in space, but you first have to get the nuclear materials off the planet. The nuclear issue is one reason Friedwardt Winterberg, back in 1971, developed the idea of using intense, relativistic electron beams to ignite fusion, a method that captured the attention of the team developing Project Daedalus for the British Interplanetary Society. In such ways do the problems of one propulsion concept play into the revised thinking that fuels the next.

Citations for Solem’s Los Alamos report and his 1993 JBIS paper are at the end of last Friday’s post. I also want to mention two other papers by Solem: “The Moon and the Medusa: Use of Lunar Assets in Nuclear-Pulse Propelled Space Travel,” JBIS Vol. 53 (2000), pp. 362-370 and “Deflection and Disruption of Asteroids on Collision Course with Earth,” JBIS Vol. 53 (2000), pp. 180-196.

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James D. Stilwell July 23, 2012 at 11:01

Medusa might be ripped apart by space rock….So indicated Alan Bond in a round about way many years ago….remember his big problem with the wrinkling effect for sails of every design…distorting every calculation you care to make….of course with robots aboard and in command it would make an interesting but costly experiment …..tracking bits of space rock will be big business in the far future….Plotting a safe lane across a 20 light year course without space rock (or sand) intervening in dense bands along the way should be done….JDS

torque_xtr July 23, 2012 at 15:38

Could the Spitzer’s, or any other space IR telescope’s spectral resolution be enough to detect infrared spectra of the planets, like it was done with Tau Boötis b? More, all the complex processing algorithms could probably improve sensitivity in the particular case of planet-hunting. Not look at the specific wavelengths for the common lines, but just analyse the whole spectrum for correlations – to look for the patterns that behave as a whole and systhematically Doppler-shift with characteristic orbital velocities and RV-predicted periods – that might enhance the sensitivity very much.

In addition, that would reveal true masses and hint at the compositions, at least distinguish between a super-earth Gl 581 d with continents and some tens of bars of CO2, and a “mini-neptune” one with a radius 2,5 times larger and H2/He signatures hinting at super-hot ocean under tens of kbars of pressure…

Daniel Suggs July 23, 2012 at 15:43

Instead of retracting and storing the sail, couldn’t it just be collapsed at the end of the tether, like a collapsed parachute? When the time comes to slow down, swing it around like a pendulum and light pressure should help open it back up. Seems much simpler than retracting and redeploying later.

Michael July 23, 2012 at 16:36

If the tethers carried a current they would create a magnetic field around them that would repel the plasma away from them protecting them, infact knit them into the canopy itself and it would protect the canopy as well, further would it not be better too have a parabolic shaped canopy, it has a better reflection profile than a circular one.

Christopher Doll July 23, 2012 at 19:46

Depending on the complexity and weight of the sail structure, it would seem prudent to carry multiple sails. Even better, install equipment and raw
materials necessary to fabricate such sails as needed.

Joy July 24, 2012 at 0:32

Regarding Gl 581, whatever planets might be there, the human interest backstory is quite interesting. I saw Geoff Marcy around 15 years ago talking about the many years he laboured in obscurity with Paul Butler working on the RV technique of extrasolar planet detection. They had to fight for credibility as their efforts were either ignored or ridiculed by colleagues who thought they had committed professional suicide. Anyway, by late 1995 they were sitting on several planet candidates detected around sunlike stars, including the still very interesting 47 Ursae Majoris b, a cool Jupiter with a 3 year orbital period.

Anyway, while the Californians were deliberately biding their time, the Swiss team of Michel Mayor & Didier Queloz jumped in with the announcement of short period hot Jupiter around 51 Pegasi and entered the history books. Ouch, particularly since the Californians had pioneered the RV technique. But the Californians have have moved on and have sought to claim other ‘firsts’. The 2010 claim of Gl 581g – first earth mass range planet in a liquid water temperature zone – was a big one for California.

The 2011 counter claim by the HARPS team (Michel Mayor & Didier Queloz again) that Gl 581g did not exist was a major rebuke. I am told that all of the volcanoes in the SF Bay Area are extinct, but I suspect steam was rising from Santa Cruz when that publication came out. Now this 2012 publication (of which Paul Butler is a co-author) has upped the ante. The analysis of the HARPS data is ruthless, and implies that the Europeans had thrown out the data points which supported the California claim. This is pretty incendiary science, a no holds barred hair pulling, eye gouging academic quarrel.

To reassert the existence of Gl 581g is a bold move for Steven Vogt and Paul Butler, they have gone all in, put all their chips in the pot. This is a scientific thunderdome. Only one team can come out of this with reputation intact. I am betting on UC Santa Cruz – Lick.

Rob Henry July 24, 2012 at 19:20

“What a wonderful approach to nuclear disarmament and the enhancement of science for the benefit of all humanity!”

This quote above from the original authors implies one of the following:
1) that they are hopelessly delusional;
2) they realise that their design is so bad, that it would only ever be used once;
3) they are hypothesising that the best way to get two superpower adversaries to reduce their stockpiles is if they retain mechanisms that would allow rapid expansion of their arsenals at short notice.

The last of these options has the advantage of discouraging any third party from trying to match them, but somehow I keep feeling that the other options apply here.

coolstar July 24, 2012 at 21:11

Joy: it was Gordon Walker and Bruce Campbell at the DAO who really pioneered precision radial velocities by using the incredibly dangerous and corrosive gas hydrogen flouride in the telescope beam. They later moved to using the CFHT at Mauna Kea. This work was so little valued that Campbell didn’t receive tenure in Canada.
Butler and Marcy switched to the much, much safer (and more effective) use of iodine. Marcy told me soon after 51 Peg was announced by Mayor and Queloz that he was HAPPY not to have been first, given the controversy surrounding its announcement.
I agree with your comments about the Gl 581g “fight”. Both groups really are all in! It may not be terribly well known that Steve Vogt is the chief designer of all the spectrographs the “California” team has used over the years at Lick (mostly on the Coude’ feed telescope at the beginning), Keck (once they became famous enough that they could get time to look at mere stars using a BIG American telescope), and now the Automated Planet Finder at Lick.

Eniac July 24, 2012 at 23:48

The second figure is very instructive about the need to leave a large hole in the canopy. As drawn, the nuclear explosion would happen right in the middle of a fairly dense array of tethers, surely vaporizing those that are within a certain distance from the fireball. This distance dictates the minimum size of the hole, and I bet it is not really small.

Somehow, it seems, the need for a hole was overlooked when the figure was drawn, which bothers me quite a bit. Maybe I am missing something?

Joy July 25, 2012 at 6:32

@coolstar
Thanks for the additional backstory, I didn’t know about Walker & Campbell and the HF, wow. I only knew HF as the incredibly toxic exhaust of a hydrogen/fluorine engine.
I did know Vogt made the spectrographs. And I am certain that he and Butler must be confident of their >0.95 refutation of the null hypothesis, otherwise it would have been better to let the matter rest. I have no idea how or when this will be resolved, but it won’t be dull!

Rob Henry July 25, 2012 at 17:56

ENIAC, you have got me thinking. If a tether bifurcates, what is the maximum angle that its two branches can be separated by before they cause a raft of other problems for the sail at their points of sail attachment? Could that support this apparent hole?

Eniac July 25, 2012 at 23:08

@Rob, I don’t think the angle of bifurcation is the real problem here. The real problem is the area of unsupported canopy between tethers, which is limited and leads to the dense forest of tethers you see in the figure. I see no way to support the canopy that is directly opposite from the capsule, unless you use a compressive structure of some sort. A ring, perhaps, that surrounds the site of explosion and keeps the tethers out of the way.

There is a brief mention of the problem in the Solem paper at the bottom of page 11, and it is dispensed with using a calculation showing that a distance of more than 18 m from the explosion should be safe for the tethers. A little hard to believe, but if true, it would certainly alleviate the problem.

Michael July 29, 2012 at 8:13

If anyone is interested here is a scanned copy link of the Medusa idea,
http://www.fas.org/sgp/othergov/doe/lanl/docs1/00189777.pdf

Mick,

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