The Probe and the Particle Beam

by Paul Gilster on April 2, 2014

For those wanting to dig deeper into Alan Mole’s 1 kilogram interstellar colony probe idea, the author has offered to email copies of the JBIS paper — write him at RAMole@aol.com. For my part, writing about miniaturized probes with hybrid technologies inevitably calls to mind Freeman Dyson, who in his 1985 title Infinite in All Directions (Harper & Row) discussed a 1 kilogram spacecraft that would be grown rather than built. Here’s Greg Matloff’s description of what Dyson whimsically called ‘Astrochicken’:

Genetically engineered plant and animal components would be required in Astrochicken. Solar energy would power the craft in a manner analogous (or identical) to photosynthesis in plants. Sensors would connect to Astrochicken’s 1-gm computer brain with nerves like those in an animal’s nervous system. This space beast might have the agility of a hummingbird, with ‘wings’ that could serve as solar sails, sunlight collectors and planetary-atmosphere aerobrakes. A chemical rocket system for landing and ascending from a planetary surface would be based upon that of the bombardier beetle, which sprays its enemies with a scalding hot liquid jet.

The passage is from Matloff’s Deep Space Probes (2nd ed., Springer, 2006), which goes on to discuss the need to master nanotechnology so that we can manipulate objects on the scale of atoms. He even speculates, citing Alan Tough, that nanotechnology could produce a hyperthin communication antenna for relaying information to Earth from an interstellar probe, constructing it from resources in the destination star system. And he cites Anders Hansson’s 1996 paper in JBIS that sketches an interstellar Astrochicken, one with

…miniaturised propulsion subsystems, autonomous computerised navigation via pulsar signals, and a laser communication link with Earth. The craft would be a bioengineered organism. After an interstellar crossing, such a living Astrochicken would establish orbit around a habitable planet. The ship (or being) could grow an incubator/nursery using resources of the target solar system, and breed the first generation of human colonists using human eggs and sperm in cryogenic storage.

Infrastructure for the Probe

Just as Project Icarus is an attempt to update the Daedalus design of the 1970s, Alan Mole’s work re-examines ideas like these in light of recent work. Yesterday we looked at the trends he thinks make probes with this degree of miniaturization possible. For getting the probe to destination at 0.1 c, he relies on a magnetic sail which draws on studies performed by Dana Andrews in the 1990s. Andrews was talking about a 2000 kg payload, but Mole’s 1 kg payload could be accelerated at 1000 g to cruising speed, with an acceleration distance he calculates at only 0.3 AU, using 1/9th the kinetic energy of the Andrews probe.

To push the magnetic sail a particle beam produced by an accelerator is demanded. Here is Mole’s description:

The probe interacts with the beam by having a magnetic sail, a loop of superconducting wire. This is ejected from the probe and a large current introduced. The magnetic field repels all parts of the wire, so it naturally inflates to a circular loop. For one kg and 0.1 c the acceleration distance is only 0.3 AU and the loop diameter…is just 270 m. After cruise, it is possible to slow the probe by reintroducing a current into the sail and using “friction” from interstellar magnetic fields. This method seems to scale successfully.

The beam generator in Earth orbit would be powered by beamed power from the Earth’s surface. Mole describes the installation as ‘an orbital solar power farm in reverse,’ with the advantage of tapping directly into Earthbound power resources. He points out that NASA drew up studies of a Solar Power Satellite system in 1981, coming in at a cost of about $4 billion to beam power to Earth by microwaves. The Mole plan reverses the process to power an orbital beam generator that would accelerate the magsail, with an estimated beam generator cost of $17 billion.

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Image: We’ve long imagined beaming power down to Earth to tap the Sun’s abundant energies. But is there a case for beaming power up to drive an interstellar beam technology? Credit: Mafic Studios/National Space Society.

Mole believes the cost of such a beam generator might actually come in considerably lower than $17 billion, but the appendix to his paper shows a wide range of possible costs. In a recent email, Mole wrote me that he is offering a stipend of $5000 (negotiable) for a suitable expert to design and produce cost estimates for the beam generator. Those interested should apply to Mole directly at the email address given at the top of this article. The cost estimates are significant, to say the least, and getting them right should illustrate the value of moving to a smaller probe.

In terms of energy use, by Mole’s figures, a 2000 kg probe of the sort Dana Andrews discussed in his 1994 paper, would require 560 times the US capacity for power generation today. A 1 kg probe would require 28 percent of the US generating capacity, making it far more feasible for a civilization at our stage of development over the next fifty years. The author adds:

Large probes and worldships inspire readers to imagine the vast civilizations that could afford them, but not to start work in hopes of seeing them launched in the readers’ lifetimes. In contrast, a 1 kg probe is plausible for the present civilization. If discussion begins now such a probe could be launched in fifty years.

Sail technologies, whether beamed by microwave, laser or particle beam, come to the fore in discussions like these because we’re already beginning to build experience with solar sails in space. Laboratory experiments have shown that sail beaming works, with the added benefit — shown in Greg and Jim Benford’s experiments — that materials can undergo ‘desorption,’ providing additional acceleration. Now we need to learn how well particle beams can drive a magnetic sail because, like all these sail concepts, this one requires no propellant aboard the spacecraft, a huge plus given the challenge of pushing even a 1 kilogram payload to a tenth of lightspeed.

The paper we’ve been discussing is Mole, “One Kilogram Interstellar Colony Mission,” Journal of the British Interplanetary Society Vol. 66, No. 12, 381-387 (available at the site). The Dana Andrews paper referred to above is Andrews, “Cost considerations for interstellar missions,” Acta Astronautica 34, pp. 357-365,1994. The Hansson paper is “Towards Living Spacecraft,” JBIS 49 (1996), 387-390.

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{ 13 comments… read them below or add one }

Andrew Palfreyman April 2, 2014 at 15:58

The power problem seems manageable without depleting any terrestrial resources, by employing solar statite mirrors (I come up with an inverted cone plus an angled ring) and perhaps solar-pumped lasers feeding off the resultant parallel beam. Many petawatts are available with this approach.

Wojciech J April 3, 2014 at 5:08

It’s the usual paradox-if you have that kind of nanotechnology, you don’t need to colonize other planets anymore. Call it Wojtek’s law-technology that allows interstellar colonization, makes it redundant ;)

don wilkins April 3, 2014 at 8:26
Michael April 3, 2014 at 15:16

1 kg spacecraft

A big issue with a 1 kg spacecraft is the optics, the lens and focuser are heavy and require great accuracy, not the sort of instrument that can under go great acceleration.

‘To push the magnetic sail a particle beam produced by an accelerator is demanded…’

If you can get your magnetic sail (loop) to go through Jupiter’s magnetic field you will encounter charged particle in excess of 10-20% the speed of light and lots of them. That is a big push.

May be one day we could control the flux of charged particles entering Jupiter’s magnetic field as a natural particle accelerator to launch our probes into the deep space.

‘..a loop of superconducting wire. This is ejected from the probe and a large current introduced.’

The current would need topping up as a magnetic force will attempt to stop it flowing and it will need keeping supercool (sunshield?)

‘For one kg and 0.1 c the acceleration distance is only 0.3 AU and the loop diameter…is just 270 m. After cruise, it is possible to slow the probe by reintroducing a current into the sail and using “friction” from interstellar magnetic fields.’

It might be best wound in during the cruise phase to protect the wire loop and then reopened on nearing the target star system.

Alan Mole April 4, 2014 at 13:07

Michael remarks: “it will need keeping supercool (sunshield?)”

Actually high temperature superconductors are good to 93K and appropriate thermal design can keep it that cool. (Narrow rectangular cross section, narrow face towards the sun with low absorptance coating. )

Details from the paper:

“Appendix A Thermal Design for Temperature of 93 °K at 1 AU

Andrews and Zubrin[15] selected YBa2Cu3O7 as a superconductor with good electrical and mechanical properties for use in a magsail in the inner solar system. Its critical temperature (Tc) is 93 K. It is shown below that this temperature can be achieved at 1 AU by appropriate geometry and thermal coatings without recourse to external cooling.

Consider a 1 cm length of ribbon cable 0.2 cm by 5 cm. The size is selected for convenience in calculation; the actual sail cross section would be much smaller but have the same proportions. The sail is painted black so emissivity is 0.95, except for the sunward face which is coated with silverized teflon with an absorptance of 0.09. The solar constant is 0.136 W/cm2 so the energy absorbed on the front face is:

.09 x .2 cm x 1 cm x .136 W/cm2 = .0024 W.

Neglecting the front and rear faces, the area of the top and bottom radiating faces is 10 cm2. The resulting temperature is

T = (Energy/(emissivity x Area x Stefan-Boltzmann Constant)1/4

T= (.0024 W/( .95 x 10 cm2 x 5.67 x 10-12 J s-1 cm-2 °K-4))1/4 = 81 °K

This is well below the required Tc of 93 °K. The temperature can be lowered further by making the cable even thinner to decrease the frontal area or by coating the front with a dielectric mirror with an absorptance of less than .01 instead of the .09 used above. Thus a magsail of YBa2Cu3O7 can be made cold enough to operate at 1 AU. It is not necessary to go to the asteroid belt; the sail can be launched from space near Earth. ”

And in fifty years better superconductors, even room temperature ones, may be available.

Michael April 4, 2014 at 14:27

@Alan Mole April 4, 2014 at 13:07

‘Actually high temperature superconductors are good to 93K and appropriate thermal design can keep it that cool. (Narrow rectangular cross section, narrow face towards the sun with low absorptance coating. )’

Perhaps a wedge shape (facing sun) with reflective coating would do better, then there will be minimum heating and allow lower temperature higher super conductive currents lowering mass. Deployment of such as device (loop) and adjustable retraction could be used to bleed energy back into the control systems as it moves through space (interaction with ions) to power the on-board systems reducing the mass of energy generators needed. Very easy to do.

P.S I sent a contact e-mail to the one given, did you receive it?

Eniac April 5, 2014 at 16:39

A 1 kg probe would require 28 percent of the US generating capacity

the loop diameter…is just 270 m

I have not done the math, but I am pretty sure that that much power focused on such a small area would dwarf the sunshine and make things way to hot for superconductivity, no matter how you paint or shape the wire.

And how much current can a 1 km wire that weighs in at less than 1 kg really conduct, under the best of circumstances? Being a fraction of a millimeter thick: not enough, would be my guess.

Rob Henry April 5, 2014 at 20:32

Eniac says, “I have not done the math, but I am pretty sure that that much power focused on such a small area would dwarf the sunshine”

The article says “ Mole’s 1 kg payload could be accelerated at 1000 g to cruising speed, with an acceleration distance he calculates at only 0.3 AU” so, given the stated cruising speed is 0.1c, we have…
acc = 10^4 m/s/s
X = 4.5 x 10^10 m
u = 0
v = 3 x 10^7 m/s

Those figures are self-consistent from school level kinematics, and yield a time of acceleration of 2100s (t = ROOT(X/acc)), and energy of 4.5 x 10^14 J/kg, so the minimum power (at 100% efficiency) is 2.1 x 10^11 W/kg sustained for 35 min. I have no idea what the radiative area is supposed to be but if it is 1m^2/kg, then the minimum (blackbody) temperature is 44,000K. Even if we have 1000m^2/kg the temperature is 7,800K. High temperature superconductance indeed!

Michael April 6, 2014 at 3:53

@Rob Henry April 5, 2014 at 20:32

‘I have no idea what the radiative area is supposed to be but if it is 1m^2/kg, then the minimum (blackbody) temperature is 44,000K. Even if we have 1000m^2/kg the temperature is 7,800K. High temperature superconductance indeed!’

The energy carried by the particles is converted to momentum for the spacecraft via the magnetic field interaction and I assume the spent particle moves off to the side or is deflected, there is no contact with the loop. But with that amount of energy been used de-poling of the superconductor is a possibility.

Eniac April 6, 2014 at 9:59

Rob: It could be up to 1,000,000 m^2/kg, where we would still have ~ 1200K or so…

A 1kg wire that is 1 km long will not conduct more than ~ 10 A, given optimistic assumptions about known superconductors. The magnetic field inside such a loop would be I/R, or about 30 mT. This may just be enough to deflect a relativistic beam, but not by a wide margin. The effective area could be on the order of a km^2.

Rob Henry April 6, 2014 at 19:14

Eniac, interesting. I didn’t think that magnetic sails had anything like the radiative area of light sails. Your 1000,000 m^2/kg would be 50,000 times greater than standard paper (even if you gave it just to illustrate the heat dissipation problem.). The energy I gave was for the vessels acceleration, but the beam from which it gains this is travelling much faster than the final velocity of 0.1c. This would mean, that from transfer of momentum considerations alone, the magnetic field would have to intercept a beam of much higher energy density. I don’t know too much about these situations, but wouldn’t it be a couple of orders higher, and thus about 2 x 10^13 W/kg??

Eniac April 7, 2014 at 21:25

Rob, I would not say an order of magnitude. If you loop is large enough (and at 1 km it appears to be), you could turn a mildly relativistic beam around 180 degrees, harvesting twice its momentum. Of course, you could not really turn every particle around, more likely you’ll get an isotropic scattering distribution, which should give you a similar efficiency as if the beam was stopped to a halt, without the thermal penalty if the plasma all dissipates into space. However, you can’t really keep some particles from hitting the coil, and your calculation should accurately reflect the thermal effect of that.

Michael April 9, 2014 at 13:02

@Eniac April 7, 2014 at 21:25

‘If you loop is large enough (and at 1 km it appears to be), you could turn a mildly relativistic beam around 180 degrees, harvesting twice its momentum… However, you can’t really keep some particles from hitting the coil, and your calculation should accurately reflect the thermal effect of that.’

I can’t see the particles turning 180 degrees, they would interact with the field slowing down and will move around it or become trapped within the field eventually impacting the loop heating it unless it is ‘like’ charged or the build up dissipated. Maybe a second alternating charged beam could be sent to neutralise the first pulse and so on repeating.

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