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Don't Rule Out Interstellar Probes
by Gerald Nordley

Author's Note:

The Intersection Panel

One of the problems in not getting around to writing a paper I promised a couple of years ago is work that doesn't get in the literature. Three years ago, I organized a panel at "Intersection," a world Science Fiction Convention at which we designed a starship to carry a crew of twenty on an arbitrary journey of 20 years (12 years on board). Our radiation safety guy, Steven Howe of Los Alamos National Labs, came up somewhat surprising conclusion that it was ambient cosmic radiation and not the radiation of passage that drove shielding considerations, and he wanted two meters of water or the equivalent around the people. Our ship ended up with three redundant hulls and weighing 10,000 tonnes.

We didn't have time on the panel to do the engineering trades needed to beat an order of magnitude out of that; it seemed simpler to just let our solar power station-makers consume a few more asteroids. Part of my problem with doing the paper is that the panel was far too conservative on the medical side (ruling out cold sleep or genetic engineering) and while we came up with an interesting point design, it is driven by technology assumptions that are already proving obsolete.

But I regret that Steve's radiation input isn't in the literature.

Anyway, radiation exposure issues for interstellar travel appear to be about the same as for long term solar system missions, and may not generate that many papers for that reason.

Many of us concerned with interstellar travel don't bother with trip times in the thousands or millions of years because, if sent, such missions would be passed by faster missions built a few years later. Based on the criterion of soonest arrival at a target star, one probably doesn't send a mission at all until one's technology is up to sending it at better than half the speed of light, and in that regime, stars are "only" on the order of twenty years apart or so.

I will at this point make a somewhat Clarkian prediction that the first human probe to reach the Alpha Centauri system will spend about the same on-board time in transit as the surviving ship of the Magellan expedition took to circle the Earth or the Galileo Probe took to reach Jupiter. Most conventional wisdom in the community would be an order of magnitude more than that (get data back within a human lifetime).

By the way, JBIS (the Journal of the British Interplanetary Society), while liberal, is a peer-reviewed publication and is probably the best source of innovative thinking on interstellar travel issues.

Cosmic Radiation and Shielding

There seems nothing wrong with Dr. (Mario) Zadnik's description of radiation environment, nor its affects on the kind of probe he assumes. However, anyone sending interstellar probes would be well aware of all that and design their probes accordingly.

It is somewhat presumptuous of us to discuss the technology of races with millions of years more experience in space travel than ourselves (and, perhaps, even more presumptuous to project our current or near-term technological limitations on them). But if our understanding of the laws of physics is approximately correct, we can use that and our own limited experience to bound the problem and guess at enough solutions to establish some confidence that the problem can be solved.

Like our own cells, the CPU's of intelligent probes utilizing nanotechnology would be expected to have error checking and correction mechanisms. As an example of what is possible, consider the radiation toleration abilities of certain bacteria. This alone probably solves the problem, but there is more fun to be had.

Magnetic fields can be used to deflect charged particles and lasers can charge any particles that aren't already charged. The magnetic fields can be generated by superconducting loops that need little en route power to maintain their fields.

The energy requirements for interstellar travel are large, but not in relation to the energy available from any central star, energy which is accessible to advanced spacefaring civilizations. What follows is fairly crude, unoptimized and cursory, but it should give a rough idea of what a civilization like ours might be able to do, starting in a half-century or so.

We can already see the beginnings of the development of self replicating machines. Such machines could use space resources to build solar power stations in space and copies of themselves. The exponential growth of such power resources would provide the energy and power needed for interstellar travel at relativistic velocities in a few decades. Other options for obtaining the energy needed, notably mining giant planet atmospheres for fusion power fuel, have been proposed as well.

The mass of material needed to build the solar power satellites would be the equivalent of a few asteroids. To show this, we start with the energy requirements. The kinetic energy requirements of a spacecraft moving at a gamma of 2 (.866c) are equal to its rest mass energy. A thousand-tonne spacecraft would thus have a kinetic energy of:

At one gravity, it would take about a year to get up to speed, for an average power consumption of about:

Solar flux at the orbit of Venus is about 2500 W/m^2. If an advanced civilization could convert 4/10 of that (1000W/m^2) to kinetic energy of a starship, it would need a collection area (per starship) of:

If each square meter of collection area (burdened by its share of power conversion and transfer hardware) had a mass of a kilogram, the arrays would weigh, collectively, 3E12 kg, or 3 billion tonnes (3E9). Let's say 30 billion tonnes to allow for necessary inefficiencies and tradeoffs. At a density of 2.5 tonnes per cubic meter, and asteroid of that mass consumed would have a volume of:

This would form a cube a little over 2 km on a side.

Since mass around a planetary system is about the only scarce resource for a technologically advanced civilization, the mass of the energy collection hardware gives us a rough order of magnitude estimate of the economic "cost" of sending a thousand tonne spacecraft up to gamma = 2. We don't yet know the full extent of our own asteroid belt, but spacewatch discovers hundreds of new kilometer-sized belt asteroids with every observing run and the total mass of the belt is now estimated to be on the order of 2E18 tonnes. Excavation of material from the Moon or Mercury is another option.

There are a variety of propulsion systems already in the literature that could be applied to relativistic flight. The best, in my view, involve projecting particles or pellets from a base anchored to a large asteroid. The pellets strike a spacecraft which reflects them and absorbs their momentum and kinetic energy. In principal, by proper choice of the projection velocity, nearly all the energy of this stream of mass can get into the spacecraft. The particles can be guided by lasers along their route, or, given advances in nanotechnology, steer themselves to their collision. Photon-pushed light sails are another option. The trick is to leave most of the propulsion system hardware mass and momentum at rest.

Deceleration can be accomplished by laying out a deceleration mass path (kind of like a runaway truck lane) outside the target system, or, if no one has been there yet, a mass beam could be projected from a star system beyond the target system. Alternatively, 999 tonnes of the thousand-tonne spacecraft could be devoted to a deceleration system, leaving only a tonne of probes in the target system. These would be self-replicators that could build up the infrastructure needed to decelerate larger spacecraft. Kuiper belt objects contain enough mass and hydrogen (for fusion power) to handle this. In a few decades of time, thousands of tonnes of alien equipment could be in place in the outskirts of a target planetary system without ground-based inhabitants knowing anything about it.

(Incidentally, the mass needed for deceleration of any probe would make excellent shielding for microcircuits, the equivalent of a few meters of water. Unshielded exposure time would be dominated by the time resident in the space of the target system, not the journey there.)

At a gamma of 2, and allowing a light year and 2 years for the acceleration and deceleration on each end, the journey from a star like Tau Ceti would take about 14.5 years in our frame of reference and 8 years in the traveler's frame of reference.

Our descendants (and any alien observers) will inevitably regard the above analysis as quaint as they will have much better tricks up their sleeves. But it is fairly easy to see from the limited knowledge we have accumulated on such subjects that there is no requirement for alien interstellar spacecraft to withstand "millions of years" of uncorrected cosmic radiation damage.

The real question before us is the one that Fermi asked: "Where are they?"

Approach to Impacts of passage ( design cruise velocity gamma = 2 ):

Particles big enough to detect with laser radar: Target individually and deflect with laser pressure. Unlikely to encounter one on nearby star journeys.

Neutral particles too small to detect: Ablative, nested, forward pointing, multilayer grazing-incidence shielding. The shielding is destroyed at the impact point, but deflects the particle debris. The chance of two particles striking in the same place is negligible.

Neutral hydrogen atoms: Ionize with laser

Charged particles: Deflect with magnetic field and or absorb in the 4 pi steradian cosmic radiation shielding.

In addition to these measures, if a toroidal magsail reflecting a propulsion beam is used it would allow a small fraction of the incident mass to escape forward along the magnetic axis, clearing a path through the interstellar medium ahead of it.

If a given mass is to be delivered for the first time at rest in the target system, many, many times that mass will be needed for deceleration, and that mass will be available for shielding.

If bacteria in the cores of nuclear reactors can handle the error correction problem in packages the size of, well, bacteria, it seems that a sufficiently advanced culture will be able to do so too.

However, larger circuits are harder with respect to radiation damage. We actually know quite a lot about how to handle this even without physical error correction. As a retired AF officer, I have to be careful about what I say, however, one should consider some of the environments our (US) national communications satellites must be designed to survive.

Magnetic Shielding

(SETI Institute scientist Dr. Peter) Bachus writes: "A magnetic field strong enough to protect the probe from cosmic rays would probably damage the electronics."

Such shields have been designed for Mars missions. They won't stop high-z, high energy rays like you probably saw in your cloud chamber, but these form a negligible part of the radiation dose. The volume protected from 1GeV protons was torroidal, and the interior fields (about 5 Tesla at loop center, I think) weren't a problem for the designers, as far as I remember. This was a small, intense, field. Larger extensive loop fields, such as those designed for ramscoops and magsails have lower peak field intensities and larger protected volumes.

Interstellar Dust

The micrometeor environment near Earth has little to do with the debris environment of interstellar space. Things are really EMPTY out there by solar system standards. There is a LOT more debris down in gravity wells. Of course, any probe visiting Earth itself will have to run the gauntlet, but while it is a concern, it hasn't gotten to the point of cancelling space shuttle flights yet.

By the way, Mauldin's "Prospects for Interstellar Travel" gives a figure of one 1E-8 to 1E-7 particle per cubic kilometer (as I understand, this is the size that gets pushed out of solar systems by starlight). A starship with a frontal area of 100 m^2 moving at 2.6E8 m/s will sweep out 2.6E10 m^3/s or 26 km^3 per second, or an average of 38 milliseconds (an eternity for modern electronics) per particle. Deflection lasers would be busy, but not impossibly so. A 1E-7 particle has a volume on the order of 1E-21 m^3 and at a density of 2E3 kg/m^3 has a mass of 2E-18 kg and a kinetic energy of 18 joules--something like a flashbulb. I was wondering if the process of deflection of this dust or its vaporization on impact (at 26 impacts per second) would create a signal detectable for SETI purposes, but I doubt very much that a signal on the order of 468 watts would be observable at interstellar distances.

The more I consider engineering approaches to interstellar flight, the more I appreciate the synergism between the microelectrionic and computational revolution we are going through and interstellar flight. It seems enabling.

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