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September 30, 2010

Centauri Dreams looks at Interstellar Space Missions

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1. ProjectIcarus is the attempt to re-examine the Project Daedalus starship study of the 1970s in light of technological developments in the intervening years

The Icarus team is looking at Uranus for mining helium-3. Getting processed helium-3 to orbit presenting perhaps the biggest technical hurdle. Double-walled hot-air balloons are chosen to keep the processing unit stable in the atmosphere, with a single-stage nuclear-thermal rocket emerging as the best solution for return to orbit. But the Icarus study is young, and the team is also considering tether concepts for atmospheric mining that do not involve braking and descent into the atmosphere at all.

The Daedalus probe needed an annual rate of 1500 tonnes of He3. If the Icarus interstellar probe was 100 times lighter then it would require an annual mining rate of 15 tonnes of He3.

Deuterium/helium-3 could be competitive with the more researched deuterium/tritium fusion alternative when Helium 3 costs between 1 – 10 Billion$ per ton or less.


2. There was an examination of the energy required for an interstellar mission and to weigh this against predictions of when those energy levels will be accessible and available to be used for space purposes.

Millis compares the annual Space Shuttle launch rate against the total annual energy consumed by the United States, finding that the maximum ratio of Shuttle propulsion energy to total US energy consumed occurred in the year 1985, equaling 1.3 x 10^-6. The average ratio over the years 1981 to 2007 is 5.5 x 10^-7. Millis then takes the maximum ratio (over an order of magnitude greater than the average ratio) to calculate the earliest opportunity for future missions. What he calls the Space Devotion Ratio is thus 1.3 x 10^-6.


How soon until Earth becomes a Kardashev Type I civilization — one capable of mastering all the energy reaching the Earth from the Sun? Acknowledging the wide span of uncertainty in the result, Millis pegs the earliest year this could occur as 2209, with a nominal date of 2390 and a latest date of 6498. A constant growth rate is assumed, which balances depletion of natural resources against unforeseen advances in new energy sources, leaving growth rates relatively stable.

When could we launch a 10 ton interstellar probe to Alpha Centauri based on these calculations? Assume 75 years as the maximum travel time that might be acceptable to mission scientists and assume a rendezvous rather than a flyby mission, acknowledging the need to acquire substantial amounts of data at the destination.

As to propulsion options, Millis works with two possibilities, the first being an ideal case that assumes 100% conversion of stored energy into kinetic energy of the vehicle (think ‘idealized beam propulsion’ or even some kind of space drive), the second being an advanced rocket with an exhaust velocity of 0.03c.

The result: The earliest launch for a 75-year probe is 2247, with a nominal date of 2463. This assumes idealized propulsion; i.e., a breakthrough technology like a space drive. Fall back on advanced rocket concepts and the energy requirements are much higher, with the nominal launch date of the probe now becoming 2566, the earliest possible date being 2301.


The Project Icarus site recently evaluated gas core nuclear reactors.

The performance of a gas core nuclear rocket has often been touted as being on the order of 3000 sec Isp and, optimistically, even higher. Practical considerations such as structural heating probably limit the achievable Isp to something closer to 1500 sec. This is still a substantial improvement over solid core nuclear rockets and probably makes the idea worth pursuing.


Adam Crowl writes about the needs for shields to protect the Icarus probe from collisions with material in space.

The Icarus project is also taking a detailed look at solar sails.

Currently the analysis is that solar sails are not going to be useful for the acceleration and deceleration of the main craft. Solar sails may have a role to play in other aspects of assisting communications or deployment of sub-probes in the target system.


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