My friend Tibor Pacher has taken our interstellar bet to a new level, publishing a lengthy letter on the subject in the current Spaceflight, a journal published by the British Interplanetary Society. Tibor, remember, had made a prediction I found outlandish: That “the first true interstellar mission, targeted at the closest star to the Sun or even farther, will be launched before or on 6 December 2025, and will be widely supported by the public.” I dissented, and we went public with the bet on the Long Bets site. Our funds are in the hands of the Long Now Foundation, with all proceeds going to good causes (details on the site).

But while I have enjoyed tweaking Tibor about the bet, it must be said that he has a solid motivation for going so far out on the speculative limb. The visionary founder of peregrinus interstellar, Tibor hopes to provoke discussion and keep people thinking. Along those lines, then, let’s look at his recent letter. One of the mission specs was a flight time of 2000 years or less to the star of choice. Assuming this is Proxima Centauri simply because of its, well, proximity, we arrive at a minimum average mission velocity of about 650 kilometers per second. That can be compared to Voyager 1’s 17.1 km/s to get an idea of the upgrade in velocity needed, but as we’ve noted in these pages before, the right kind of sail employing a Sun-diver maneuver might get at least close to that speed.

Useful data along the way? Tibor names the targets of opportunity: A craft traveling at 650 km/s gets out to the Kuiper Belt in about a year and reaches the heliosheath at 100 AU. Year two takes it out of the heliosphere entirely, while years five to ten are of note because they take us to the distance of the Sun’s gravitational focus, where Sol acts as a unique lens to magnify distant starlight. Recall that unlike optical lenses (where the light diverges after the focus), a gravitational lens has a focal line that extends to infinity. In other words, separations greater than 550 AU (where the gravitational lensing effect is first available) still offer unique observational possibilities.

Greg Matloff writes about this in his Deep Space Probes book (Springer/Praxis, 2000), noting that beyond 550 AU, the electromagnetic radiation from the occulted object under study is amplified by a factor of 108. And note this:

The ‘spot radius’ (distance from the centre line of the image at which the image intensity gain falls by a factor of 4) has been calculated…to be about 11 km for a Sun-spacecraft separation of 2,200 AU.

In other words, an outbound probe making lensing studies has a long observational run ahead of it. On the other hand, on a Proxima Centauri trajectory, what exactly will it be looking at? This is a rhetorical question, as I don’t know what is exactly on the opposite side of the Sun from this trajectory. Maybe one of our resident astronomers can fill me in.

Somewhere around year 20 of the Pacher probe’s mission it reaches the Oort Cloud, an area of obvious interest that may, in fact, extend halfway to the target star. We might also mention the Pioneer anomaly, for an outbound Proxima Centauri probe can obviously be studied in terms of anomalous acceleration along its route. Two thousand years after launch, the probe reaches the Proxima Centauri system, but for those who object that surely faster probes would have passed it along the way, I can only agree with Tibor that such a probe would get much done along its route before that happens, given a properly configured mission.

No, I won’t object to a 2000-year Centauri mission, powered perhaps by sail technology, on the grounds that it would quickly become obsolete. We don’t know what the future holds, and pushing the state of the art will teach us many things we would not otherwise have learned. Tibor and I have grounds for a bet, though, on the audacious idea that this mission might fly by December of 2025. If that occurs, I suspect advances in nanotechnology will be a large part of the story, but I will still be amazed if Tibor is the one to pop the Champagne cork when we meet seventeen years from now in Budapest to seal the deal.