Let's set the record straight! For altogether too many years now, growing numbers of unfounded myths about interstellar messenger probes have sprouted, weedlike, in the well-manicured gardens of SETI. As noted radioastronomer Edward Purcell once remarked, "All this stuff about traveling around the Universe in spaceships belongs back where it came from, on the cereal box." To help clear out the crabgrass, I have marshalled together a dozen arguments that conclusively rebuke the many myths of interstellar flight and probes.
(1) Probes are more expensive than radio signals. To receive interstellar communications that arrive by radio or by messenger probe, we must search using either radiotelescopes or optical telescopes, either of which costs about the same. But what if you're the one who's sending? The costs to transmit observable radio signals or observable probes, over the duration of an entire exploration program covering, say, the nearest million stars, also are comparable. For example, operating a 1-gigawatt beacon for 1 million years takes exactly the same amount of energy as sending out a fleet of 100-kg 10%c (10% of the speed of light) probes to each of the same million target stars at the rate of one per year. During this time the, beacon will probably elicit no response to its call and thus yields zero knowledge, whereas the probe fleet is certain to have returned, comprehensive data on a million nearby solar systems even if no intelligent life is found; the former approach seems more cost-effective. Further, if the probe is self-replicating only one initial device need be sent out, a strategy that costs thousands of times less energy.
(2) Interstellar flight takes too much energy. A self-powered interstellar probe that first accelerates to 10%c coasts to its destination, then decelerates to zero velocity, requires a mass-ratio (initial/final mass) of 1.2 for a photon rocket or 5.0 for a fusion rocket. These are hardly excessive, considering the mass-ratio of about 21 for the Space Shuttle. It's true you get a fusion mass-ratio of 1 billion at 86%c, but who needs to go this fast?
(3) Probe launch energy is huge by human standards. Calculations showing that the energy required to launch spacecraft to the stars is equal to thousands of times the current total U.S. power consumption or will cost 100 Gross National Products are really irrelevant to SETI. These computations chauvinistically presume current human society to be the standard of comparison for the entire Universe. Yet almost certainly any race capable of transmitting either radio signals or interstellar probes for SETI purposes must be far in advance of ourselves, possibly on the level of a Kardashev Type II civilization (utilizing a major fraction of the energy output of their sun). Launching a 100,000-ton vehicle on a one-way trip at 1 g acceleration to a destination 100 light-years away requires an equivalent relative energy expenditure for a Type II civilization as the launching of a few Saturn V rockets represented to American society a decade ago. Active interstellar exploration by advanced cultures will require commitment but hardly an outrageous sacrifice.
(4) Probe launch energy is better spent on other things. The energy required to send probes, maintain a beacon, or to receive information across interstellar distances via artifacts or radio waves is about the same, so this objection would apply equally strongly to radio-SETI efforts. Buying the "good life" is a subjective purchase and a cultural relative that for many includes active cosmic exploration.
(5) Interstellar flight takes too long. Travel time for either radio waves or 10%c messenger probes is 100-10,000 years, long compared to individual human lives for all but the nearest stars. Probes can establish a rapid information exchange with the locals immediately upon arrival (or any time afterwards), which more than offsets the slight disadvantage of slower initial transmission speed. Probes eliminate the time wasted in waiting for radio messages to crawl back and forth between the stars.
(6) Radio technology is less complex than probe technology. Although radio technology seems less complex, it is also less competent than probe technology for interstellar contact and communication - and technical competence and simplicity are at least of equal importance in engineering. It's equally easy for us to receive probe or radio messages, so from our point of view we don't especially care if one or the other is more difficult to send. And really, are probes all that much more difficult to send? Even on Earth the first gas reaction jet and first rockets were built 2200 years and 600 years ago, respectively, predating the Invention of radio by many centuries. The Project Daedalus starship study suggests that we probably should regard radio and interstellar flight technologies as virtually simultaneous technological developments in the time frame of interstellar communication.
(7) Probe data retrieval requires radio listening antennas. Bernard Oliver once argued that an 80-light-yearr network of 1000 probes, each equipped with a 10-meter X-band transmitter, can report back at 1 bit/sec to the senders only if 1000 100-meter dishes are maintained to monitor the continuous transmissions. Since a Cyclops-like listening array must be constructed by the senders anyway, asks Oliver, why should they send probes that will cost more than the proposed radio searches? Well, in the first place all probes needn't 136 monitored simultaneously because usually they'll have nothing new to report. Ronald Bracewell assures me that one 100-meter dish should be enough to maintain a serial monitoring schedule with no data loss for the fleet. Second, if everyone is listening, who is sending? Remember Radio waves and probes cost about the same to send or receive.
(8) Long flight time Implies technological obsolescence upon arrival. Interstellar exploration is necessarily a long-term effort, so any probe detected is undoubtedly the product of a very mature technology, long developed and near perfectly adapted for the task assigned to it. Probes needn't be state-of-the-art for us to detect them, and as long as the device does its job the senders will be satisfied too.
(9) Probes aren't reliable enough for very long-term missions. For years companies like RCA have been manufacturing fairly complex integrated circuit chips with a mean-time-to-failure of 100 million hours, or about 10,000 years. Whole systems are less reliable, of course. But the Daedalus design assumes a starship reliability of 99.99% (the same as NASA's Project Apollo). This works out to a survival probability of 99.9% after 1000 years and 81.9% after 100,000 years.
(10) A search for probes is a passive strategy. This is true only if we sit and wait for visiting extraterrestrial probes to initiate contact An active SETI search for probes is not passive.
(11) The number of possible artifact sites is infinite. The number of radio frequency bins also is infinite. However, just as there are "magic frequencies" in the same microwave window, there are "magic orbits" where probes are more likely to be found (e.g., Icarus, 1980 [Ref]).
(12) Probes are unlikely because radio is so clearly superior. I would argue exactly the opposite. First, extraterrestrial artifacts have a clear advantage in acquisition efficiency. Microwave beacons may radiate otherwise useful energy and information for millennia or longer without gaining any new knowledge in return. Messenger probes, on the other hand, become independent exploratory agents upon launch. They are the only way we know of to seek out either non-technological intelligent life or non-intelligent lifeforms. If a communicative species is located, the probe enters into a dialogue with it at no further cost to the original senders. Here again probes sire superior in terms of communications feedback - they can engage in a true conversation with the indigenous species, an almost instantaneous, complex interaction between cultures. By comparison, radio wave messages appear little better than sterile data swaps with extraordinarily poor bit rates. Finally, using artifacts gives the overwhelming advantage of military security to the senders. The probe may initiate surveillance or contact without disclosing the identity or whereabouts of its creators. On the other hand, radio beacons are an invitation to disaster at the hands of unknown predatory alien civilizations in this case the senders give up the position of their home star at great risk for more speculative benefits.
The real myth, you can see, is the notion that messenger probes are inherently inferior to radio waves for interstellar communication.