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136 a SETI Energy Hypothesis that states that a technologically advanced civilization uses electromagnetic energy as a means to remotely explore the universe and to detect or communicate with other advanced civilizations, and a SETI Artifact Hypothesis that states that a technologically advanced civilization has undertaken a long-term program of interstellar exploration The authors of the SETI 2020 report recognized that we could search for alien artifacts or interstellar spacecraft. SETI has sought signals instead, not because these other approaches are without merit, but simply because in electromagnetic signaling the speed is very high and the cost is very low. We should keep our robotic eyes open for both, Tarter and Chyba proposed.” Television journalist and novelist Richard Burke-Ward proposed an ambitious broadening of SETI to look for or contact alien artifacts. The search for alien machines might even be incorporated into the Drake equa- tion. The probability of finding a functioning probe would depend on the prevalence of intelligent species in our galaxy, the likelihood of probes being sent to other stars, and the life spans of the probes (we might qualify this; a dead probe still could be detectable). There are several possible forms of evidence: a beacon advertising the probe’s presence; communications between drones; heat traces from pro- pulsion or waste energy radiation; small objects near the Earth or other planets; space-time distortion; anomalous modulation of human commu- nications systems; traces of construction on the Moon or other bodies.”! Scientists already have scanned the lunar gravitational libration (Lagrange) points for objects that might have been placed there to take advantage of those relatively stable positions. They found nothing larger than a few meters in size.” Dyson pointed out that a starship braking from a high velocity would leave behind it a long straight trail of hot plasma that should be a source of persistent broad-band radio emission; radio astronomers could watch for tracks in the sky. The effects of more exotic propulsion systems such as matter—antimatter engines might be detectable over great distances. Others believe that alien machines will be very difficult to detect. “No matter how awesome the starship might be in a terrestrial context,” wrote Viewing and his colleagues, “in its own environment—interstellar space— it is virtually invisible.” The Clarks proposed looking for tritium that would be associated with fusion propulsion, gamma-ray emissions from a nuclear-powered probe, or gravitational waves from other propulsion systems. They cautioned that these techniques could detect only a small subset of possible alien probes.” We may be able to detect only those probes employing technology not very far advanced beyond our own. The same may be true for our efforts to detect electromagnetic signals. Direct Contact