Which Way from Here?

On March 3, 1972, the spacecraft Pioneer 10 was launched from its pad at Cape Canaveral, Florida, bound for the giant planets Jupiter and Saturn. In addition to its plethora of scientific instruments, the craft also carried a small, gold-plated plaque with a unique message engraved on it. The plaque (Fig. 29.1) showed a man and woman against the outline of the spacecraft, a sketch of the solar system with the trajectory of the spacecraft indicating where it had originated from, and the positions of fourteen prominent pulsars in our galaxy from which the location of our solar system could be determined. Also shown was a diagram of the hydrogen atom, whose frequency and wavelength of radiation could be used as a universal clock and yardstick. By comparing the wavelength to the number shown in binary code next to the woman’s image, an alien scientist could infer the size of those who sent this cosmic messenger. Similarly, by comparing the observed frequency of each pulsar with the data on the plaque, the time that has elapsed since the launch of the craft could be deduced (based on the fact that a pulsar’s frequency decreases steadily at a known rate). Thus Homo sapiens sent an identifying message across the cosmic ocean, announcing to any alien civilization our existence in this niche of our galaxy.