Robotic Emissaries to the Stars

Five spacecraft from Earth are currently moving out into the stars. After their operations cease, these spacecraft will continue to coast indefinitely. None of them will come close to any known stars, and it is unlikely they will encounter any star systems for at least for millions of years.

This illustration shows the trajectory of the various planetary probes leaving the solar system.

Voyager 1 and 2 and New Horizons will return data measuring the nature of interstellar space after their planetary missions are completed. By 2011 Voyager 1 had reached more than 110 Astronomical Units (AU) from the Sun, or 110 times the Sun-Earth distance. At the same time Voyager 2 reached more that 90 AU from the Sun. After picking up velocity from flybys of Jupiter and Saturn, Voyager 1 has the greatest velocity and is leaving the solar system at about 3.5 AU per year.

Voyager 2 has a slightly lower velocity, at 3.1 AU annually. New Horizons, after its gravity assist at Jupiter, approached the Kuiper Belt at 3.37 AU, encountered Pluto and other bodies in the area. The two Pioneer probes, although no longer functioning, are leaving the solar system at 2.6 and 2.5 AU per year, respectively.

It is very unlikely that any of these spacecraft will ever been seen by any alien civilization, even billions of years in the future. However, four of the outgoing spacecraft contain messages to possible alien space travelers. During planning stages of the Pioneer 10 and 11 missions,NASA officials decided to add a small plaque as a greeting to any alien civilization that may come across the spacecraft in the distant future. The plaque was affixed to the underside of the two spacecraft. The two Voyager spacecraft included a gold-covered phonograph record and a cover with instructions for its use. Encoded on the record are 115 images showing scenes from Earth, audio greetings in several languages, and musical selections.

The fifth spacecraft leaving the solar system, New Horizons, contains several objects including a sample of the ashes of Clyde Tombaugh, the discoverer of Pluto. However, it does not carry any material intended to greet alien civilization.

This pioneer plaque was carried by the Pioneer 10 and 11 spacecraft leaving the solar system. Illustrations of a human male and female were drawn to be representative of diverse ethnic backgrounds. The spacecraft is drawn behind them for scale. The solar system appears along the lower edge. Each planet (plus Pluto) is listed with its average relative distance from the Sun. Distances are listed in binary numbers in units of 1/10th the Mercury distance. The diagram uses numerical figures to explain locations and other information. This presents a challenge when attempting to communicate with an alien civilization, which would certainly use numerical and linguistic systems very different to anything with which humans are familiar. The diagram in the upper left shows atomic hydrogen, by far the most abundant element in the universe. It shows a hydrogen atom undergoing a shift in its electron energy level. This change emits electromagnetic radiation, and is the most common such emission in the universe. This is used as a frequency and distance scale represented by the binary number 1. Converging lines in the left show the position of the Sun relative to 14 pulsars in the Milky Way and the center of the galaxy. Pulsars are very dense remnants of exploded giant stars, and they rotate at very stable frequencies. The frequency of each pulsar is listed in binary numerals relative to the frequency of hydrogen emission. The average human height is listed relative to the wavelength of hydrogen emission.