Perhaps, you think, it’s time to reassess the conjecture that there’s intelligent life on Earth.

Spacecraft from the Earth have now flown by dozens of planets, moons, comets, and asteroids—equipped with cameras, instruments for measuring heat and radio waves, spectrometers to determine composition, and a host of other devices. We have found not a hint of life anywhere else in the Solar System. But you might be skeptical about our ability to detect life elsewhere, especially life different from the kind we know. Until recently we had never performed the obvious calibration test: to fly a modern interplanetary spacecraft by the Earth and see whether we could detect ourselves. This all changed on December 8, 1990.

Galileo is a NASA spacecraft designed to explore the giant planet Jupiter, its moons, and its rings. It’s named after the heroic Italian scientist who played so central a role in toppling the geocentric pretension. It is he who first saw Jupiter as a world, and who discovered its four big moons. To get to Jupiter, the spacecraft had to fly close by Venus (once) and the Earth (twice) and be accelerated by the gravities of these planets—otherwise there wasn’t enough oomph to get it where it was going. This necessity of trajectory design permitted us, for the first time, to look systematically at the Earth from an alien perspective.

Galileo passed only 960 kilometers (about 600 miles) above the Earth’s surface. With some exceptions—including pictures showing features finer than 1 kilometer across, and the images of the Earth at night—much of the spacecraft data described in this chapter were actually obtained by Galileo. With Galileo we were able to deduce an oxygen atmosphere, water, clouds, oceans, polar ice, life, and intelligence. The use of instruments and protocols developed to explore the planets to monitor the environmental health of our own—something NASA is now doing in earnest—was described by the astronaut Sally Ride as “Mission to Planet Earth.”

Other members of the NASA scientific team who worked with me on Galileo’s detection of life on Earth were Drs. W. Reid Thompson, Cornell University; Robert Carlson, JPL; Donald Gurnett, University of Iowa; and Charles Hord, University of Colorado.

Our success in detecting life on Earth with Galileo, without making any assumptions beforehand about what kind of life it must be, increases our confidence that when we fail to find life on other planets, that negative result is meaningful. Is this judgment anthropocentric, geocentric, provincial? I don’t think so. We’re not looking only for our kind of biology. Any widespread photosynthetic pigment, any gas grossly out of equilibrium with the rest of the atmosphere, any rendering of the surface into highly geometrized patterns, any steady constellation of lights on the night hemisphere, any non-astrophysical sources of radio emission would betoken the presence of life. On Earth we have found of course only our type, but many other types would have been detectable elsewhere. We have not found them. This examination of the third planet strengthens our tentative conclusion that, of all the worlds in the Solar System, only ours is graced by life.

We have just begun to search. Maybe life is hiding on Mars or Jupiter, Europa or Titan. Maybe the Galaxy is filled with worlds as rich in life as ours. Maybe we are on the verge of making such discoveries. But in terms of actual knowledge, at this moment the Earth is unique. No other world is yet known to harbor even a microbe, much less a technical civilization.

The visions we offer our children shape the future. It matters what those visions are. Often they become self-fulfilling prophecies. Dreams are maps.

I do not think it irresponsible to portray even the direst futures; if we are to avoid them, we must understand that they are possible. But where are the alternatives? Where are the dreams that motivate and inspire? We long for realistic maps of a world we can be proud to give to our children. Where are the cartographers of human purpose? Where are the visions of hopeful futures, of technology as a tool for human betterment and not a gun on hair trigger pointed at our heads?

NASA, in its ordinary course of doing business, offers such a vision. But in the late 1980s and early ‘90s, many people saw the U.S. space program as, instead, a succession of catastrophes—seven brave Americans killed on a mission whose main function was to put up a communications satellite that could have been launched at less cost without risking anybody; a billion-dollar telescope sent up with a bad case of myopia; a spacecraft to Jupiter whose main antenna—essential for returning data to Earth—did not unfurl; a probe lost just as it was about to orbit Mars. Some people cringe every time NASA describes as exploration sending a few astronauts 200 miles up in a small capsule that endlessly circles the Earth and goes nowhere. Compared to the brilliant achievements of robotic missions, it is striking how rarely fundamental scientific findings emerge from manned missions. Except for repairing ineptly manufactured or malfunctioning satellites, or launching a satellite that could just as well have been sent up in an unmanned booster, the manned program has, since the 1970s, seemed unable to generate accomplishments commensurate with the cost. Others looked at NASA as a stalking horse for grandiose schemes to put weapons into space, despite the fact that an orbiting weapon is in many circumstances a sitting duck. And NASA showed many symptoms of an aging, arteriosclerotic, overcautious, unadventurous bureaucracy. The trend is perhaps beginning to be reversed.

But these criticisms—many of them surely valid—should not blind us to NASA triumphs in the same period: the first exploration of the Uranus and Neptune systems, the in-orbit repair of the Hubble space telescope, the proof that the existence of galaxies is compatible with the Big Bang, the first close-up observations of asteroids, mapping Venus pole to pole, monitoring ozone depletion, demonstrating the existence of a black hole with the mass of a billion suns at the center of a nearby galaxy, and a historic commitment to joint space endeavors by the U.S. and Russia.

There are far-reaching, visionary, and even revolutionary implications to the space program. Communications satellites link up the planet, are central to the global economy, and, through television, routinely convey the essential fact that We live in a global community. Meteorological satellites predict the weather, save lives in hurricanes and tornados, and avoid many billions of dollars in crop losses every year. Military-reconnaissance and treaty-verification satellites make nations and the global civilization more secure; in a world with tens of thousands of nuclear weapons, they calm the hotheads and paranoids on all sides; they are essential tools for survival on a troubled and unpredictable planet.

Earth-observing satellites, especially a new generation soon to be deployed, monitor the health of the global environment: greenhouse warming, topsoil erosion, ozone layer depletion, ocean currents, acid rain, the effects of floods and droughts, and new dangers we haven’t yet discovered. This is straightforward planetary hygiene.