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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 patible 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 mitment to joint space endeavors by the U。S。 and Russia。
There are far…reaching； visionary； and even revolutionary implications to the space program。 munications 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 munity。 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。
Global positioning systems are now in place so that your locale is radio…triangulated by several satellites。 Holding a small instrument the size of a modern shortwave radio； you can read out to high precision your latitude and longitude。 No crashed airplane； no ship in fog and shoals； no driver in an unfamiliar city need ever be lost again。
Astronomical satellites peering outward from Earth's orbit observe with unsurpassed clarity—studying questions ranging from the possible existence of planets around nearby stars to the origin and fate of the Universe。 Planetary probes from close range explore the gorgeous array of other worlds in our solar system paring their fates with ours。
All of these activities are forward…looking； hopeful； stirring and cost…effective。 None of them requires 〃manned〃* spaceflight。 A key issue facing the future of NASA and addressed in this book is whether the purported justifications for human spaceflight are coherent and sustainable。 Is it worth the cost？
* Since women astronauts and cosmonauts of several nations have flown in space； 〃manned〃 is just flat…out incorrect。 I've attempted to find an alternative to this widely used term； coined in a more unselfconsciously sexist age。 I tried 〃crewed〃 for a while； but in spoken language it lends itself to misunderstanding。 〃Piloted〃 doesn't work； because even mercial airplanes have robot pilots。 〃Manned and womanned〃 is just； but unwieldy。 Perhaps the best promise is 〃human；〃 which permits us to distinguish crisply between human and robotic missions。 But every now and then； 1 find 〃human〃 not quite working； and to my dismay 〃manned〃 slips back in。
But first； let's consider the visions of a hopeful future vouchsafed by robot spacecraft out among the planets。
VOYAGER 1 AND VOYAGER 2 are the ships that opened the Solar System for the human species； trailblazing a path for future generations。 Before their launch； in August and September 1977； we were almost wholly ignorant about most of the planetary part of the Solar System。 In the next dozen years； they provided our first detailed； close…up information on many new worlds—some of them previously known only as fuzzy disks in the eyepieces of ground…based telescopes； some merely as points of light； and some whose very existence was unsuspected。 They are still returning reams of data。
These spacecraft have taught us about the wonders of other worlds； about the uniqueness and fragility of our own； about beginnings and ends。 They have given us access to most of the Solar System—both in extent and in mass。 They are the ships that first explored what may be homelands of our remote descendants。
U。S。 launch vehicles are these days too feeble to get such a spacecraft to Jupiter and beyond in only a few years by rocket propulsion alone。 But if we're clever （and lucky）； there's something else we can do： We can （as Galileo also did； years later） fly close to one world； and have its gravity fling us on to the next。 A gravity assist； it's called。 It costs us almost nothing but ingenuity。 lt's something like grabbing hold of a post on a moving merry…go…round as it passes—to speed you up and fling you in some new direction。 The spacecraft's acceleration is pensated for by a deceleration in the planet's orbital motion around the Sun。 But because the planet is so massive pared to the spacecraft； it slows down hardly at all。 Each Voyager spacecraft picked up a velocity boost of nearly 40；000 miles per hour from Jupiter's gravity。 Jupiter in turn was slowed down in its motion around the Sun。 By how much？ Five billion years from now； when our Sun bees a swollen red giant； Jupiter will be one millimeter short of where it would have been had Voyager not flown by it in the late twentieth century。
Voyager 2 took advantage of a rare lining…up of the planets： A close flyby of Jupiter accelerated it on to Saturn； Saturn to Uranus； Uranus to Neptune； and Neptune to the stars。 But you can't do this anytime you like： The previous opportunity for such a game of celestial billiards presented itself during the presidency of Thomas Jefferson。 We were then only at the horseback； canoe； and sailing ship stage of exploration。 （Steamboats were the transforming new technology just around the corner。）
Since adequate funds were unavailable； NASA's Jet Propulsion Laboratory （JPL） could afford to build spacecraft that would work reliably only as far as Saturn。 Beyond that； all bets were off。 However； because of the brilliance of the engineering design and the fact that the JPL engineers who radioed instructions up to the spacecraft got smarter faster than the spacecraft got stupid—both spacecraft went on to explore Uranus and Neptune。 These days they are broadcasting back discoveries from beyond the most distant known planet of the Sun。
We tend to hear much more about the splendors returned than the ships that brought them； or the shipwrights。 It has al； ways been that way。 Even those history books enamored of the voyages of Christopher Columbus do not tell us much about the builders of the Nina； the Pinta； and the Santa Maria； or about the principle of the caravel。 These spacecraft； their designers； builders； navigators； and controllers are examples of what science and engineering； set free for well…defined peaceful purposes； can acplish。 Those scientists and engineers should be role models for an America seeking excellence and international petitiveness。 They should be on our stamps。
At each of the four giant planets—Jupiter； Saturn； Uranus； and Neptune—one or both spacecraft studied the planet itself； its rings； and its mo