respectively。 He courageously testified before Congress in the face of a politically generated order from the White House Office of Management and Budget （this was in the Reagan years） to exaggerate the uncertainties and minimize the dangers。 His calculation on the explosion of the Philippine volcano Mt。 Pinatubo and his prediction of the resulting temporary decline in the Earth's temperature （about half a degree Celsius） were right on the money。 He has been a force in convincing governments worldwide that global warming is something to be taken seriously。
How did Hansen get interested in the greenhouse effect in the first place？ His doctoral thesis （at the University of Iowa in 1967） was about Venus。 He agreed that the high radio brightness of Venus is due to a very hot surface； agreed that greenhouse gases keep the heat in； but proposed that heat from the interior rather than sunlight was the principal energy source。 The Pioneer 12 mission to Venus in 1978 dropped entry probes into the atmosphere； they showed directly that the ordinary greenhouse effect—the surface heated by the Sun and the heat retained by the blanket of air—was the operative cause。 But it's Venus that got Hansen thinking about the greenhouse effect。
Radio astronomers； you note； find Venus to be an intense source of radio waves。 Other explanations of the radio emission fail。 You conclude that the surface must be ridiculously hot。 You try to understand where the high temperatures e from and are led inexorably to one or another kind of greenhouse effect。 Decades later you find that this training has prepared you to understand and help predict an unexpected threat to our global civilization。 I know many other instances where scientists who first tried to puzzle out the atmospheres of other worlds are making important and highly practical discoveries about this one。 The other planets are a superb training ground for students of the Earth。 They require both breadth and depth of knowledge； and they challenge the imagination。
Those who are skeptical about carbon dioxide greenhouse warming might profitably note the massive greenhouse effect on Venus。 No one proposes that Venus's greenhouse effect derives froth imprudent Venusians who burned too much coal； drove fuel…inefficient autos； and cut down their forests。 My point is different。 The climatological history of our planetary neighbor； an otherwise Earthlike planet on which the surface became hot enough to melt tin or lead； is worth considering—especially by those who say that the increasing greenhouse effect on Earth will be self…correcting； that we don't really have to worry about it； or （you can see this in the publications of some groups that call themselves conservative） that the greenhouse effect itself is a 〃hoax。〃
（3） Nuclear winter is the predicted darkening and cooling of the Earth—mainly from fine smoke particles injected into the atmosphere from the burning of cities and petroleum facilities—that is predicted to follow a global thermonuclear war。 A vigorous scientific debate ensued on just how serious nuclear winter might be。 The various opinions have now converged。 All three…dimensional general circulation puter models predict that the global temperatures resulting from a worldwide thermonuclear war would be colder than those in the Pleistocene ice ages。 The implications for our planetary civilization—especially through the collapse of agriculture—are very dire。 It is a consequence of nuclear war that was somehow overlooked by the civil and military authorities of the United States； the Soviet Union； Britain； France； and China when they decided to accumulate well over 60；000 nuclear weapons。 Although it's hard to be certain about such things； a case can be made that nuclear Winter played a constructive role （there were other causes； of course） in convincing the nuclear…armed nations； especially the Soviet Union； of the futility of nuclear war。
Nuclear winter was first calculated and named in 1982/83 by a group of five scientists； to which I'm proud to belong。 This team was given the acronym TTAPS （for Richard P。 Turco； （even B。 Toon； Thomas Ackerman； James Pollack； and myself）。 Of the five TTAPS scientists； two were planetary scientists； and the other three had published many papers in planetary science； The earliest intimation of nuclear winter came during that same Mariner 9 mission to Mars； when there was a global dust storm and we were unable to see the surface of the planet； the infrared spectrometer on the spacecraft found the high atmosphere to be warmer and the surface colder than they ought to have been。 Jim Pollack and I sat down and tried to calculate how that could e about。 Over the subsequent twelve years； this line of inquiry led from dust storms on Mars to volcanic aerosols on Earth to the possible extinction of the dinosaurs by impact dust to nuclear winter。 You never know where science will take you。
PLANETARY SCIENCE fosters a broad interdisciplinary point of view that proves enormously helpful in discovering and attempting to defuse these looming environmental catastrophes。 When you cut your teeth on other worlds； you gain a perspective about the fragility of planetary environments and about what other； quite different； environments are possible。 There may well be potential global catastrophes still to be uncovered。 If there are； I bet planetary scientists will play a central role in understanding them。
Of all the fields of mathematics； technology； and science； the one with the greatest international cooperation （as determined by how often the co…authors of research papers hail from two or more countries） is the field called 〃Earth and space sciences。〃 Studying this world and others； by its very nature； tends to be non…local； non…nationalist； non…chauvinist。 Very rarely do people go into these fields because they are internationalists。 Almost always； they enter for other reasons； and then discover that splendid work； work that plements their own； is being done by researchers in other nations； or that to solve a problem； you need data or a perspective （access to the southern sky； for example） that is unavailable in your country。 And once you experience such cooperation—humans from different parts of the planet working in a mutually intelligible scientific language as partners on matters of mon concern—it's hard not to imagine it happening on other； nonscientific matters。 I myself consider this aspect of Earth and space sciences as a healing and unifying force in world politics； but； beneficial or not； it is inescapable。
When I look at the evidence； it seems to me that planetary exploration is of the most practical and urgent utility for us here on Earth。 Even if we were not roused by the prospect of exploring other worlds； even if we didn't have a nanogram of adventuresome spirit in us； even if we were only concerned for ourselves and in the narrowest sense； planetary exploration would still constitute a superb investment。
CHAPTER 15 THE GATES OF THE WONDER WORLD OPEN
The great floodgates of the wonder…world swung open。
—HERMAN MELVILLE； MOBY DICK； CHAPTER 1 （1851）
Sometime ing up； perhaps just around the corner； there will be a nation—more likely； a consortium of nations—that will work the next major step in the human venture into space。 Perhaps it will be brought about by circumventing bureaucracies and making efficient use of present technologies。 Perhaps it will require new technologies； transcending the great blunderbuss chemical rockets。 The crews of these ships will set foot on new worlds。 The first baby will be born somewhere up there。 Early steps toward living off the land will be made。 We will be on our way。 And the future will remember。
TANTALIZING AND MAJESTIC； Mars is the world next door； the nearest planet on which an astronaut or cosmonaut could safely land。 Although it is sometimes as warm as a New England October； Mars is a chilly place； so cold that some of its thin carbon dioxide atmosphere freezes out as dry ice at the winter pole。
It is the nearest planet whose surface we can see with a small telescope。 In all the Solar System； it is the planet most like Earth。 Apart from flybys； there hav