planet Saturn, where in 1979 it would observe at close range this lightest of the planets (it could float on water), its mysterious rings, and its 4800-kilometer-diameter moon Titan.

Going in the other direction, Mariner 10 left Earth on 3 November 1973, headed inward toward the sun. In February 1974 it passed Venus, gathering information that confirmed the inhospitable character of that planet. Then, using Venus's gravitational force as propulsion, it charged on toward the innermost planet, Mercury. On 29 March 1974, Mariner 10 flew past Mercury, providing man a 5000times closer look at this desolate, crater-pocked, sun-seared planet than had been possible from Earth. Using the gravitational field of its

Venus was photographed from 720 000 kilometers by Mariner 10.

A large, fresh impact crater on Mercury was photographed by Mariner 10 from 34 000 kilometers. The crater, 120 kilometers across, looks similar to many on the moon, but because Mercury has a gravitational field 2.3 times as strong as the moon's, material ejected at impact is not hurled nearly as far on Mercury.

host planet to alter course, Mariner 10 flew out in a large elliptical orbit, circled back by Mercury a second time on 21 September 1974, and a third time on 16 March 1975. The cumulative evidence pictured a planet essentially unchanged since its creation some 4.5 billion years ago, except for heavy bombardment by meteors, with an iron core similar to Earth's, a thin atmosphere composed mostly of helium, and a weak magnetic field.

Fascinating as was the information on our fellow-voyagers in the solar system and as important as the long-range scientific import might be, Congress and many government agencies were much more intrigued with the tangible, immediate-return, Earth-oriented program that began operations in 1972. On 23 July ERTS 1 (Earth Resources Technology Satellite) was launched into polar orbit. From that orbit it would cover three-quarters of the earth's land surface every 18 days, at the same time of day (and therefore with the same sun angle for photography), affording virtually global real-time information on developing events such as crop inventory and health, water storage, air and water pollution, forest fires and diseases, and

recent urban population changes. In addition it depicted the broadarea-and therefore undetectable by ground survey or aircraft reconnaissance-geologic patterns and coastal and oceanic movements. ERTS 1 also interrogated hundreds of ground sensors monitoring air and water pollution, water temperature and currents, snow depth, etc., and relayed information to central collection centers in near real-time. The response was instantaneous and widespread; foreign governments, states, local governments, universities, and a broad range of industrial concerns quickly became involved in both the exploration of techniques to exploit these new

ERTS (Earth Resources Technology Satellite) photograph of the Washington-Baltimore area in October 1972. Green, red, and infrared images from the satellite were combined at Goddard Space Flight Center. Healthy crops and trees come out bright red in the infrared. Cities and industrial areas show as green or dark gray; clear water is black or dark blue. Washington is to be seen slightly left of center on the Potomac River; Baltimore is at the top center on Chesapeake Bay.

wide-area information sources and in real-time use of the data for pressing governmental and industrial needs. Some 300 national and international research teams pored over the imagery. For the first time accurate estimates were possible of the total planting and growth status of wheat, barley, corn, and rice crops at various times during the growing season; real-time maps versus ones based on data that would have been collected over a period of years; timber cutting patterns; accurate prediction of snow run-off for water management; accurate, real-time flood damage reports. Mid-term data included indications that the encroachments of the Sahara Desert in Africa could be reversed by controlled grazing on the sparse vegetation in the fringe areas; longer range returns suggested promise in monitoring strip mining and subsequent reclamation and in identification of previously unknown extensions of Earth faults and fractures important to detection of potential earthquake zones and of associated mineral deposits.

Like the experimental communications satellites of the early 1960s, the Earth-resources satellites found an immediate clientele of governmental and commercial customers clamoring for a continuing inflow of data. The pressure made itself felt in Congress; on 22 January 1975, Landsat 2 (formerly ERTS 2) was orbited ahead of schedule to ensure continuation of the data that ERTS 1 (renamed Landsat 1) had provided for two and a half years, and a third satellite was programmed for launch in 1977. This would give confidence to experimental users of the new system that they could securely plan for continued information from the satellite system.

The Earth-resources program had another important meaning. It was a visible sign that the nature and objectives of the space program were undergoing a quiet but dramatic shift. Where the moon had been the big target during the 1960s and large and expensive programs had been the name of the game, it became increasingly clear to NASA management as the decade ended that the political climate would no longer support that kind of a space program. The key question now was, "What will this project contribute to solving everyday problems of the man-in-the-street?" One by one the 60s-type daydreams of big, away-from-Earth projects were reluctantly put aside: a manned lunar base, a manned landing on Mars, an unmanned “Grand Tour" of several of the planets. When the Space Shuttle finally won approval, it was because of its heavy dedication to studies of our Earth and its convincing economies in operation.

Another sign of the times was that NASA was increasingly

becoming a service agency. In 1970 NASA for the first time launched more satellites for others (ComSatCorp, NOAA, DoD, foreign governments) than for itself. Five years before only 2 of 24 launches had been for others. Clearly this trend would continue for some years.

Meanwhile Apollo was running its impressive course. Apollo 12 (14-24 November 1969) repeated the Apollo 11 adventure at another site on the moon, the Ocean of Storms. One attraction of that site was that Surveyor 3 had been squatting there for two and a half years. A pin-point landing put the LM within 183 meters of the Surveyor spacecraft. In addition to deploying scientific instruments and collecting rock samples from the immediate surroundings, Astronauts Conrad and Bean cut off pieces from Surveyor 3, including the TV camera, for return to Earth and analysis after 30 months of exposure to the lunar environment.

Apollo 13 was launched 11 April 1970, to continue lunar exploration. But 56 hours into the flight, well on the way to the moon, there was a thump back in the service module behind the astronauts. An oxygen tank had ruptured. Pressure dropped alarmingly. What was the total damage? Had other systems been affected? How crippled was the spacecraft combination? The backup analysis system on Earth sprang into action. Using the meager data available, crews at contractor plants all over the country simulated, calculated, and reported. The verdict: Apollo 13 was seriously, perhaps mortally, wounded. There was not air or water or electricity to sustain three men on the shortest possible return path to Earth. But, ground crews and astronauts asked simultaneously, what about the lunar module, a self-contained spacecraft unaffected by the disaster? The lunar landing was out of the question anyway; the lifesaving question was how to get three men around the moon and back to Earth before their life-supporting consumables ran out. Could the LM substitute for the command module, supplying propulsion and oxygen and water for an austere return trip? The simulations said yes. Apollo 13 was reprogrammed to loop around the moon and set an emergency course for Earth return. The descent engine for the LM responded nobly; off they went back to Earth. It was a near thing-powered down to the point of minimum heating and communication, limiting activity to the least possible to save oxygen. Again the flexibility and depth of the system came to the rescue; when reentry was safely within the limited capabilities of the crippled Apollo, the "lifeboat" LM was fondly jettisoned along with the wounded service module. Apollo 13 reentered safely.

The next flight was delayed while the causes and fixes for the