In December 1971, President Nixon's Office of Mangement and Budget canceled the National Aeronautics and Space Adminstration's plans to take a $750 million Grand Tour of the five outer planets in the solar system.

Frantically, NASA looked far a way to replace the Grand Tour with a cheaper mission. What it came up with, what President Nixon approved and what will play out a fresh crescendo Wednesday, is the Voyager mission that will end up costing $460 million and missing only Pluto, the most distant of the planets.

Two Voyager spacecraft left earth in 1977, both bound for Jupiter. Each has encountered Jupiter, survived its hazardous radiation belts and the first of the two is about to encounter Saturn.

Voyager I will pass by Saturn late this wee, then fly out of the solar system. Voyager 2 will reach Saturn next August, then press on toward a Jan. 24, 1986, encounter with Uranus and meeting with Neptune in August 1989.

The last two encounters depend somewhat on Congress, which has not approved all of the $75 million it will take to keep people on the payroll for the Uranus encounter or any of the $50 million needed to keep fewer people around for the Neptune mission. But nobody in the space business likes to imagine Congress withholding these funds.

"The next time the outer planets are lined up like this will be 175 years from now," one astronomer said. "That literally means we probably can't attempt another flight to Uranus and Neptune until the 22nd century."

To hear space scientists tell it, the Voyager mission to the outer planets is worth every penny.

"On a cost-per-world-basis," said Cornell University's Dr. Carl Sagan, "the cost of Voyager has been about a penny for every person living on earth." "

So far, Voyager's findings have been impressive. It discovered two new moons around Jupiter and has already found three new moons near Saturn. It found volcanoes on Jupiter's moon Io bigger than any on earth. It found a ring around Jupiter and has already discovered that the six rings around Saturn really number more like 120.

Who cares? Scientists certainly care. They are here in droves at the Jet Propulsion Laboratory where the flight of Voyager is directed and they come from at least 30 states and a dozen foreign countries. One planetary astronomer on the Voyager team has made 74 round trips the last two years from Great Britian to witness to these events.

"Not many scientists get an opportunity like this to make history," said Dr. Garry Hunt of the University of London. "My goal in life is to understand planetary atmospheres as best I can. There is no time like the present in which to do that."

What is next after Voyager? The space agency plans to send a Galileo spacecraft in 1983 to orbit Jupiter and drop a probe into its turbulent atmosphere. Beyond that, in 1986 it wants to send a similar spacecraft to orbit Saturn and probe Titan, the largest of Saturn's 15 moons, the largest in the solar system and the only one that has a significant atmosphere.

The space agency also would like to land a spacecraft on Mars to dig up a soil sample and bring it back home. Failing that, it wants to send a roving vehicle across Mars to get a better understanding of what cut billion-year-old canyons so deep they dwarf the Grand Canyon. The cost of all this would be well over $2 billion in the next 10 years. The debate about whether to do it is about to begin.