On its convoluted journey to Jupiter, the Galileo spacecraft swung by Earth on Saturday, giving scientists the first glimpse of what it might be like to be aboard an alien spacecraft flying through the solar system.
At 12:35 p.m., only four-tenths of a second behind schedule and a mere five miles off course, the $1.5-billion nuclear-powered spacecraft flew flawlessly less than 600 miles above the Atlantic Ocean.
“It was virtually perfect,” said William J. O’Neil, Galileo’s project manager at the Jet Propulsion Laboratory in Pasadena. “We could have missed by 100 miles or more . . . and we’d still be on our way to Jupiter.
“It’s hard to communicate the elation one feels with these encounters,” said Clayne M. Yeates, science and mission design manager at JPL.
Saturday’s encounter with Earth was like no previous space mission because it allowed scientists for the first time to observe the Earth and moon in the same way and with the same instruments that are used to study other planets in the solar system. Eventually, scientists hope to be able to make observations and detailed comparisons among planets never before possible in space science.
Two extremely sharp photographs taken Saturday by Galileo, one of the Earth and the other of the moon, gave researchers a hint of the wealth of information they can expect when they sift through all the data recorded by the probe during the flyby.
Whatever comes of these recorded images, Galileo will surely go down in history as “the first time any spacecraft has returned to fly by the Earth after visiting another planet,” O’Neil said. “It’s the first confirmed interplanetary visitor to Earth.”
It will be a week before the spacecraft completes its photographs and atmospheric measurements of the Earth and years before all the analyses are complete. Yeates jokingly said that at least one JPL scientist already has “observed intelligence on Earth” from the pictures he has seen.
Although clearly giddy with enthusiasm at how well the Earth encounter went, scientists at JPL realize that they have many more hurdles to cross before the six-year mission to Jupiter can be labeled a success.
The next major step will take place Oct. 29, when Galileo passes by the asteroid Gaspra, where it will take pictures and make measurements. A year later, on Dec. 8, 1992, it will have its second encounter with Earth, sweeping the skies just 200 miles above South Africa.
Both swings around the Earth, as well as a rendezvous with Venus last February, were designed so the spacecraft can build enough speed to make the long trip to Jupiter.
In a complicated maneuver known as “gravity assist,” the spacecraft makes use of the gravity of planets to alter its orbit around the sun and build up speed along the way.
As Galileo raced by Earth, its speed climbed from 67,240 m.p.h. to 78,860 m.p.h. in its orbit around the sun. When Galileo scoots by Earth two years from now, its velocity will be boosted 8,280 m.p.h. to a fiery 87,190 m.p.h.--fast enough to begin the long voyage to Jupiter, about 480 million miles from the sun.
When Galileo arrives at Jupiter late in 1995, it will have journeyed 2.5 billion miles and begin the first long-term study of the solar system’s largest planet. As it enters the vicinity of that planet in the summer of that year, the spacecraft will drop a probe into Jupiter’s stormy atmosphere before going into orbit for a 20-month tour.
Galileo was named for the 17th-Century Italian scientist who discovered Jupiter’s four major moons with one of Earth’s first telescopes.