Culminating a six-year adventure marked by disaster, seat-of-the-pants ingenuity and several triumphs, the Galileo space mission will soar to its big day Thursday when it sends a 746-pound probe parachuting into Jupiter.
If all goes well, the probe’s radios will crackle to life six hours before it plunges through the thin gases of the Jovian upper atmosphere at 106,000 miles an hour beginning at 4:56 p.m. Chicago time.
NASA officials called the countdown “white-knuckles time,” admitting they don’t know for certain what will happen. They have high hopes of salvaging a $1.4 billion mission that took 20 years to get off the ground and then needed a new computer system installed on the fly after the major antenna failed to open.
On Wednesday, though, they were thinking nothing but good thoughts about the instruments on board Galileo and its probe.
“The data they will return promise to revolutionize our understanding of the Jovian system and reveal important clues about the formation and evolution of the solar system,” according to the space agency.
If NASA’s math is right–after a slingshot, 2.3-billion-mile voyage that sent Galileo boomeranging around Venus and Earth to gain enough momentum to make it to Jupiter–the probe will strike Jupiter’s atmosphere at an angle of only 8 degrees to the horizon, steep enough so it won’t skip out again into space, yet shallow enough to survive the heat and jolting deceleration of the entry.
Blazing like a tiny sun and twice as hot (28,000 degrees), the probe must survive flying through a nuclear fireball. After the heat shield falls away, a parachute will unfurl to slow it to 100 m.p.h. in just two minutes.
If it survives, the probe will float more than 125 miles through Jupiter’s roiling atmosphere of hydrogen and helium while being buffeted by boiling white clouds of frozen ammonia, methane and water vapor.
It may encounter hurricane winds of up to 200 m.p.h. and lightning and heavy rain at the base of the water clouds believed to exist within the planet’s atmosphere.
The probe will have only 75 minutes to gather data about the giant gas ball–half planet, half star–and relay it up 133,000 miles to the mother ship before vaporizing from heat and pressure.
Galileo will receive the probe data within 50 seconds and store the precious material–humanity’s first look inside one of the outer planets–in the spacecraft tape recorder and in computer memory for later transmission to Earth.
After receiving the probe’s data, Galileo will turn away to prepare for a crucial 49-minute burn of its main rocket engine that will insert the spacecraft into a looping, 22-month orbit around Jupiter to look for clues about the formation of the solar system.
“In many ways, Jupiter is like a miniature solar system in itself,” said Wesley T. Huntress, associate administrator for space science at NASA headquarters in Washington.
“Within Jupiter’s constellation of diverse moons, its intense magnetic field and its swarms of dust and charged particles, the Galileo mission should uncover new clues about how the sun and the planets formed and how they continue to interact and evolve.”
Many scientists believe the largest and most massive planet still holds, in a pristine state, much of the material from which the sun and planets were formed about 4.6 billion years ago.
According to Richard Young, a probe project scientist, “Jupiter gives us a Rosetta stone for what the early solar system was like.” Galileo will examine temperatures, magnetic fields, radiation, cloud characteristics and gravity.
Mostly, however, the scientists want another crack at observing Jupiter’s 13 spectacular moons, which were examined by the Voyager spacecraft in 1979. Galileo will study them closely some 385 years after its namesake, Galileo Galilei, discovered them.
Some are icebergs, some mudballs, some rock, some water. Their surface features range from great plains pockmarked by craters to the salt-strewn beds of forgotten seas. Four rotate backwards, and two wobble.
Scientists will be paying special interest to Europa, whose slippery, cracked landscape seems to have an atmosphere and the possibility of underground oceans. The discovery of water, and hence the possibility of primitive life, would make all the misery scientists have undergone over this mission melt away.
Launched on Oct. 18, 1989, from the shuttle Atlantis, Galileo embarked on its complex series of “slingshot trajectory” planetary flybys, in the process passing and making memorable snapshots: the moon’s north pole; lightning on Venus; the first good look at two asteroids, Gaspra and Ida. But the key triumph came when Galileo directly observed the impact of comet Shoemaker-Levy 9 on Jupiter on the planet’s far side, an event unavailable to earthbound telescopes.
In the meantime, Galileo scientists wrestled with a nightmare: the high-gain antenna’s failure to unfold.
The low-gain antenna, which works fine, dribbles data back to Earth. The high-gain antenna, shaped like a 16-foot-wide umbrella, was intended to send information from Jupiter in a virtual flood of computer data–135,000 computerized bits per second.
In April 1991, scientists discovered they couldn’t open the high-gain antenna. Failure was blamed on three of 18 umbrella-like “ribs” that wouldn’t release from the antenna’s central column, apparently because some small pins lost lubrication.
Through 1992 and 1993, scientists devised ways to use the low-gain antenna to transmit data–albeit much less data, and much more slowly–in an effort to save the mission.
In the process Galileo’s onboard computers received a brain transplant: They were completely reloaded with rewritten software.
Now, in the best of outcomes, the orbiter will end up sending home highly-compressed dribbles of data at a rate 100 times slower than the high-gain antenna.
Scientists will receive only about 1,000 pictures, instead of the 50,000 that had been originally hoped for. But much of the data they seek is chemical, not photographic, and they still predict the spacecraft will be able to fulfill about 70 percent of its scientific mission.
