Voyager 2 Aims to End Tour in Grand Fashion

The Grand Tour is almost over.

The venerable Voyager 2 spacecraft is sailing through the back yard of the solar system at more than 42,000 m.p.h., closing in on the last encounter of the most incredible journey of all.

The automated craft is plunging through an area nearly 3 billion miles away where no visitor from Earth has ever been before, aiming toward a spectacular flight over the cloud tops of Neptune at 9 p.m. on Aug. 24. No one has ever seen Neptune other than as a ball of telescopic fuzz, but by the time Voyager is through, thousands of close-up photos and warehouses full of data will have been sent back to Earth.

Scientists will watch the encounter with rapture, knowing that nothing like this will happen again for decades, perhaps even centuries.

For many years they will study the images and examine the data and rewrite the textbooks as Voyager 2 soars out into the Milky Way to wander among the stars, possibly forever. Its twin, Voyager 1, is already on its way to study emissions from distant stars, its journey to the planets completed.

For most people, the fantastic flight of the Voyagers will end later this month when Voyager 2 whips past Neptune and pays a quick visit to its extraordinary moon, Triton. The spacecraft has already discovered four new moons, including three announced Thursday.

But the electronic camera that has allowed armchair scientists to tour the solar system’s four outer planets will soon grow quiet, ending a 12-year odyssey that is without parallel.

“The picture show will be over,” said Edward C. Stone, vice president of Caltech and chief scientist on the Voyager project.

Fitting End

The Neptune encounter should be spectacular, and many of those final pictures will be shown live on television, providing a fitting end for a most unusual spacecraft. The one-ton Voyager is the most sophisticated robot ever built, composed of 65,000 parts, including an on-board computer that has the electronic circuitry equivalent to 2,000 color TV sets. In recent years, Voyager 2 has been electronically rebuilt by controllers at the Jet Propulsion Laboratory in Pasadena as it sailed through the solar system, making photography possible under circumstances comparable to taking pictures in a dark cellar.

The craft’s computer is sophisticated enough for the Voyager to heal itself when trouble arises. It can even shut off important electronic parts during times of radiation peril, or freeze its orientation toward Earth if for some reason it appears in danger of drifting into a position where contact with Mission Control would be lost.

Stone expects the two Voyagers to send scientific data on the composition of interstellar space back to Earth for another quarter of a century, but there will be no more Neptunes, no more moons like Jupiter’s Io with its potent volcanoes, no more mad dashes through the rings of Saturn. It will be the middle of the 22nd Century before the outer planets once again present themselves in such an alignment that would make a similar mission possible.

Neptune will grow ever dimmer as Voyager 2 dips below the ecliptic plane in which the planets travel around the sun, signaling the end of an era.

A Sad Day

“I’m going to be sad,” admitted Charles E. Kohlhase, who for the last 14 years has worried about every detail of the mission. As manager of the Voyager mission planning office at JPL, Kohlhase well remembers the early days when he and his colleagues wondered among themselves: “Is this really going to work?”

Yet work it did, “better than any of us expected,” Stone observed.

The magnitude of that fact was underscored by astronomer Carl Sagan, a member of the Voyager’s imaging team, during a recent speech to JPL employees.

“This will surely be in the history books 1,000 years from now,” Sagan said.

“Voyager is a unique event in solar system exploration,” he added. “But an era is drawing to an end.”

The climax is not without peril, however. Even a small piece of dust could severely damage Voyager during its daringly close flyby.

Antenna Network

Throughout the encounter with Neptune, 38 different antennas on four continents will be used to receive transmissions from Voyager. Many of them have been enlarged, and some have been tied together to form a network of antennas to monitor the weak signal.

The total cost of both Voyager missions is about $865 million, according to Kohlhase.

That means the Grand Tour, as the National Aeronautics and Space Administration likes to call it, has cost every American 20 cents per year, he added. And in 12 short years it has produced more information about the solar system than had been learned throughout history.

Although this month will bring the end of the planetary odyssey, Voyager will go out in spectacular form. Consider this:

- Neptune is 2.71 billion miles away, so far that Voyager will be on its own throughout the close encounter. It will take four hours and six minutes for Voyager’s radio transmissions, traveling at 186,000 miles per second, to reach Earth. That means that if an object should suddenly loom in Voyager’s path as it zips past Neptune at 17 miles per second, the spacecraft would have hit it hours before ground controllers even learn of the danger. There will be no opportunity to correct its course from Earth.

- Voyager is so distant that by the time its radio signal reaches Earth it will be reduced to the strength of only one ten-quadrillionth of a watt. That is 20 billion times weaker than the power used to operate a digital wristwatch.

- The amount of sunlight that reaches Neptune is about 1,000 times less than that reaching Earth, making it extremely difficult to capture clear images. So engineers have come up with a system that will allow the entire spacecraft to “nod” so that its camera can remain fixed on the target as the craft speeds past, permitting longer exposures. Each electronic image will be composed of more than 5 million bits of data, so as the craft whips past Neptune it will create a continuous string of data nearly 3 billion miles long, reaching back to Earth.

- Voyager will pass within 3,000 miles of Neptune’s north pole, closer than it has come to any other object in the solar system. That is, quite literally, right over the cloud tops.

- Neptune’s gravitational pull at that close range will change Voyager’s course by 90 degrees, sending it off to encounter one of the strangest objects in the solar system five hours later. Voyager is to pass 24,000 miles from the surface of Triton, which is about the size of Earth’s moon but a very different body indeed. Triton is the only large moon in the solar system that travels backwards, orbiting Neptune in the opposite direction of the planet’s rotation. Scientists think they know why, and Voyager should tell them if they are right.

- By the time Voyager passes Neptune, 11,000 man years will have been devoted to the mission, according to JPL’s Kohlhase.

The Voyager program was initially funded to study only the planets Jupiter and Saturn. Two spacecraft were provided so that if one failed, the other could complete the mission.

Voyager 2 was launched from the Kennedy Space Center atop a Titan-Centaur rocket on Aug. 20, 1977. Sixteen days later Voyager 1 was launched on a shorter trajectory, so the second spacecraft arrived at the two planets first, reaching Jupiter on March 5, 1979, and Saturn on Nov. 12, 1980.

Voyager 1 Pays Price

Voyager 1 passed close to the two planets while Voyager 2 remained at a safer distance. Both encounters by both spacecraft were an enormous success, but Voyager 1 paid a price--it passed through an intense radiation belt surrounding Jupiter, and several instruments aboard the craft were severely damaged.

“If you had been riding the spacecraft you would have received a lethal dose of radiation,” Kohlhase said. In fact, he added, the amount of radiation would have been 1,000 times greater than humans can endure.

So a somewhat crippled Voyager 1 was allowed to end its planetary sojourn and zip on out toward the stars. But Voyager 2 was still in excellent condition, just as it is all these years later, and mission planners at JPL did not want to let go.

NASA agreed to provide funding for the rest of the Grand Tour, and Voyager 2 used the gravitational field of Saturn to fling itself out toward Uranus.

Date With Miranda

On Jan. 24, 1986, Voyager 2 zipped passed Uranus, but the planet itself ended up being upstaged by one of its moons, Miranda. Miranda, it turned out, had strange contours on its surface that to this day have not been fully explained.

Like Uranus, Neptune may well end up playing second fiddle. The planet will appear bluish in color because of the methane in its atmosphere, and it will have more atmospheric texture than Uranus. But since it is essentially a huge ball of hydrogen and helium gas, there are no surface features to observe other than clouds. There will never be a “landing” on Neptune, because there is no land.

But Triton, Neptune’s largest moon, is another story entirely and it has fascinated scientists ever since it was discovered in 1846--just one month after Neptune itself was detected. A smaller moon, Nereid, was discovered in 1949, and Voyager has since found four more. That, however, is not very many moons compared to the other great planets, and that shortage has long troubled scientists because they can see no reason why Neptune should have so few.

Wrong-Way Moon

Once every 5.88 days, Triton completes an orbit around Neptune, but it travels in the wrong direction. Moons form in the equatorial plane around planets, and in all other cases they move in the direction of the planet’s rotation.

“When planets form they have a disk and small satellites, and small satellites orbit in the same direction as the planet,” said Stone, who has been the Voyager’s chief scientist throughout the mission.

Why is Triton traveling in the opposite direction in a cockeyed orbit that carries it far above and below the planet’s equator?

A team of Caltech scientists led by astrophysicist Peter Goldreich have argued that Triton is not a natural part of the Neptunian system at all and that the reddish-brown moon was actually a chunk of rock and ice wandering through the solar system that collided with one of Neptune’s moons. That spectacular collision would have destroyed the other satellite and sent Triton careening off on a very irregular orbit, so the argument goes.

If Triton was a wandering “planetesimal” that collided with a Neptunian satellite, it would have entered a highly elliptical orbit, passing as close as five Neptune radii from the planet and then moving out to more than 100 radii in an elongated orbit.

Sweeping a Path

That irregular orbit would have caused Triton to sweep through those altitudes, smashing into any other moons that happened to be in the area.

“Triton would have devoured the regular satellite system, other than those that are closer than about five radii,” Goldreich said.

Eventually, Triton’s orbit would grow circular, but not before “almost any object in its path would have been destroyed,” he added.

During that period, which probably lasted up to a billion years, Triton would have been subjected to immense stresses due to the changes in Neptune’s gravitational pull as the moon plunged close to the planet, and then zipped far, far away. The stresses, called “tidal forces,” would have been great enough to liquefy the core of the satellite, and possibly its surface.

Voyager should reveal whether this theory is true, according to both Stone and Goldreich. Furthermore, anyone who follows Voyager’s discoveries during August will have a chance to test the theory for themselves.

Test of Theory

If that scenario is correct, Voyager should not find new moons between five and 100 radii of Neptune. The four that have been discovered so far are in orbits close to the planet and would not have been affected by Triton’s grand sweep.

It would be an immense problem for the theory if Voyager discovered a lot of moons in the area that Triton should have swept clear, but at this point that appears improbable.

Moreover, if Triton was liquefied during the time when it would have been sweeping up other moons, there should be far fewer craters on its surface than on other moons throughout the solar system. Thus the surface of Triton should be reasonably smooth, although some cratering could have occurred after the surface hardened.

If the surface of Triton is indeed much smoother than the surface of the Earth’s moon, and if a spate of new moons are not discovered beyond five radii, chalk one up for the theorists and think of Triton as a captured wanderer.

But if Triton is heavily cratered and many other moons are discovered circling Neptune, scratch Goldreich, et al., and look for another explanation for Triton’s peculiarities. That, however, is considered unlikely.

More Than a Rock

No matter who is right, Voyager’s view of Triton should be sensational. Evidence suggests that Triton has pools of frozen nitrogen on its surface, so this should not be just a dull hunk of rock orbiting a distant planet.

First, however, Voyager has to get there and, despite its success record over the last 12 years, the rest of the journey is not without hazards. Scientists believe there should be little debris in Voyager’s path. But no one can be absolutely certain of that.

“We are barreling in there really close,” said JPL’s Kohlhase. “We think we are outside the rings, and above the atmosphere. But are we going to make it? Are we running out of good fortune?”

Only Voyager knows for sure.