For Voyager, It’s Too Late to Turn Back Now

The Voyager spacecraft fired its thrusters and changed course for the last time Monday morning, tossing its fate to the interstellar winds.

There is virtually nothing that can be done now if danger should arise on the last leg of its journey to Neptune.

“This was the last maneuver,” said Voyager project manager Norman Haynes.

Fortunately, engineers at the Jet Propulsion Laboratory believe the course they have charted will keep the aging craft out of harm’s way, setting the stage for “a week of great drama,” JPL director Lew Allen told scientists and reporters from around the world who have gathered at the Pasadena laboratory awaiting the encounter.

Thursday night Voyager should pass clear of the partial ring arcs around Neptune’s equator, and then zip over the planet’s north pole just 3,000 miles above the cloud tops. Friday, it is to fly past Neptune’s largest moon, Triton, ending a spectacular tour of the four outer planets.

“This is the final movement of the Voyager symphony,” said chief scientist Edward Stone. “We’re in for a good ride.”

Voyager continued to send back data on the distant, gaseous planet and its “intriguing” moon, Triton. Images transmitted are growing slightly sharper.

“Everything we see is new,” said Bradford Smith of the University of Arizona, leader of the imaging team.

The smorgasbord of discoveries, which will intensify as the week wears on, included images of Triton that reveal atmospheric phenomena that Smith likened to “snowstorms.”

Triton travels in a highly inclined orbit that carries it far above and below Neptune’s equator, and the amount of sunlight that falls on either pole varies according to where Triton is in its orbit. By a bit of good fortune, Voyager will arrive at Triton when its south pole is enjoying what passes for “summer.”

Triton looks slightly pink in images sent back by Voyager, but its northern hemisphere has patches that have a trace of blue, suggesting that Voyager has arrived at an opportune time to witness one of the moon’s dynamic processes.

Smith said it appears that at Triton’s south pole, the uneven distribution of heat is thawing frozen methane, which rises into the atmosphere and drifts north. The volatile cloud eventually settles back on the moon’s northern surface as a “snowstorm” of frozen natural gas.

The images also reveal that Triton is brighter than had been expected, indicating that it is reflecting more solar energy. That also means it is colder than expected. Some scientists had speculated that Triton might have lakes and oceans of liquid nitrogen, but Smith dashed those conjectures Monday.

“The temperature (of Triton) is well below liquid nitrogen,” he said, so any nitrogen on its surface will be frozen.

Triton fascinates scientists because it is the only large moon in the solar system that orbits in the opposite direction of its planet’s rotation. “It is a most intriguing moon,” Stone said. “It orbits backwards.”

That has led scientists to speculate that Triton did not form when Neptune was formed, but just happened to be passing by and got captured by the planet’s gravity.

“Perhaps it was a giant comet which had the misfortune of coming too close to Neptune,” Stone said.

If so, Triton should have cleaned out a wide swath of lesser objects as it settled into a stable orbit, colliding with everything in its path.

Voyager has so far found nothing that would disprove that. The four new moons it has discovered are too close to the planet to have been hit by Triton, and no new moons have been found in the area that Triton should have swept clear.

Late Monday Voyager began taking pictures of Neptune’s other major moon, Nereid. Voyager’s course will keep it nearly 3 million miles away from Nereid, so these will not be close-ups, but they will offer the best view anyone has ever had of that distant body.

Voyager will arrive at Neptune a little earlier than had been planned. It turns out that Neptune is slightly closer to Earth than scientists had thought. That will shave 4 1/2 minutes off Voyager’s 12-year journey, making its arrival time 8:55 p.m. Pacific Time on Thursday.

Neptune, by the way, is presently 2,742,542,051 miles from Earth.