An obscure little moon that is almost invisible to the most powerful telescopes on Earth appeared in danger of stealing Neptune's thunder as the Voyager spacecraft raced toward tonight's historic encounter with the distant planet.
Images of Neptune's oddball moon, Triton, indicate part of the satellite's surface may be light blue in color, while much of the rest of it is pink.
"If it is indeed blue, it's the only thing that's blue on any satellite that we have seen" in the entire solar system, said Brad Smith, head of the imaging team. Smith and others suggested that the color may indicate the existence of glacier-like formations composed of frozen methane.
Explaining that puzzle was just one of scores of problems confronting scientists whose joy over the flawless mission seemed unbounded Wednesday.
"We have people literally jumping up and down" with excitement, Smith said.
The one-ton Voyager will arrive at Neptune at 8:56 tonight, climaxing a 12-year odyssey that is without parallel in the history of space exploration. But Neptune is so far away--2.7 billion miles--that it takes more than four hours for its signals to reach Earth, so it will be Friday morning before scientists get their first real close look at the encounter.
This will be the fourth and final planetary flyby for the aging craft.
Voyager 2, as it is officially known, and Voyager 1, its sister craft that whipped off toward outer space after visiting Jupiter and Saturn, have provided scientists with more information on the outer solar system than they had been able to learn throughout history.
Tonight, however, is the ultimate payday.
Prior to Voyager, Neptune was the least-known major object in the solar system. Over the next few days, as scientists at NASA's Jet Propulsion Laboratory pore over data from the encounter, much of its mystery will be stripped away. But Wednesday, Neptune and its moons and unusual rings were giving up their secrets grudgingly.
Scientists have long been fascinated by Triton, which is a little smaller than Earth's moon, because it orbits Neptune in the opposite direction of the planet's rotation. No other major moon in the solar system travels backward.
Wednesday, Triton revealed that it has a few more surprises.
Color images of Triton, which are only about 1/10th as sharp as the photos that are expected over the weekend, reveal patches on the surface that appear to be light blue.
"That seems to be real," Smith said. "That suggests some real excitement here Friday morning and again Saturday" as better images are analyzed and released.
The blue region, however, is probably not the ocean of liquid nitrogen that some scientists had hoped to find at Triton. It turns out that the temperature there is around 307 degrees below zero, and that is so cold that nitrogen would be frozen.
"We don't know of any materials that would be blue that we would expect to see at Triton," Smith said.
It could be that Triton has the rough equivalent of glaciers of methane ice, sprinkled with crystals of methane snow.
On Earth, the sky looks blue because air molecules scatter sunlight as it enters the atmosphere. Something similar to that may be happening on the surface of Triton.
Smith noted that "very small solid particles" could do the same thing on the surface of Triton as gas molecules do in the Earth's atmosphere
The ice crystals "would be very small, and they could scatter the light and tend to look somewhat bluish."
In most areas, however, the surface appears pink. Triton is constantly being bombarded by high-energy particles from space, changing the chemical and physical characteristics of the surface and giving it a pinkish hue.
Scientists will have to wait at least another day to get images that are sharp enough to tell them about Triton's geology, but enough information has been collected by Voyager to give them some strong hints.
Caltech physicist Edward Stone, the Voyager project's chief scientist, said the surface of Triton is primarily methane ice.
"Methane ice is quite soft and will flow," he said. "It's not rigid like the water ice we've seen at other satellites."
So it could be that the surface has some interesting contours. No matter how it looks, scientists are not likely to be disappointed.
Meanwhile, JPL wizards who "fly" the Voyager confirmed that the maneuver that they put the craft through Monday morning has put them "right smack-dab in the middle" of the place they wanted to be, according to Norm Haynes, Voyager project manager.
Flight controllers wanted to change Voyager's course so that the Earth and the sun will both be aligned on this side of Triton when the spacecraft passes on the far side.
Known as a "double occultation," that alignment will permit scientists to study Triton's atmosphere by determining which wavelengths of light from the Earth and the sun are absorbed by Triton's atmosphere. Different chemical elements absorb different wavelengths.
That required an ever so gentle nudge, and Haynes said Wednesday it had been a total success.
"We are predicting a 99.9% probability that we will hit an Earth-sun occultation," he said.
Voyager has demonstrated rather dramatically the difficulty that scientists have had in studying Neptune from Earth. Astronomers with ground-based telescopes had accumulated strong evidence of three ring arcs and at least a hint of three more for a total of six.
Voyager, however, has found only one. The errors in ground observations were most likely due to uncertainties about the Neptunian system that even included the planet's exact position, Stone said, but Neptune also seems to have played a bit of a trick on some of the observers.
It seems that two telescopes trained on Neptune at exactly the same time detected what appeared to be a ring arc, a streamer of particles that could be many thousands of miles long. But against almost impossible odds, they seem to have been looking at the planet at the exact moment that a passing star was eclipsed by one of the newly discovered tiny moons, falsely indicating the presence of an arc.
For such a small moon to block out the light from a star at just the time that astronomers were looking it was termed "an improbable event" by Smith.
"But it does really seem that that is what happened," he said, incredulously.
TV WILL SHOW FLYBY--Astronomers and insomniacs can view Neptune on TV tonight as Voyager 2 zips past at 38,000 m.p.h. Calendar, Page 1.