Voyager 2 probably will be asked to take one last photo before it disappears into the darkness of space to wander among distant stars--a "snapshot" of the sun and its family.
Sources in the National Aeronautics and Space Administration said Monday they expect officials to approve a plan to order the Voyager to glance over its shoulder sometime next spring for one last look at home.
The "family photo" could be snapped next March or April after the Voyager finishes a number of other objectives related to its encounter with Neptune and the outer solar system. The exact cost has not been determined, but it would require that some members of the Voyager team be retained for a while longer than is now planned.
"It would be wonderful to look back from Voyager and see the sun as little more than a point of light," said astronomer Carl Sagan, who originated the family photo idea. "There is Mars, that little red dot. There is Venus, that little yellow dot. And that little blue dot, that's us."
Such a snapshot, the Cornell University scientist said, would put things in an "epochal perspective," showing the sun and its family as just "points of light."
It could be the last photo by the Voyager, which is expected to fly through the Milky Way galaxy for millions of years.
Sagan said the Apollo moon program showed just how dramatic such a picture could be.
"I think one of the most powerful and important gifts of Apollo were those scenes of Earth as a fragile sphere," Sagan said.
"They gave a tremendously strong sense of the fragility and the vulnerability of the Earth. Now, here we are on the frontier of the universe."
Meanwhile Monday, scientists at NASA's Jet Propulsion Laboratory in Pasadena said they have finally found the phenomenon on Neptune that produces the northern and southern lights on Earth, but it has left them more puzzled than before. And oddly enough, they not only found it on Neptune, they found it on Neptune's moon Triton as well.
On Earth, the lights are produced when electrically charged particles from space enter the planet's area and become trapped by its magnetic field. The particles spiral along the magnetic field lines and into the atmosphere at the magnetic poles, causing the auroral activity that produces the lights.
The lights are seen over the North and South poles because the Earth's magnetic field is aligned closely with the planet's spin axis. That is what scientists had expected to find at other planets because Earth's magnetic field is created by internal fluids that move in concert with the planet's spin.
The motion of the fluids creates what amounts to a bar magnet running north and south through the center of the Earth, and that forms the magnetic field.
But Voyager scientists have found that Neptune's magnetic field is inclined about 50 degrees to the planet's spin axis. In other words, Neptune's bar magnet is lying on its side.
No one knows why, and it has complicated the picture enormously.
"This is something we weren't counting on," said Ralph McNutt, a physicist with the Massachusetts Institute of Technology.
Since Neptune's "bar magnet" is askew, the planet's magnetic field "wobbles around the spin axis" as the planet rotates.
That generates a magnetic field that is so confused that the northern and southern lights seem to be popping up everywhere, not just over the magnetic poles.
Further complicating the picture is the fact that Triton wanders in and out of Neptune's "radiation belt" of charged particles, according to physicist Andy Chen of Johns Hopkins University.
Scientists had not expected to find southern lights when they looked at Triton's south pole, but that's exactly what they found anyway.
"We do in fact see it," said Roger Yelle of the University of Arizona.
Chen said the lights on Triton apparently are caused by the interaction of Triton's atmosphere with Neptune's radiation belt.
No one standing on Triton's cold surface would be able to see them, however. The lights there occur at frequencies that the human eye cannot see. Voyager's ultraviolet sensors detected the lights.
Scientists studying Triton's thin atmosphere reported Monday that the odd moon would be extremely inhospitable to any visitors.
"Triton is the coldest body we have seen in the solar system," Yelle said.
Triton, which is the only large moon in the solar system that orbits in the opposite direction of its planet's rotation, is pockmarked with a wide range of geological features and scientists are growing more comfortable with the notion that many of them are of volcanic origin.
"We're very excited about icy volcanism," said Torrence Johnson of the Jet Propulsion Laboratory.
As planetary geologists have studied the images of Triton's south pole, they have become increasingly convinced that seasonal variations may play a role in that process. Ice volcanoes are believed to be currently active in Triton's southern hemisphere, which is now in a decades-long springtime.
Johnson theorized that the slightly warmer temperature may cause some of the ice in that region to vaporize, reducing the thickness of the ice that covers liquid nitrogen below the surface.
That, in turn, weakens the "lid" on the volcano, allowing the liquid nitrogen to migrate toward the surface. Nitrogen must remain under pressure to be a liquid, and as it approaches the surface the pressure lessens.
At some point the nitrogen would "explode" as it turned into a gas.