Voyager 1 Leaving Solar System, Entering Interstellar Space

[Guest guest]

May 31, 2005: After nearly 28 years of touring the giant planets and

beyond, NASA's Voyager 1 has now reached the outermost antechamber

of the solar system, a final interlude before it departs.

 

"We're now in the final lap of the race to get to interstellar

space," said Dr. Edward C. Stone, the project scientist for the two

Voyager spacecraft, which were launched in 1977 and continue

operating on their plutonium power sources.

 

In New Orleans last week, at a meeting of the American Geophysical

Union, scientists reported that last December Voyager 1 passed

through a boundary, called the termination shock, that is similar to

a sonic boom. Although silent to human ears, the tenuous gas

particles in outer space do bounce back and forth in fast-moving

sound waves. At the termination shock, the speed of particles

streaming out from the Sun suddenly drops from supersonic - 700,000

to 1.5 million miles per hour - to subsonic, slowed by the pressure

of interstellar particles pushing on the solar system.

 

Voyager 1, about 8.7 billion miles from the Sun and more than three

times as far as Pluto, has entered a region of space known as the

heliosheath.

 

Dr. Stone said that a phenomenon similar to a termination shock

could be seen when water flowing out of a faucet hits the flat

bottom of a sink. Where the water hits, "It's very thin and very

fast," he said. But as the flow spreads outward, it becomes too

sparse to continue pushing the water in front, and the water

suddenly piles up.

 

"You'll notice that a ring forms around the water in the bottom of

the sink," Dr. Stone said. "The water gets very thick and slow. That

thick region is the heliosheath."

 

The crossing occurred Dec. 16, the one day last year that no

listening time on NASA's Deep Space Network was devoted to the

Voyagers. On Dec. 15, "We're clearly in the solar wind," said Dr.

Norman F. Ness, a professor at the University of Delaware's Bartol

Research Institute and the principal investigator for Voyager's

magnetometer. On Dec. 17, "we're clearly in the heliosheath," he

said.

 

Two years ago, a team led by Dr. Stamatios M. Krimigis of the

Applied Physics Laboratory at Johns Hopkins University had made a

similar claim. Dr. Krimigis and his colleagues described a drop in

the speed of the solar wind in August 2002 and a sharp rise in the

number of high-energy particles bombarding the spacecraft,

suggesting that Voyager 1 had crossed the termination shock. The

solar wind appeared to speed back up six months later. The

acceleration could have occurred if stronger solar winds had pushed

the termination shock boundary outward, back beyond Voyager 1.

 

Other scientists disagreed. Dr. Ness referred to the earlier claim

as "the alleged crossing of the termination shock" and said that a

crucial characteristic of a crossing - a large, sustained rise in

the magnetic fields, caused by the piling up of solar wind

particles - did not occur. The controversy arose in part because the

instrument that directly measured the solar wind was knocked out in

1980 as the spacecraft passed through the radiation belts of Saturn.

 

Voyager 2 is expected to pass through the termination shock in a few

years. Its solar wind instrument is still working.

 

Dr. Krimigis said last week that he now agreed that the 2002 event

was not a termination shock crossing. "We thought it may be the

termination shock, but we were quite guarded in our interpretation,"

he said.

 

What appears to have happened, Dr. Stone said, is that the shock

boundary was expanding outward at about the same speed as Voyager 1

and that particles accelerated by the boundary and funneled inward

by magnetic fields were occasionally hitting the spacecraft. "We

were sort of surfing the shock, but not crossing it," he said.

 

In mid-2004, after the maximum of the Sun's 11-year cycle, the

boundary started collapsing inward again, he said, allowing Voyager

1 to finally catch up.

 

Other puzzles remain. Astrophysicists had thought that the

termination shock boundary provided the kick for certain high-speed

particles seen in the inner solar system. But Voyager 1 detected no

change in the numbers of these particles after it crossed the

termination shock. "There must be another source," Dr. Stone

said. "We'll try to understand where that source is."

 

In the heliosheath, the average velocity of the solar wind particles

continues to slow until, at the edge of the solar system, they are

overwhelmed by the interstellar wind - made up of particles that

flow between stars. The heliosheath may stretch several billion

miles, so Voyager 1 is expected to take about a decade to reach that

final crossing into the interstellar medium.

 

Voyager's plutonium power source is expected to last until 2020, but

whether NASA will still be listening when that moment comes is

uncertain. As part of the shift in priorities toward sending people

back to the Moon and then to Mars, a proposed budget for NASA next

year cuts about $20 million of the $70 million spent for about a

dozen missions, including the Voyagers, that continue past their

original designed lifetimes. The Voyagers cost about $4 million a

year.

 

Dr. Richard R. Fisher, deputy director of NASA's Earth-Sun division,

said last week that he was looking to transfer some money from other

areas to the extended missions and that an independent review later

this year would prioritize which missions should be continued. A

final decision will be made next year, he said.

 

Dr. Ness said NASA would be foolish to cut the financing. "The risk

is very low," he said. "The return is very high."

 

SOURCE: The New York Times. Voyager 1 Approaching Edge of the Solar

System. By KENNETH CHANG.

URL: http://www.nytimes.com/2005/05/31/science/space/31voyager.html