Voyager 1 Spacecraft Becomes Most Remote Human-Built Object -- Probe Moves Toward `Termination Shock' At 39,000 Mph

If the Voyager 1 space probe could glance over its shoulder toward home, it would see the receding dot of a sun only one five-thousandth as bright as it looks from Earth. The planets would be so many pinpricks dispersed across a vast black canvas.

But the spacecraft is hurtling at 39,000 mph toward "termination shock," the uncharted zone where the sun's magnetic influence ends in a collision with the interstellar wind.

Twenty-one years after leaving Earth, Voyager 1 has become the most remote human emissary in space. It is 6.5 billion miles from Earth in the dark, chill hinterlands of the solar system. Its radio transmissions, flashing across space at the speed of light (186,000 miles per second), take almost 10 hours to reach Earth.

In a few years, scientists expect Voyager 1 to become the first human-made object to push its way into the interstellar frontier beyond the sun's influence.

"The fact that the spacecraft is still returning data is a remarkable technical achievement," said Edward Stone, Voyager project scientist and director of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., site of interstellar mission control.

Time capsule onboard

On the remote chance that Voyager might encounter extraterrestrial intelligence on its long sail toward infinity, scientists in the 1970s outfitted it with a time capsule described by the late author and scientist Carl Sagan as a "bottle (thrown) into the cosmic ocean," carrying the story of Earth. Recorded on a gold-plated copper disk are sounds (thunder, crickets, ocean surf, a mother's kiss), images (cell division, human anatomy) and greetings in 55 languages. There is music, from Chuck Berry to Bach.

Voyager 1 was launched from Cape Canaveral on Sept. 5, 1977. Along with its sister craft, Voyager 2, it conducted the first reconnaissance of the outer solar system, swooping near Jupiter, then Saturn.

To steer it near Saturn's large moon Titan, trajectory engineers at JPL used Saturn's gravity to bend Voyager's path northward out of the ecliptic plane - the imaginary disk on which all the planets except Pluto circle the sun. (Pluto takes a more eccentric, egg-shaped path.) The other Voyager craft has turned southward and is about 5.1 billion miles from Earth.

Voyager has several important assignments to accomplish. But because of the vast distances involved, "the mission today presents an unequaled technical challenge" in spacecraft operations, said JPL's Ed Massey, project manager for the Voyager interstellar mission. For example, the round trip required to send a command signal and then receive Voyager's response takes most of a day, he said.

The craft is able to operate in the deep cold so far from the sun because of nuclear batteries, which convert heat from the natural radioactive decay of plutonium-238 to electricity. Onboard computer software enables the craft, in emergencies, to place itself in a safe state quickly and handle various emergencies.

The reach of the sun

Instruments on both Voyager craft have sent hints that the next big event of the mission is about to begin. They have sensed changes in their environment that scientists believe herald the fringes of the outer boundary of the sun's magnetic field, known as the heliopause.

Like inhabitants of a submarine moving through water, the sun and planets travel through space in a magnetized bubble called the heliosphere. This bubble is created by the solar wind, a continuous flow of electrically charged subatomic particles emitted by the sun and traveling outward in all directions at velocities up to 1 million or 2 million mph.

The walls of the bubble form when the solar wind eventually runs into an opposing pressure in the larger medium through which it is moving. This opposition is thought to come primarily from the interstellar wind, emitted by exploding stars and in other events, which fills much of the space between the stars like a patchy, electrically charged fog.

When these two realms clash, the solar wind is suddenly slowed and deflected, creating the boundary known as the termination shock zone. Space physicists have waited eagerly for their first crack at studying this mysterious borderland.

"The data coming back from Voyager now suggest that we may pass through the termination shock in the next three to five years," Stone said.

Voyager data on the size of the heliosphere, Stone said, should reveal much about the interstellar wind and the galactic magnetic field outside the bubble.

Voyager is expected to sail unharmed through the boundary layers with enough power to keep it operating until about 2020. By then, it will be 14 billion miles out.