Wednesday, 5 November 2003

Voyager I may have reached heliopause

Voyager I, a 26-year-old NASA probe and the most distant man-made object from Earth, may have reached the heliopause just over a year ago. The heliopause is the region of space where the dominant substance is the cool gas and dust left over from ancient supernova. In the words of an Associated Press article published today ("Voyager May Be at End of Solar System"), every star "sends out a stream of highly charged particles, called the solar wind, that carves out a vast bubble around the solar system. Beyond the bubble's ever-shifting boundary, called the termination shock, lies a region where particles cast off by dying stars begin to hold sway."

So how would we know if Voyager has reached the heliopause?

Scientists have long theorized that a shock wave exists where the hot solar wind bumps up against the thin gas of the interstellar medium. A similar shock wave precedes aircraft flying faster than the speed of sound, causing a sonic boom.

In space, the violent encounter slows the solar wind from supersonic velocity to subsonic speed, and causes a pileup of particles.

As they accumulate, the particles increase in temperature. Also, as they skip back and forth across the shock boundary, they are accelerated and energized. Id.

Two groups of scientists are pouring over the stream of data that the spacecraft sends back to Earth, and they disagree on whether it indicates that Voyager has reached this uncharted region of space. Unfortunately, "[t]he one instrument that could measure the solar wind velocity [directly] and give somewhat of a definitive answer ceased working years ago." Id.

The precise location of the heliopause has long been a subject of speculation. As the first spacecraft to reach the region, Voyager I and its younger brother, Voyager II, are expected to provide valuable data. Measurements taken by the robotic vehicles and sent back to Earth will go a long way toward resolving this particular mystery. This may be the last major scientific contribution by this pair of probes — a fine cap to two stellar scientific careers.

By the way, NASA reminds us this week that Voyager I reached a distance from Earth today of 90 AU. AU stands for "astronomical unit," and it is defined as the average distance of Earth from the sun — or approximately 8.4 billion miles or 13.5 billion kilometers.

Posted at 11:26:27 AM | Permalink

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