In many cultures, Friday the 13th is occasionally considered bad luck, but at least for 2008, Friday, June 13 will mark a momentous (if only mildly inaccurate) anniversary. On June 13, 1983, the space probe Pioneer 10 became the first man-made object to leave the solar system… sort of.

What Pioneer 10 actually did on that date was pass into trans-Neptunian space, beyond the reach of the farthest known planet (be that planet Neptune or Pluto, for those of you who refuse to stop calling the latter a bona fide planet). That’s not technically the end of the solar system, as there’s always the Oort Cloud containing the most distant (about one light year away) objects still in orbit of our sun, and the heliopause, where the power of our sun’s solar winds give way to the thrust of stellar wind.

Pioneer 10 is nowhere near reaching either of these boundaries, but it was the first human construct to leave the planetary zone of our local space behind and brave the wilds of the Kuiper Belt, so June 13, 1983 is a significant astronautic date regardless of the media’s mislabeling the event. Pioneer 10 is also still occasionally referred to as the most distant man-made object in existence, but while the probe did once hold that title, Voyager 1 overtook it on February 17, 1998 and — barring some external intervention — will be the more far-flung human construct for the foreseeable future.

Rather than focus on what Pioneer 10 is not, let us remember what it was and is: One of the most historically significant and technically accomplished spacecraft ever built. Pioneer 10 was the first probe to get up close and personal with Jupiter, sending back the first highly detailed photos of the Jovian gas giant in 1973. In 1997, after its official mission profile was completed, Pioneer 10 became a training article for space probe flight controllers, so they could learn how to acquire radio signals from weak and distant sources. At the same time, Pioneer 10‘s telemetry readings became component data in a Deep Space Network study into chaos theory.

Thus, for various purposes, Pioneer 10 was tracked continuously from its launch in 1972 until it broadcast its last known signal in 2003. From that vast data set, a physics-defying mystery emerged that continues to intrigue and baffle scientists to this day. It’s known appropriately as the Pioneer Anomaly.

WHAT IS THE STRANGE, PHYSICS-DEFYING MYSTERY KNOWN AS THE PIONEER ANOMALY?

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What is the scientific mystery known as the Pioneer Anomaly, a physics-defying conundrum that is named in part after the Pioneer 10 space probe?

The Pioneer Anomaly is the shorthand name given to the unexplained deceleration observed in the progress of both Pioneer 10 and its sister probe Pioneer 11. As both spacecraft travel away from the sun, they are losing velocity at a rate more significant than can be accounted for by all known factors. Put simply, Pioneers 10 and 11 are slowing down too fast, and nobody knows why.

The explanation could be as simple as an unknown fuel leak common to the design of both space probes, but that seems unlikely. Virtually every unmanned spacecraft to spend significant time in space — and travel significant distances — has shown some evidence of the Pioneer Anomaly. Pioneer 10 and Pioneer 11 are simply the first craft to amass enough telemetry data to explicitly expose the phenomenon.

At present, the Pioneer Anomaly represents a sunward acceleration of (8.74 ± 1.33) × 10-10 m/s2 for both its namesake spacecraft. Put more directly, some unknown force is shaving about 5,000 kilometers off the distance the Pioneers should be putting between themselves and the sun every year.

A range of possible explanations have been put forth to explain away the Pioneer Anomaly, none yet proven. These include the aforementioned fuel leak, unforeseen radiation pressure from the Pioneersradioisotope thermoelectric generators, collective drag from stellar wind and cosmic dust, gravitational pull from theoretical dark matter sources, or even a simple observational error from deep space radar tracking systems. That’s without even addressing the more “heretical” explanations, many of which suggest that gravity is not actually a constant force, but one that varies in certain exotic situations.

The Pioneer spacecraft were uniquely suited to highlight their namesake Anomaly, as they don’t have the course-correcting thrusters that later probes use to stay on target and factor out any minute and unexplained dragging forces. Both Pioneers have ceased communications with Earth, so their six collective decades of data are nearly all we presently have to analyze the mystery that bears their name. If and when this astrophysical puzzle is ever solved, it will be another feather in the caps of these two historic space probes.

That’s not just a legendary scientific legacy, but some long-lived and light year-spanning Geek Trivia.

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