Date: April 25, 2011
Title: Goodbye, Pioneer Anomaly
Podcaster: Kate Becker
Links: SpaceCraft science writing: www.spacecrafty.com
Inside NOVA: http://www.pbs.org/wgbh/nova/insidenova/author/kathryn-becker/
The Daily Camera: www.dailycamera.com
Social media: http://www.facebook.com/katembecker, www.twitter.com/kmbecker
Description: One of the solar system’s unsolved mysteries is finally put to rest. So why are we disappointed?
Bio: Kate Becker is a story researcher for NOVA, the public television science documentary series, and an astronomy columnist for the Daily Camera in Boulder, Colorado. Kate studied physics at Oberlin College and astronomy at Cornell University, and she’s had the good fortune to observe with the Arecibo Observatory in Puerto Rico and the Very Large Array in New Mexico—two of the very best places on this pale blue dot of a planet, if you ask her. Kate lives in Brookline, Massachusetts.
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Transcript:
Say you want to make a scary movie. Here’s my advice: Don’t show the monster.
Show the monster’s shadow. Let us hear the monster scuttling through the walls. Give us the horrified faces of the characters who just saw the monster. But as soon as you show us some green scaly thing—no matter how brilliant the computer animators who created it—well, you might as well have shown us the zipper on the costume.
That’s because, sometimes, it’s the mystery itself that’s thrilling. And that’s why I, for one, am sad to see one the Pioneer anomaly.
For decades, the Pioneer anomaly was one of our solar system’s unsolved mysteries. It started with the launch of the Pioneer 10 and 11 spacecraft in 1972 and 1973. The Pioneers are grand old lions of space exploration. Pioneer 10 was the first spacecraft to pass through the asteroid belt and visit Jupiter; Pioneer 11was the first probe to visit Saturn. Both went on to explore the outskirts of our solar system, and will one day travel on into interstellar space.
In anticipation of some future encounter with alien life, the Pioneers were equipped with metal plaques bearing messages from Earth. You’ve probably seen a picture of the Pioneer plaques somewhere: Two naked humans, a woman and a man with his hand raised in greeting, plus stylized representations of the solar system, the sun’s location in the galaxy, and an energy transition in a hydrogen atom.
Think about this too hard and you’ll probably conclude that the probability of intelligent extraterrestrials ever encountering these plaques in the void of space is something like the probability of winning the lottery while simultaneously being struck by lightning and giving birth to septuplets—but it’s the thought that counts. And maybe that thought—that Pioneer 10 and 11 were our emissaries into the vast unknown—can help explain what happened next.
As Pioneer 10 and 11 traveled deeper into space, they continued to transmit weak radio signals. But when scientists looked closely at those signals, they discovered something strange: The Pioneers were slowing down ever-so-slightly, as if the sun were tugging on them a little harder than it should have been. Engineers checked and rechecked the data, but the aberration persisted.
The “Pioneer anomaly” has attracted lots of attention. Professional and amateur space detectives have pounced on the case, starting with mundane explanations, like computational rounding errors, and progressing to stranger ones. As years passed and the anomaly still could not be explained, some theorists began to speculate that we were actually seeing previously unknown laws of physics at work, or that the Pioneers had uncovered flaws in the law of gravity.
Then, a team of physicists from Portugal stepped up and spoiled the fun—I mean, solved the mystery. Applying a technique called Phong shading, which is more commonly used to make light play realistically over 3D computer graphics, they precisely calculated how waste heat from each spacecraft was reflecting off the spacecraft’s own antenna. This reflection creates a tiny force that pushes the spacecraft off course.
This wasn’t a completely new idea—a scientist from the Jet Propulsion Laboratory had suggested some years earlier that heat radiating off the spacecraft could have tweaked their trajectories—but the new analysis predicts that the force caused by the heat reflection could change the speed of the spacecraft just enough to match the observed data.
So that’s it. No new physics, no rewriting gravity. No aliens accidentally nudging the Pioneers off course while ogling the naked plaque people. We’ve seen the monster: And it is heat reflection.
If this were one of those scary movies, you’d be groaning now and ditching your leftover popcorn into the trash bin. But maybe you’d also be asking if there’s a sequel: And in this case, there just might be.
Because the Pioneer anomaly isn’t the only weird thing going on in the solar system. There’s also the flyby anomaly. The flyby anomaly has given strange, unexplained kicks of energy to spacecraft like Galileo, NEAR, Cassini, and Rosetta when they’ve swung by Earth looking for a gravitational slingshot effect. The flyby anomaly is so well studied that physicists can write an equation to predict its magnitude for a given flyby: It’s related to the angle at which the spacecraft approaches and recedes from Earth’s equator. But the cause of the anomaly is still unknown.
Will the conclusion of this mystery be another letdown—another shadow revealed to be ordinary old physics in a monster suit? Probably. But history tells us that sometimes discrepancies that look like nitpicking to some do turn out to be revolutionary. Think of the precession of Mercury: A tiny gap between the observed precession and the predictions of Newtonian physics was wide enough to permit an entirely new physics that we now know as General Relativity.
And that’s why, next time there’s a scary movie showing, we’ll still line up to buy tickets. Sure, the ending might be cheesy. Sure, the Junior Mints cost ten dollars. But this time, maybe we’ll see a monster that’s worthy of its shadow.
This is Kate Becker with the 365 Days of Astronomy Podcast. Want to get in touch? Email me at kate at spacecrafty dot com, or find me on Twitter—I’m @kmbecker—or Facebook, where I’m katembecker. Wishing you clear skies.
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365 Days of Astronomy
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