Spacecraft velocity discrepancies show there are still fundamental things we don’t know

Friday, February 29th, 2008

Ten years ago, scientists observed a discrepancy in the velocity of the space probe Pioneer 10. It was going slightly faster than anyone could explain. Every possible physical effect we can think of, including gravitational influence from all bodies in the solar system, the effect of solar wind, possible magnetic fields, possible outgassing of fuel vapors from the spacecraft itself, etc., have been taken into account and considered, and yet we still cannot explain Pioneer 10′s unexpected velocity. Even the known unknown (to borrow Rumsfeld’s terminology) of dark matter, if it exists, cannot explain this acceleration. And now, scientists have expanded the search for discrepancies to the rest of our spacecraft, and finding them in every single one.

Are you spooked? Because I am. A velocity discrepancy of 13 mm/sec for the NEAR spacecraft doesn’t sound like much, but compound that over the course of many years and NEAR’s predicted orbit and where it will actually be will diverge (luckily it has thrusters on it). But that’s not really the significant part of this. The precision of the test is 0.1 mm/sec, so we the discrepancy is significant, and because it’s happening to all of our spacecraft, we know it is repeatable. There is some fundamental aspect of this universe that we do not understand. It’s simultaneously spooky and exciting. With Newton’s theory of gravity, we thought we had the macro scale all figured out until we began finding discrepancies in our observations (particularly with the orbit of Mercury). Then with Einstein’s theory of relativity, we again thought we had everything all figured out.

Now there are more experimental discrepancies.

Sure, it’s possible, maybe even likely, that the discrepancies we observe in our spacecraft are caused by phenomena we already know about, but do not understand fully (such as larger variations in the solar wind than expected). The discrepancy’s effect, weakest along the ecliptic (the plane connecting the Sun’s equator and the orbits of the planets), and strongest for spacecraft in highly inclined orbits, gives us some intriguing clues. But what if we exhaust all of the “normal” possibilities? What if we get right down to it and end up having to revise relativity itself in order to explain these new findings? Maybe gravity doesn’t work exactly how we thought over large distances? The possibilities are frightening and amazing. I’m excited by the possibility of seeing the theory of gravity revised again in my lifetime.

And Happy Leap Day everyone. How fun to learn this news on the weirdest of days.