Specialized circuits could open sets of thermal louvers to release excess heat. Most of each probe’s control and science instruments were located on the back of this dish. The RTGs were ultrareliable: They contained no moving parts, and the 88-year half-life of the fuel meant the spacecraft would have adequate power for decades.īig Dish: The largest single component on both Pioneer 10 and Pioneer 11 was the high-gain antenna, a 2.74-meter-wide reflector that was designed to be able to communicate with Earth from well beyond the orbit of Mars. These generators converted heat from the radioactive decay of plutonium-238 pellets directly into electricity using bimetallic thermocouples. So each spacecraft carried a set of four radioisotope thermoelectric generators (RTGs), positioned a safe distance away from the spacecraft’s sensitive instruments on two long booms, a bit like a pair of rabbit ears. Out by Jupiter, the intensity of sunlight is about 4 percent of what it is in Earth orbit, far too dim to power the solar panels of the day. But Pioneer 10 and 11’s designers nevertheless reckoned they had what they needed to make these missions work. No one knew how risky it would be to cross the asteroid belt between Mars and Jupiter or how difficult it would be to withstand the zones of intense radiation that encircle the gas giant. When Pioneer 10 and 11 were conceived in the mid-1960s, no spacecraft had yet been sent past the orbit of Mars. When it comes to understanding the Pioneer anomaly, it helps to start more or less at the beginning. Our effort to identify the source of this anomaly is in part a detective story and part a cautionary tale about the importance of preserving seemingly uninteresting data. Now, after a dedicated search for lost spacecraft records, detailed study of onboard sensor and navigational data, and careful modeling of the spacecraft and their trajectories, we believe we finally know the answer. But none of the proposed explanations has been entirely satisfactory, and there has always been the possibility that some more mundane mechanism-perhaps some aspect of the spacecraft themselves-was responsible. Others thought the spacecraft might have unearthed the first evidence of extra dimensions or exotic “ mirror matter.”Īll told, hundreds of papers have been written that attempted to explain the Pioneer anomaly with new physics. ![]() Some suspected the two spacecraft were being tugged by an unseen cloud of dark matter or else were exposing a breakdown in the laws of gravitation at large distances. ![]() The magnitude of the Pioneers’ deceleration was in just the right ballpark to suggest that we might be seeing a manifestation of this same expansion within our own solar system. Studies of distant stellar explosions had just shown that the universe is expanding at an accelerating rate. When the discovery of this strange anomaly was finally announced in 1998, more than 15 years after it was first detected, it didn’t take long for potential explanations to emerge. And no one could figure out where it came from. No one would have blinked an eye if it hadn’t been for one troubling detail: For years, as the spacecraft sped deeper and deeper into space, that tiny discrepancy stuck around. It would have shown up as a simple correction, the sort that spacecraft navigators routinely apply to accommodate fuel leaks and other small, transient deviations in spacecraft behavior. ![]() Such a small effect didn’t seem out of place at first. The magnitude of this deceleration was minuscule, just one ten-billionth of the gravitational acceleration that we experience on Earth’s surface. About 11 years later, Pioneer 10 became the first spacecraft to cross the orbit of Neptune, then the farthest planet from the sun. , atop a three-stage launch vehicle, which accelerated the spacecraft to a speed of nearly 52 000 kilometers per hour, a record for the time. The 258-kilogram spacecraft launched on 2 March 1972 from Cape Canaveral, Fla. Building Up: Engineers put some of the finishing touches on Pioneer 10 at a TRW facility in Redondo Beach, Calif.
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