The Pioneer Anomaly. Something’s up in deep space: the Pioneer spacecraft, now out of contact, have shown an unexplained Doppler drift, indicating sunward acceleration, effectively decelerating the probes cumulatively. The effect may be be nongravitational, and could be explained by any number of factors: an undiscovered twist in Newtonian physics, localized cosmological contraction issues, or just venting gas. Other deep space probes may have experienced the anomaly as well, and a new mission could explore the puzzle; but for now, all we have is past Pioneer data, and that’s stored on old 9 track tape which can only be read by antique readers. What’s to be done? (Also see Pioneer Odyssey for a nostalgic romp through those early days of deep space exploration. And NASA, bring back the original Pioneer home page plz, kthx.)
Followup link: Planetary Society’s Pioneer Anomaly FAQ.