  Launched more than thirty years ago and now drifting in space with no further
contact, the Pioneer 10 and 11 spacecraft are currently at the center of a
small but developing concern: are they under the influence of an anomalous
acceleration that reflects new, yet unknown physics, or merely experiencing
some unexpected technical problem ? The proposals vary from basic engineering
principles to extra spatial dimensions, but most probably an answer can only be
obtained after a dedicated mission is underway.
