Pioneer's Final Riddle
  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. 
O. Bertolami and J. P\'aramos