Is an unknown force having an influence (1) on Pioneer 10 and 11, NASA’s oldest space probes? The probes have been travelling through space for over thirty years, but scientists have now (2) come to the conclusion that their current position does not correspond (3) to the predictions of where they should be. The probes appear to be (4 under the control of something other than the sun’s gravity. Scientists are unsure of the cause (5) of the probes’ slowness, but have narrowed (6) down the possibilities to three main options. They (7) put forward their suggestions at a conference this week. It is possible, but unlikely, (8) that both probes are malfunctioning in the same way at the same time; our understanding of gravity may be wrong, or an unknown force in the universe is (9) probably the cause of a difference to the speed of the craft. Scientists say they will need to carry (10) out more research before they can determine what the cause is.