8 Biggest, Weirdest Mysteries In Space

3. The Pioneer Anomaly

The Pioneer 10 and 11 probes were launched back in 1972 and 1973 and were sent on their merry way throughout the solar system to explore deep space. Obviously, launching expensive equipment into outer space requires a lot of careful route calculation to ensure that they don't go careening into a stray asteroid or planet, and so the various effects of gravity and solar winds had to be taken into account.

Despite this, however, NASA noticed that something was forcing both probes off course at a rate of 386,000 km per 10 billion km of travel, resulting in a very slight acceleration towards the sun.

One thought is that the probes may be emitting small amounts of infrared radiation from one side, pushing them slightly off course, but that hypothesis can only account for about 30% of the error. It could be that Einstein might have got his sums slightly wrong when devising relativity, it could be that this is an as-yet-undiscovered effect of dark matter. Whatever it is, something is slowing our spacecraft down.