The mystery object is, in fact, the asteroid Toutatis. It was snapped last week by China's Chang'e 2 probe when it flew by the lump of rock. The three-mile-long asteroid was 4.4 million miles from Earth when Chang'e 2 passed it by, missing the tumbling rock by just 2 miles.
Chang'e 2 was originally sent into space to map the moon. After 8 months doing that, it was sent off farther into space—and capturing this image marks the success of its first non-lunar mission. That means the satellite has now set a record for the most distant Chinese space probe ever, and by January it will reach a distance of more than 6 million miles from Earth. [Space Flight Now]