CAPE CANAVERAL, Florida (AFP) – After a 2.5-year journey across the solar system, the US space probe Epoxi readied to swing by comet Hartley 2 for an up-close study of its icy, volatile surface, NASA said Wednesday.

"This flyby will mark the fifth time in history that a spacecraft has been close enough to image the heart of a comet, more commonly known as the nucleus," the US space agency said in a statement.

Epoxi's closest approach to Hartley 2, at some 700 kilometers (435 miles), was expected at 1902 GMT on Thursday.

The probe -- its full name Extrasolar Planet Observation and Deep Impact Extended Investigation -- began its mission in 2005 with a scientific first by dropping a projectile on comet Tempel-1 to study the plume it lifted off its surface.

After the Deep Impact study and further astronomic observations, NASA in 2007 decided to send the spacecraft to an encounter with Hartley-2.