Miami - An unmanned NASA spacecraft travelling to a distant asteroid veered toward Earth on Friday for a gravitational slingshot manoeuvre that will better aim it toward the Sun-orbiting space rock, Bennu, the US space agency said.The gravity-boost took place about halfway through the journey of the spacecraft, known as OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security - Regolith Explorer)."Catch you on the flip side!" said the NASA Twitter account for OSIRIS-REx, just before it made its closest approach to Earth at 12:52 (16:52 GMT).The mission launched last year from Cape Canaveral, Florida. Its goal is to collect a sample from Bennu in 2018, and return it to Earth for further study in 2023.Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, Tucson, described the gravity-assist as "a clever way to move the spacecraft onto Bennu's orbital plane using Earth's own gravity instead of expending fuel".The spacecraft zipped over Antarctica at a distance of 17 000km, using Earth's gravity to shift its trajectory so it can eventually meet up with Bennu.Bennu is a primitive, carbon-rich asteroid, the kind of cosmic body that may have delivered life-giving materials to Earth billions of years ago.The asteroid's orbit around the Sun is tilted six degrees in comparison to Earth's.NASA cautioned that during the gravity assist, OSIRIS-REx must swing through a region of space that contains Earth-orbiting satellites."NASA has taken precautions to ensure the safety of the spacecraft as it flies through this area," said the space agency."The mission's flight dynamics team designed a small manoeuvre that, if necessary, could be executed a day before closest approach to change the spacecraft's trajectory slightly to avoid a potential collision between OSIRIS-REx and a satellite."OSIRIS-REx was expected to lose communications with Earth for about an hour during the flyby. "The spacecraft will be too low relative to the southern horizon to be in view with either the Deep Space tracking station at Canberra, Australia, or Goldstone, California," explained Mike Moreau, the flight dynamics system lead at NASA's Goddard Spaceflight Centre.An update on its progress is expected later Friday.