A gas cloud on its way towards the super-massive black hole in the
  Galactic Centre
  Measurements of stellar orbits provide compelling evidence that the compact
radio source Sagittarius A* at the Galactic Centre is a black hole four million
times the mass of the Sun. With the exception of modest X-ray and infrared
flares, Sgr A* is surprisingly faint, suggesting that the accretion rate and
radiation efficiency near the event horizon are currently very low. Here we
report the presence of a dense gas cloud approximately three times the mass of
Earth that is falling into the accretion zone of Sgr A*. Our observations
tightly constrain the cloud's orbit to be highly eccentric, with an innermost
radius of approach of only ~3,100 times the event horizon that will be reached
in 2013. Over the past three years the cloud has begun to disrupt, probably
mainly through tidal shearing arising from the black hole's gravitational
force. The cloud's dynamic evolution and radiation in the next few years will
probe the properties of the accretion flow and the feeding processes of the
super-massive black hole. The kilo-electronvolt X-ray emission of Sgr A* may
brighten significantly when the cloud reaches pericentre. There may also be a
giant radiation flare several years from now if the cloud breaks up and its
fragments feed gas into the central accretion zone.
