Russia's space agency has so far failed to contact its $13-million unmanned Phobos-Grunt Mars probe, which it lost contact with immediately after launch early on Wednesday -- and may end up as the 18th in a long string of unsuccessful Russian missions to the Red Planet.

"So far all efforts to communicate with the craft have been unsuccessful," lead mission scientist Alexander Zakharov of Moscow's Space Research Institute told Reuters.

The probe is stuck in a dangerously low orbit, officials said, and if uncontacted could eventually crash down to Earth. Should the craft fail, it will join a surprisingly long string of  Russian missions to Mars that have failed, even as U.S. rovers have logged hundreds of hours on the Red Planet. Meanwhile, NASA plans to launch a $2.5 billion rover designed to assess the planet's suitability for life later this month, toward the end of a launch window for Mars flights that comes every 780 days.

If Phobos-Grunt eventually crashes back to Earth, its cargo may become a problem: The craft carries a full payload of toxic hydrazine and nitrogen tetroxide fuel and small cargo of radioactive cobalt-57. 

It is unclear how much of it will survive the fiery plunge through the atmosphere.

17 failed missions to Mars since 1960

  *   1960        Korabl 4         Didn't reach Earth orbit
  *   1960        Korabl 5         Didn't reach Earth orbit
  *   1962        Korabl 11       Earth orbit only; spacecraft broke apart
  *   1962        Mars 1            USSR Radio Failed
  *   1962        Korabl 13       Earth orbit only; spacecraft broke apart
  *   1964        Zond 2            Radio failed
  *   1969        Mars 1969A   Launch vehicle failure
  *   1969        Mars 1969B   Failure Launch vehicle failure
  *   1971        Kosmos 419  Achieved Earth orbit only
  *   1971        Mars 2           Orbiter arrived, but no useful data and Lander destroyed
  *   1971        Mars 3           Orbiter obtained approximately 8 months of data, returned only 20 seconds of data
  *   1973        Mars 4           Flew past Mars
  *   1973        Mars 6           Occultation experiment produced data and lander failure on descent
  *   1973        Mars 7           Missed planet; now in solar orbit.
  *   1988        Phobos 1       Orbiter lost en route to Mars
  *   1988        Phobos 2        Lost near Phobos
  *   1996        Mars 96         Launch vehicle failure     

(Source: NASA, Missions to Mars)