The Phobos-Grunt unmanned spacecraft cannot be controlled at a low-Earth orbit, where it is currently stuck, because of its design features, a Russian space expert said on Saturday.

The Mars probe was launched from the Baikonur Space Center in Kazakhstan on Wednesday, but its engines failed to put it on course for the Red Planet. The craft, designed to bring back rock and soil samples from the Martian moon Phobos, is currently moving along the so-called "support orbit."

All attempts to receive a signal from the spacecraft have so far been unsuccessful.

"The control of the [Phobos-Grunt] spacecraft at a low orbit has not been incorporated in its design at all," said Igor Lisov, the editor-in-chief of the industry magazine Cosmonautics News.

"Ground-based stations cannot monitor the probe's movements while it is orbiting the Earth with the current rotational velocity," Lisov said. "All that's being done at present is just an improvisation."

According to Lisov, the possibility of a two-way communication with the spacecraft has been envisioned only after the first firing of the on-board engine and a significant reduction of the rotation velocity.

However, the Phobos-Grunt has never even fired its booster after failing to align itself toward solar and stellar reference points, making the possibility of any contact with the spacecraft highly unlikely.

Vladimir Popovkin, the head of Russia's space agency Roscosmos, said on Wednesday engineers had two weeks to restart the probe's booster before its batteries ran out.

In the worst case scenario, the probe could fall on Earth in about four weeks after a slow decent.