Data from the Hayabusa probe, now hovering several miles from the Itokawa asteroid, did not indicate that the vessel had fired a metal projectile onto the asteroid's surface during its landing as previously thought, said Seiji Oyama of Japan's space agency, JAXA.
"Now we just won't know till Hayabusa comes back to Earth, and we open it up," Oyama said.
JAXA had announced on Nov. 26 that the Hayabusa appeared to have briefly touched down on Itokawa, fired a metal projectile and collected the dust that was kicked up, then lifted off again to transmit data to mission controllers.
The landing on the asteroid about 180 million miles from Earth was Hayabusa's second, following a faulty touchdown in mid-November. JAXA lost contact with the probe during that attempt and didn't even realize the probe had landed until days later — long after it had lifted off.
Still, there was a "slight possibility" the impact of Hayabusa's two landings released enough particles for the probe to collect, according to Oyama.
The probe, launched in May 2003, is due to land in the Australian Outback in June 2007. But a technical glitch could delay its arrival.
Hayabusa experienced trouble with its thruster after taking off from Itokawa the second time, forcing JAXA to shut down the ship's engines.
The agency has until December 10 to fix the problem, before Hayabusa must start its journey back to Earth. Any delay would make a return by June 2007 unlikely, because the probe's orbit around the sun would take it away from Earth for two years.
If the Hayabusa does return to Earth with extraterritorial material, it would be the first successful mission to return asteroid samples to Earth, JAXA said. The 2001 NASA probe of the asteroid Eros did not collect surface samples.
JAXA hopes that examining asteroid samples will help unlock the secrets of how celestial bodies formed, because their surfaces are be largely unchanged over the eons, unlike those of larger bodies such as planets or moons.