Japan's Hayabusa probe returns to Earth on Sunday, releasing a capsule into the atmosphere that scientists hope will contain samples from asteroid Itokawa.
If the 40cm-wide conical disc survives the blistering heat of the descent and can deploy a parachute, it should touch down in Australia at about 1400 GMT.
No-one really knows whether Hayabusa succeeded in grabbing samples from Itokawa's surface during a 2005 visit.
Its capture mechanism did not work as designed, analysis has shown.
However, Japanese space agency (Jaxa) officials remain confident.
They believe a lot of dust would have been kicked up when Hayabusa landed on the space rock, and some of this must have found its way inside the probe.
"At the moment of touch-down, Hayabusa collided with the surface and stayed on the surface for 30 minutes, so we believe some material will have found its way into the sample container," senior project team member Professor Hitoshi Kuninaka told BBC News.
If the Hayabusa capsule is confirmed to contain even just dusty fragments from the asteroid, it would represent a scientific first and a remarkable triumph for the Japanese who have had to battle a succession of technical problems.
