This weekend a Japanese spacecraft is due to return to Earth, possibly carrying the first sample taken directly from the surface of an asteroid.

The Japan Aerospace Exploration Agency (JAXA) launched the craft—dubbed Hayabusa, or "falcon" in Japanese—in 2003. The probe rendezvoused with the small near-Earth asteroid Itokawa in 2005, where it lingered for several months before heading back toward home.

After a round-trip journey of four billion miles (six billion kilometers), Hayabusa is expected to land in Australia around midnight on June 13 (10:30 a.m. ET on June 12).

Despite a few glitches along the way—including problems with the instruments designed to take samples—it's possible Hayabusa managed to collect a small amount of asteroid dust and gas.

Hayabusa's sample container was designed to hold up to ten milligrams of material, but "we'll be very happy if we get a cubic millimeter back," said team member Trevor Ireland, a planetary scientist at the Australian National University (ANU) in Canberra.

"That's much smaller than a match head ... but we can pretty much tell what's going on from that amount of material." (Related: "Water Discovered on an Asteroid—A First.")

Scientists won't know for sure whether they'll be any precious cargo to study until the container is retrieved, Ireland said.

"We're literally in the dark about whether we've got something there to analyze or not."

Hayabusa's Return Will Make Twin Fireballs

With its final course adjustments now complete, Hayabusa principal investigator Junichiro Kawaguchi said things are proceeding smoothly.

"There are currently no problems. ... Everything is on course," Kawaguchi said.

If all continues to go as planned, Hayabusa's sample return capsule will separate from the main body when the spacecraft is about 25,000 miles (40,000 kilometers) from Earth.

Both objects are then expected to enter the atmosphere above a sparsely populated military-testing zone in the South Australia (map) outback known as the Woomera Prohibited Area.

Roughly the size of England, Woomera "is a fairly biggish sort of landing target, but [JAXA] is narrowing it down to about 20 or 30 square kilometers [7.7 to 11.5 square miles]" within that zone, ANU's Ireland said.

As Hayabusa and its return capsule streak through the upper atmosphere, they will appear as twin, incandescent fireballs visible for about a minute to anyone within roughly 100 to 200 miles (200 to 300 kilometers) of the reentry point. The spacecraft's trajectory, however, won't take it over any heavily populated areas.

Hayabusa's refrigerator-size main body will most likely be incinerated during reentry. But the 16-inch (40-centimeter) return capsule is equipped with a high-tech heat shield and a parachute to help it land safely.

About an hour after touchdown, a helicopter carrying JAXA team members will try to locate the capsule from the air and record its GPS coordinates. Due to safety reasons, the return capsule will not be recovered until dawn the next day.

At that point, the helicopter will return to the site with a retrieval party, which will include Hayabusa team members as well as Aboriginal land owners.

"Native [Aboriginal] title holders will be among the first to go out and see the landing site," Ireland said.

Within 24 hours of its retrieval, the capsule will be flown to a specially-built JAXA facility in Japan to be opened sometime the following week.

"We're not really set up in Australia to ... minimize the risk of contamination and maximize the returns" of the capsule opening, Ireland said.

First "Pure" Asteroid Sample?

The sample-return part of Hayabusa's mission is also full of suspense. During the course of its seven-year trip, the spacecraft has suffered a string of mishaps.

For instance, en route to the asteroid Itokawa, the craft's solar cells were damaged by a powerful solar flare, limiting its energy supply and prolonging the time it took to reach the asteroid. Once "parked" near the asteroid, an altitude adjustment error sent the craft's exploration robot MINERVA tumbling into space.

What's more, during one of Hayabusa's few successful landings on Itokawa, a metal-projectile system crucial for collecting asteroid samples failed to deploy.

However, scientists think it's possible some gas and dust may have been funneled into the spacecraft's collection chamber during its landing. (Find out what Hayabusa has already revealed about Itokawa's "shaking rubble pile" makeup.)

And if there's anything inside the capsule, Hayabusa's cargo could give scientists a first glimpse into what asteroids are really like. The findings could offer new insight into the formative years of our solar system, scientists say, because asteroids are the rocky leftover building blocks of planets and moons.

"We don't currently have any asteroid samples," Ireland said.

Technically, what scientists have now are meteorites—usually small asteroid chunks—that have survived the passage through Earth's atmosphere but have been irrevocably scarred in the process.

"Their original surfaces are pretty much lost," Ireland said. "This will be the first time we can actually look at the surface of an asteroid and look for things such as space weathering, and how the solar wind interacts with rocky material, and so forth."

A preliminary group of international scientists, including Ireland, has already been selected to perform the initial examination of any "pure" asteroid sample inside the capsule.

"All of the inspection initially will be done in Japan," Ireland said.

If there is enough of the sample to go around, in about a year JAXA will distribute some of the precious bits of space rock to other research centers around the world.

Building a Better Heat Shield

Even if Hayabusa's sample container comes up empty, the mission could still be a success: The state of the capsule after reentry could lead to engineering advances that will benefit future space missions, said Peter Jenniskens of the NASA Ames Research Center in California.

Although not officially part of the Hayabusa team, Jenniskens and colleagues will be watching the reentry from aboard a modified NASA DC-8 research airplane flying at an altitude of about 39,000 feet (about 12,000 meters).

"We hope to have a really good view of the reentry," Jenniskens said. "The aircraft allows us to be above the weather. ... If you observe from the ground, you have a lot of dust and water vapor between you and the capsule." (Related pictures: "NASA Guts 747, Adds 17-Ton Telescope.")

The aircraft will be about 125 miles (200 kilometers) from the Hayabusa fireballs and thus in no danger of being struck.

"We're positioning the aircraft so that the capsule is coming more or less toward us," Jenniskens said. This will give the plane's instruments a good view of the capsule's heat shield, which is expected to pulsate between red, orange, and yellow during reentry.

The team will later use the exact colors to estimate the temperatures the capsule experienced during different points of the reentry process and to evaluate the heat shield's performance.

NASA's research plane will also play an important role in the capsule-retrieval process.

"We are providing eyes for JAXA in case there is [cloud] cover," Jenniskens said. "We'll be able to confirm the reentry path ... and help determine how fast the capsule penetrated into the atmosphere and where the endpoint of the trajectory is expected to be."

ANU's Ireland added that, even after seven years of anticipation, he and his colleagues will wait however long it takes to find out what's inside the capsule.