Empty or not, Hayabusa has whetted scientists’ appetites for more sorties to near-Earth asteroids. Credit: JAXA

Tempering curiosity with caution, Japanese planetary scientists and their US colleagues are carefully preparing to open the recovery capsule from the Hayabusa spacecraft, which returned to Earth on 13 June following a spectacular high-speed re-entry over the Australian outback. If they're lucky, they may find a speck or two of material from asteroid 25143 Itokawa that could hold clues to the early days of the Solar System. But even if Hayabusa has returned empty, it will have given a boost to future asteroid missions.

Last week, the outer chamber of the 40-centi­metre-diameter aluminium capsule revealed traces of a gas that researchers say could have come from the 535-metre-long near-Earth asteroid on which Hayabusa twice touched down in November 2005. Yet the gas may simply be a bit of Earth's own atmosphere that sneaked in after the capsule landed — or it may have been emitted by the spacecraft itself.

X-rays of the inner compartment, which is still tightly closed, have shown that it hides no grains larger than 1 millimetre. This is not entirely surprising. A proof-of-concept mission, Hayabusa was designed to fire a metal projectile at the asteroid's surface to kick up fragments of rock and dust for the sample-retrieval apparatus to capture. Technical glitches prevented the projectile from firing, leaving mission scientists to hope that some asteroid dust might still have found its way into Hayabusa. "Even a small grain would provide a lot," says Paul Abell, a planetary scientist at NASA's Johnson Space Center in Houston, Texas, who collaborated on the mission.

In fact, Hayabusa has already paid dividends, Abell says. When the craft reached Itokawa, it found, rather than a coherent, solid body, a boulder-strewn 'rubble pile'. "It was one of the most heavily studied asteroids, and yet when we got there, we were very surprised," says Abell.

The mission has also inspired future efforts. "They have clearly shown us the path forward to the inevitable large-scale exploration of the near-Earth asteroid population," says Dante Lauretta, a researcher at the University of Arizona's Lunar and Planetary Laboratory in Tucson.

Japan's Aerospace Exploration Agency (JAXA) is already planning a successor mission — Hayabusa-2 — to 1999 JU3, a carbon-rich asteroid whose composition is expected to match the primitive Solar System more closely than does Itokawa's. It may also contain organic molecules that could shed light on the origins of life on Earth. Because the asteroid is an order of magnitude more massive than Itokawa, with a stronger gravitational pull, the craft will need more power­ful versions of the ion thrusters with which Hayabusa manoeuvred around Itokawa.

Design and planning for Hayabusa-2 are essentially complete, says JAXA's Hajime Yano, who will coordinate the scientific planning. But funding remains an obstacle. For the current fiscal year, the project received only ¥30 million (US$335,000) of its requested ¥1.7 billion. But on 15 June, amid excitement at the successful re-entry of the Hayabusa recovery capsule, Japan's science and technology minister promised to increase the budget. Yano hopes for a 2014 launch, which would require the estimated ¥25-billion budget to be allocated by next April. On that schedule, samples from 1999 JU3 could be in scientists' hands by 2020.

Interest in asteroid missions has also been on the rise in the United States. In June, US President Barack Obama called for a manned trip to an asteroid. And late last year, the OSIRIS-REx project, also a sample-return mission targeting a carbon-rich asteroid, was selected as a finalist in NASA's New Frontiers Program, which would provide US$650 million. Deputy principal investigator Lauretta hopes for a 2016 launch, a rendezvous with asteroid 1999 RQ36 in November 2019, and samples returned by September 2023.

Lauretta says the OSIRIS-REx team has studied the Hayabusa mission "in excruciating detail" and will extend operations at the asteroid to some 15 months compared with Hayabusa's 6 weeks. Hayabusa's troubles, he says, taught a clear lesson: "Take the time to thoroughly study the asteroid and safely design the sampling manoeuvre."

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