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An artist's impression of Hayabusa as it circled the asteroid Itokawa.

An artist's impression of Hayabusa as it circled the asteroid Itokawa.


Asteroid paper to be retracted because of faulty analysis

TOKYO—The Japan Aerospace Exploration Agency (JAXA) announced on Friday that it is asking Science to withdraw one of the 2006 papers that resulted from the Hayabusa asteroid sample return mission because of an error in the data analysis. The retraction won't affect scientists' understanding of the asteroid, however, because other papers have confirmed the study's key conclusions.

The Japanese-led team published a collection of seven papers in a special issue of Science on 2 June 2006 based on observations by four instruments as the Hayabusa spacecraft circled asteroid Itokawa in the fall of 2005. The craft later touched down to grab samples. The paper being retracted, by Tatsuaki Okada and colleagues, presents an analysis of x-ray spectra to determine the elements on the asteroid's surface. The authors concluded "that Itokawa has a composition consistent with that of ordinary chondrites." Chondrites are a type of stony asteroid.

For various reasons, the authors felt they could not rely on the calibration of the instrument done on Earth before the spacecraft was launched. To compensate, they started by assuming they would see the characteristic x-ray spectra of magnesium and silicon, elements known to be present on ordinary chondrites. They then used what they took to be the spectra of those elements to interpret the instrument's raw data. In effect, the authors jumped to a conclusion and then based their analysis on what they expected to observe. (Explanatory materials, in Japanese, are here.)

The assumption "was a big mistake for scientists, but as humans sometimes we make mistakes," says Masaki Fujimoto, who heads the solar system science division of JAXA's Institute of Space and Astronautical Science in Sagamihara. Fujimoto, who was not among the paper's authors, says the faulty analysis voids the paper's conclusion. Ironically, he says, an analysis of the dust returned to Earth later revealed that Itokawa actually is an ordinary chondrite, as was reported in a second collection of six Hayabusa-related reports published in Science on 26 August 2011.

The error was found during a routine review of the Hayabusa project results looking for lessons to apply to future missions. Investigators concluded there was no research misconduct involved. The authors are now going back over the raw data captured by the spectrometer to see if there would be merit in a new analysis.

The Hayabusa mission, launched in May 2003, has resulted in 129 peer-reviewed papers, 14 of which appeared in Science. The Hayabusa ground crew relied on ingenuity and a bit of luck to overcome damaged solar panels, engine failures, fuel leaks, and the complete loss of communications for several months during the craft's 6-billion-kilometer, 7-year journey. The sample capsule, with its precious cargo, landed in the Australian outback in June 2010. A follow-on mission, Hayabusa 2, is on track to launch in the coming winter.