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Encouraged by their success with probes to investigate Halley’s comet, in the mid- 1980s Japanese scientists at ISAS started to plan their second step in solar system exploration. In addition to technological missions that would eventually lead to the Hayabusa asteroid sample-return, they also desired science-oriented missions such as orbiters to study the atmospheres of Venus and Mars. Although Venus would be easier to reach, and a spinning orbiter based on the successful Halley probes was designed, it was decided that Mars was a more worthy scientific target. Go-ahead for this Planet-B mission was authorised by the government in the early 1990s and detailed work started in 1992. Venus exploration was left to the Planet-C mission that was launched in 2010. Planet-A had been the Suisei probe sent to Halley’s comet.