The Venus Climate Orbiter is also known as Akatsuki and Planet-C. Why is a Venus spacecraft called Planet-C? Venus is the second planet from the Sun, but C is the third letter of the alphabet.

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    $\begingroup$ "Venus is the second planet from the Sun" OR IS IT?! (it is) $\endgroup$
    – Schwern
    Mar 7, 2021 at 20:28

2 Answers 2


It's the third in a series.

Looks like there's a Planet-D (PDF link) planned for visiting Venus, too.

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    $\begingroup$ And there are also the MUSES, SOLAR, EXOS and ASTRO series counting by letters. $\endgroup$
    – asdfex
    Mar 7, 2021 at 9:59

An ISAS mission in development is known by a series and a letter. After successful launch, it gets a name, based on suggestions from its developers. One I worked on, ASTRO-E, unsuccessfully launched, was rebuilt as ASTRO-E2. Once successfully launched, it was named "Suzaku", after a mythical bird that is reborn in some stories.

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    $\begingroup$ Is ISAS the Institute of Space and Astronautical Science? isas.jaxa.jp/en $\endgroup$ Mar 7, 2021 at 17:00
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    $\begingroup$ @OrganicMarble Yes. A somewhat odd translation of 宇宙科学研究所, which is more literally "space science research institute". $\endgroup$
    – John Doty
    Mar 7, 2021 at 17:42

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