Pioneer H is an unlaunched unmanned space mission that was part of the US Pioneer program for a planned 1974 launch. Had this mission and spacecraft been launched, it would have been designated Pioneer 12; that designation was later applied to the Pioneer Venus Orbiter.

The probe would have been constructed from flight-qualified spare components intended for the Pioneer F and G probes (designated Pioneer 10 and Pioneer 11 after launch) by NASA contractor TRW.

As planning for the Pioneer 10 and 11 missions progressed, mission scientists found themselves desiring a third probe. They proposed a 1974 launch, using a spacecraft assembled from the unused spare parts. The mission would take the craft to Jupiter, where it would use the gas giant as a gravitational slingshot to travel outside the ecliptic. However, NASA management did not approve the mission, and it was never launched. In 1976 NASA transferred the craft (without RTG) to the Smithsonian. The actual physical transfer occurred in January 1977. The planned US spacecraft for an International Solar Polar Mission (1979) was cancelled in 1981. In 1990, the Ulysses spacecraft was built by the European Space Agency with the instruments split between ESA and NASA for this Out-Of-The-Ecliptic (OOE) mission—first proposed by the Pioneer 10/11 principal investigators 20 years earlier.

Current location

The spacecraft that would have been Pioneer H hangs in the Milestones of Flight Gallery at the National Air and Space Museum in Washington, D.C., serving as a stand-in for the Pioneer 10 probe.

While described in official Smithsonian records as a "replica", the spacecraft was considered fully-functional by Pioneer mission planners. Mark Wolverton quotes James Van Allen in "The Depths of Space"::"We mounted an intensive campaign to launch the flight-worthy spare spacecraft and its instrument complement on a low-cost, out-of-ecliptic mission via a high-inclination flyby of Jupiter. However, our case fell on deaf ears at NASA headquarters, and the spare spacecraft now hangs in the main gallery of the National Air and Space Museum, at 1 AU and zero ecliptic latitude."

Sources

* Mark Wolverton. [http://www.nap.edu/books/0309090504/html/179.html The Depths of Space: The Story of the Pioneer Planetary Probes] . 2004
* [http://www.nasm.si.edu/exhibitions/gal100/pioneer.html National Air and Space Museum - Pioneer 10 - Milestones of Flight]
* [http://www.nasm.si.edu/research/dsh/artifacts/SS-pioneerX.htm Assembling for NASM the Pioneer 10 replica]
* [http://adsabs.harvard.edu/abs/1979muni.iafcQ....S International Solar Polar mission, 1979]