Sol Alan Stern (born November 22, 1957) is an American engineer, planetary scientist and space tourist.

[14] In December 2008, Stern joined Blue Origin, a company that was founded by Amazon.com's Jeff Bezos as an independent representative for research and education Missions.

Stern is a principal investigator (PI) in NASA's UV sounding rocket program, and was the project scientist on a Shuttle-deployable SPARTAN astronomical satellite.

[18] He is the PI of the SWUIS ultraviolet imager, which has flown two Shuttle missions, and the SWUIS-A airborne astronomical facility.

They use the camera to search for a hypothetical group of asteroids (Vulcanoids) between the orbit of Mercury and the Sun that are so elusive and hard to see that scientists are not sure they exist.

In this position Stern directed a US$4.4 billion organization with 93 separate flight missions and a program of over 3,000 research grants.

During his tenure a record 10 major new flight projects were started and deep reforms of the research and also the education and public outreach programs were put in place.

[35][38] Stern left to avoid cutting healthy programs and basic research in order to cover cost overruns.

Griffin favored cutting "less popular parts" of the budget, including basic research, and Stern's refusal to do so led to his resignation.

I am quite comfortable with my decision to leave, rather than eviscerate innocent SMD missions that should have proceeded apace...On November 23, 2008, in an op-ed in The New York Times, Stern criticized NASA's inability to keep its spending under control.

Stern said that, during his own time at NASA, "when I articulated this problem... and consistently curtailed cost increases, I found myself eventually admonished and then neutered by still higher ups, precipitating my resignation earlier this year."

"[40] Since leaving NASA, Stern has made criticisms of the budgetary process and has advocated for revamping its public appeal.

After the IAU's decision was made he was quoted as saying "It's an awful definition; it's sloppy science and it would never pass peer review" and claimed that Earth, Mars, Jupiter and Neptune have not fully cleared their orbital zones and has stated in his capacity as PI of the New Horizons project that "The New Horizons project [...] will not recognize the IAU's planet definition resolution of August 24, 2006.

Some large satellites are of similar size or larger than the planet Mercury, e.g. Jupiter's Galilean moons and Titan.