to attention Pluto as a viable target for exploration. In 1990, because of pressure from the scientific community, including those of the Pluto Underground, engineers at NASA decided to look into concepts for a mission to Pluto. At the time, it was thought that the atmosphere of Pluto would freeze and fall to the surface during winter, and so a lightweight spacecraft was desirable, as it would be able to reach Pluto before such an event would occur. One of the earliest concepts was for a 40-kilogram spacecraft that would reach Pluto in five to six years. The idea was shortly scrapped, however, because of the infeasibility of miniaturizing scientific instruments aboard such a spacecraft to that size. Another mission concept, known as Pluto 350, was developed by Robert Farquhar of the Goddard Space Flight Center, with Alan Stern and Fran Bagenal of the Pluto Underground, who both served as study scientists for the project. Pluto 350 aimed to send a spacecraft, weighing 350 kilograms, to Pluto. The spacecraft's minimalistic design was to allow it to travel faster and be more cost-effective, in contrast to most other big-budget projects NASA were developing at the time, such as Galileo and Cassini. Pluto 350, however, would later become controversial among mission planners at NASA, who considered the project to be too small and too high-risk. An alternative plan which was considered at one point was to send to Pluto a configuration of the Mariner Mark II spacecraft, which would weigh 2,000 kilograms and cost US$3.2 billion, in sharp contrast to Pluto 350's $543 million cost. While both projects competed for approval, Pluto 350 was more favored by NASA mission planners, who were starting to adopt smaller missions such as Mars Pathfinder and NEAR Shoemaker. Pluto postage stamp In October 1991, the United States Postal Service released a series of stamps commemorating NASA's exploration of the Solar System. The series featured a stamp for all planets, displaying an image of the planet and highlighting an associated spacecraft which was sent to it. The stamp for Pluto, however, depicted a featureless sphere, presented with the phrase "not yet explored" in place of the name of a spacecraft. The stamps were unveiled in a ceremony at the Jet Propulsion Laboratory. Two scientists who attended the event, World Space Foundation president Robert Staehle] and JPL scientist Stacy Weinstein, were inspired by Pluto's status on the stamp,