Many of NASA's space projects are the top choices separately made by formal surveys of the astronomy/astrophysics, Sun and heliophysics, and planetary science communities. These three surveys are made every 10 years.
NASA's budget is suggested by the President's Office of Management and Budget each year. The U.S. Congress allocates money to the various items in the budget. Congress often modifies amounts and may add or delete funds for different projects.
Usually there are projects in preparation and/or in flight for studying at least some of the planets in the Solar System, the Sun and space weather, and stars, galaxies, and the universe. Even though a project may have been designed for a particular research area, crossover use of spacecraft in flight is common:
- The Hubble Space Telescope that was built for studying objects beyond the Solar System is also used for studying the planets, comets, asteroids, and the Moon.
- The Cassini orbiter (at Saturn) will soon be observing a transit of Venus as a "laboratory standard" for exoplanet studies like those conducted by the Spitzer Space Telescope. Deep Impact is a comet project that has been searching for exoplanet transits like those studied by Kepler.
- The Solar Dynamics Observatory was used to study the asteroid ("rock comet") 3200 Phaeton at perihelion. The Solar and Heliospheric Observatory regularly observes comets near the Sun.
NASA gauges importance based on science community recommendations.
There were several proposals for missions to Pluto. The earliest I know of were in the plans for the Grand Tour, developed in the 1960s using the TOPS spacecraft design. The Grand Tour would have permitted a gravity assist trajectory from Jupiter-Saturn-Uranus-Neptune-Pluto.
The history next is a little muddy. Four large outer planet spacecraft were planned, under the name Viking, that were to be launched on two Saturn V rockets. Pluto was included in the mission plans, with Jupiter, Saturn, Uranus, and Neptune. I believe the proposed trajectories were Jupiter-Saturn-Pluto and Jupiter-Uranus-Neptune. (In that same time period, large Mars landers called Voyagers were going to be launched on Saturn Vs too. They were down-sized and renamed Viking 1 and 2 and launched on smaller boosters. Apparently names ended up being swapped, as you'll read in the next paragraph.)
The Viking effort to the outer planets evolved into the smaller Voyager 1 and Voyager 2 spacecraft which were launched individually on smaller Titan IIIC rockets. They had trajectories called JST (Jupiter-Saturn-Titan) for Voyager 1 and JSX (Jupiter-Saturn-To be Determined) for Voyager 2, assuming they lasted longer than their primary missions to Jupiter and Saturn. VGR2 was a back-up to VGR1 and would have flown its own JST if VGR1 had not completed its mission or if Titan had been more interesting than was observed at the time (Cassini has shown us that it is MUCH MORE interesting than anyone dreamed). Titan's hazy atmosphere limited what could be done so VGR2 could have gone to Pluto or to Uranus and Neptune. The choice went to Uranus and Neptune, though an acquaintance of mine did run the trajectory long after the decision and found that VGR2 would have had to burrow through Neptune to reach Pluto.
For a more detailed account of more recent efforts leading up to New Horizons, I refer you to http://www.planetary.org/explore/topics/new_horizons/road_to_nh.html. It will recount a little of what I've described and much more about Pluto missions that were proposed but didn't get far along in development.
I cannot cite specific examples of New Horizons technology being used by subsequent missions. What I can tell you, though, is that engineers will always use "heritage" technology if it will do the necessary job. Using heritage technology saves a lot of money in development and testing, compared to brand new technology. You can be sure that New Horizons will provide heritage for instruments or engineering subsystems for future missions.
he Voyager spacecraft had several technology "firsts" that have been used on subsequent missions to the outer planets, such as the Galileo mission to Jupiter and the Cassini mission to Saturn. The most significant hardware design issue was making the electronics tolerant to the Jupiter/Jovian radiation environment. Voyager was the first spacecraft to use radiation hardened parts, protecting the electronics from damage caused by charged particles. Voyager was the first spacecraft to have a programmable command computer, which could be modified in flight and be updated as the mission went on. This is the standard design for spacecraft today. Voyager was also the first planetary spacecraft to employ Xband as a downlink frequency. Planetary exploration has depended upon X-band ever since. Finally, Voyager made extensive use of the systems engineering discipline of "fault protection" which was conceived during the Voyager development. Today fault protection is used on all missions.
Suzanne Dodd, Voyager Project Manager, Tom Gavin, Chris Jones, JPL Associate Director for Flight Projects and Mission Success, Matt Landano. And Joseph Savino
"The Voyager Interstellar Message Committee, which included astronomers, writers and artists, was trying to paint as full a portrait of life on Earth as possible -- every part of the record, the music made by crickets, whales and humans, the pictures, the sounds -- each part was chosen to add some additional information about who we are. So, young and old, cultures of the east, west, south and north, ancient and modern, night and day, and so forth, all of it is represented by some element of the Voyager Message.
Space is mostly empty so the possibility that either Voyager will collide with another world and be discovered there is tiny. More likely, (but still a very small possibility), is that space-faring extra-terrestrials (should they exist) will detect and intercept one of the spacecraft. Now, if you are smart enough to develop the means for space travel, then, you are smart enough to figure out our message."
Ann Druyan, Creative Director, Voyager Interstellar Message Project
Carl Sagan's Co-Writer and Widow
Voyager detects individual atoms that don't reveal anything directly about life. But they do tell us about the origin of the building blocks of life.
Ed Stone, Voyager Project Scientist and Merav Opher, Voyager Guest Investigator Boston University