Vision and Voyages for Planetary Science in the - Solar System ...

As discussed in more detail below, the Discovery program remains vibrant and highly valuable,
allowing the science community to propose a diverse range of low-cost missions with short development
times and focused science objectives.
The New Frontiers program fills the middle ground between the small and relatively inexpensive
Discovery missions and the much larger and more costly Flagship missions. Inspired by the success of
the Discovery program, New Frontiers missions are also selected via a competitive process and led by a
principal investigator (PI). In contrast to the Discovery program, New Frontiers solicitations are more
strategic, restricting proposals to a small number of specific mission goals. New Frontiers missions
address focused science goals that cannot be implemented within the Discovery cost cap, but do not
require the resources of a Flagship mission.
Flagship missions are more expensive than the New Frontiers cost cap, and can cost up to several
billion dollars. They are strategic in nature and are designed to address a wide range of important
scientific objectives at high-priority targets. They often involve multi-agency and international
cooperation. Because of their scientific breadth and high cost they are not PI-led, but typically carry a
large and sophisticated payload of instruments, most of which are led by individual PIs. Some also carry
“facility” instruments that typically are provided by the institution that builds the spacecraft. Despite their
high costs, flagship missions are consistently characterized by high science return per dollar.
BALANCE AMONG MISSION COST CLASSES
The issue of finding the optimum balance among small, medium and large missions has been
addressed in a recent NRC/SSB study. 4 The report of a subsequent NRC workshop touched on balance in
the context of the decadal survey process: “[The discussion] reinforced the concept of the decadal survey
as a strategic package. Decadal studies need to provide the best balance of scientific priorities and
prioritized missions.” 5
The challenge is to assemble a portfolio of missions that achieves a regular tempo of solar system
exploration and a level of investigation appropriate for each target object. For example, a program
consisting of only Flagship missions once per decade may result in long stretches of relatively little new
data being generated, leading to a stagnant community. Conversely, a portfolio of only Discovery-class
missions would be incapable of addressing important scientific challenges like in-depth exploration of the
outer planets.
Mission classes are differentiated not only by their costs but also by the time scale of execution,
span of technology, and involvement of the scientific community. Flagship missions like Viking, Galileo,
and Cassini ordinarily have a ~10-year development cycle. They require very capable launch vehicles.
They involve large teams of investigators and a complex of supporting institutions. Each Flagship
mission is unique in terms of its scientific objectives and frequently also in terms of the spacecraft used,
so each is often a new development with little use of heritage hardware.
New Frontiers missions, while still complex and challenging, can be executed on timescales
significantly less than a decade. These missions have less extensive, more focused scientific objectives
than Flagship missions and typically take advantage of technological developments from recent prior
missions. The institutional arrangements are less complex and the launch vehicle requirements less
demanding.
Discovery missions can respond rapidly to new discoveries and changes in scientific priorities.
Rapid mission development (~3 years) is feasible, providing opportunities for student participation, rapid
infusion and demonstration of technology and a rapid cadence of missions pursuing scientific goals.
These missions are executable using relatively small launch vehicles.
In studying any given object, there is a natural progression of mission types, from flyby to orbital
investigation to in situ exploration to sample return. The missions early in this progression are generally
simpler and less costly than the later ones. So, because the long-term goals of planetary science involve
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