Copyright © National Academy of Sciences. All rights reserved.
Solar and Space Physics: A Science for a Technological Society
RECOMMENDATIONS
95
BOX 4.7 A BRIEF HISTORY OF THE EXPLORER PROGRAM
The Explorer program is arguably the most storied scientific spaceflight program in NASA’s history. Since
1958, when the first U.S. satellite, Explorer I, discovered Earth’s radiation belts, the program has produced a
wealth of information about the nature of Earth’s space environment and properties of the universe. UHURU,
IMP, ISEE, DE, SAMPEX, ACE, TRACE, and COBE are some of the well-known missions that have yielded enormous
science return for the investment, often operating for years beyond their expected lifetimes. Science from
Explorer missions contributed to three of the Nobel Prizes awarded for NASA-directed space science. Many of
these missions have provided critical scientific measurements beyond their design lifetime to become cornerstones of the Heliophysics Systems Observatory, indispensable to basic research as well as to space weather
operations.
Examples of Explorers launched since 2000 that have made major contributions to scientific understanding pertinent to heliophysics are IMAGE, RHESSI, THEMIS, AIM, and IBEX. Each of these have made fundamental
discoveries spanning the full range of the discipline, from the edge of the heliosphere (IBEX) to flare and reconnection physics on the Sun (RHESSI) to the explosive releases of energy taking place in Earth’s magnetosphere
(THEMIS) to the enigmatic formation of ice clouds in Earth’s polar regions (AIM). In addition, the Explorer program is the home for Missions of Opportunity, including SNOE, CINDI, and TWINS, that produce science benefits
far beyond their cost. The Explorer program is a cornerstone of heliophysics and has repeatedly proven to be
one of the most cost-effective and best cost-controlled avenues for implementing space science missions.
concept studies to launch. This approach requires lean management teams, strong collaboration between
institutions and NASA centers, and continuous attention of the PI to the balance of risk and scientific reward.
The survey committee recommends that NASA accelerate and expand the Heliophysics Explorer program,
the most successful and impactful mission line in the Heliophysics program.
New Worlds, New Horizons in Astronomy and Astrophysics 9 highly recommended an increase in
the Astrophysics Explorer budget for many of the same reasons that the present survey committee does.
Augmentation of Explorer Line to Restore MIDEX
The rate of Explorer satellite development has slowed remarkably since the 2003 solar and space
physics decadal survey. 10 This decrease in selection rate is due to a major reduction in funding for the
Explorer program that occurred in 2004 rather than to any drop in the number of compelling proposals
for Explorer missions rated as selectable by NASA. The sharp funding cuts necessitated a reduction of
Explorer competitions for the SMEX class in order to preserve even a minimal overall selection cadence.
The MIDEX class historically has offered an opportunity to resolve the highest-level science questions (e.g.,
IMAGE addressed science that was originally identified for a larger solar-terrestrial probe), but this line
of Explorer competition has not been possible under the current Explorer budget, particularly given the
scarcity of medium-class launch vehicles and the cost of alternatives. A stable of competitively selected
9 National Research Council, New Worlds, New Horizons in Astronomy and Astrophysics, The National Academies Press, Washington, D.C., 2010.
10 National Research Council, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics, The
National Academies Press, Washington, D.C., 2003; and National Research Council, The Sun to the Earth—and Beyond: Panel
Reports, The National Academies Press, Washington, D.C., 2003.
Solar and Space Physics: A Science for a Technological Society
RECOMMENDATIONS
95
BOX 4.7 A BRIEF HISTORY OF THE EXPLORER PROGRAM
The Explorer program is arguably the most storied scientific spaceflight program in NASA’s history. Since
1958, when the first U.S. satellite, Explorer I, discovered Earth’s radiation belts, the program has produced a
wealth of information about the nature of Earth’s space environment and properties of the universe. UHURU,
IMP, ISEE, DE, SAMPEX, ACE, TRACE, and COBE are some of the well-known missions that have yielded enormous
science return for the investment, often operating for years beyond their expected lifetimes. Science from
Explorer missions contributed to three of the Nobel Prizes awarded for NASA-directed space science. Many of
these missions have provided critical scientific measurements beyond their design lifetime to become cornerstones of the Heliophysics Systems Observatory, indispensable to basic research as well as to space weather
operations.
Examples of Explorers launched since 2000 that have made major contributions to scientific understanding pertinent to heliophysics are IMAGE, RHESSI, THEMIS, AIM, and IBEX. Each of these have made fundamental
discoveries spanning the full range of the discipline, from the edge of the heliosphere (IBEX) to flare and reconnection physics on the Sun (RHESSI) to the explosive releases of energy taking place in Earth’s magnetosphere
(THEMIS) to the enigmatic formation of ice clouds in Earth’s polar regions (AIM). In addition, the Explorer program is the home for Missions of Opportunity, including SNOE, CINDI, and TWINS, that produce science benefits
far beyond their cost. The Explorer program is a cornerstone of heliophysics and has repeatedly proven to be
one of the most cost-effective and best cost-controlled avenues for implementing space science missions.
concept studies to launch. This approach requires lean management teams, strong collaboration between
institutions and NASA centers, and continuous attention of the PI to the balance of risk and scientific reward.
The survey committee recommends that NASA accelerate and expand the Heliophysics Explorer program,
the most successful and impactful mission line in the Heliophysics program.
New Worlds, New Horizons in Astronomy and Astrophysics 9 highly recommended an increase in
the Astrophysics Explorer budget for many of the same reasons that the present survey committee does.
Augmentation of Explorer Line to Restore MIDEX
The rate of Explorer satellite development has slowed remarkably since the 2003 solar and space
physics decadal survey. 10 This decrease in selection rate is due to a major reduction in funding for the
Explorer program that occurred in 2004 rather than to any drop in the number of compelling proposals
for Explorer missions rated as selectable by NASA. The sharp funding cuts necessitated a reduction of
Explorer competitions for the SMEX class in order to preserve even a minimal overall selection cadence.
The MIDEX class historically has offered an opportunity to resolve the highest-level science questions (e.g.,
IMAGE addressed science that was originally identified for a larger solar-terrestrial probe), but this line
of Explorer competition has not been possible under the current Explorer budget, particularly given the
scarcity of medium-class launch vehicles and the cost of alternatives. A stable of competitively selected
9 National Research Council, New Worlds, New Horizons in Astronomy and Astrophysics, The National Academies Press, Washington, D.C., 2010.
10 National Research Council, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics, The
National Academies Press, Washington, D.C., 2003; and National Research Council, The Sun to the Earth—and Beyond: Panel
Reports, The National Academies Press, Washington, D.C., 2003.