The committee’s task includes addressing the proper balance between new and extended missions. NASA’s Science Mission Directorate (SMD) is currently operating approximately 60 science missions, of which approximately three-fourths are in their extended mission phase and one-fourth in their prime phase. This complementary arrangement has proved effective in enabling all four mission divisions to achieve scientific goals that could not have been reached with either primary or extended missions alone.
An example of a scientific goal that could only have been reached with both prime and extended missions concerns the magnetized plasmas that fill near-Earth space and produce long-range interactions that can be understood only by taking measurements at widely distributed observing points and continuing to monitor them over decades. By extending missions beyond their prime lifetime and adding additional spacecraft every few years, NASA’s Heliophysics Division has created what is referred to as the Heliophysics System Observatory (HSO), a network of spacecraft that monitors the entire heliosphere with a special emphasis on a volume of space with a radius 200 times that of Earth’s orbit. In 2016, the HSO, which includes the STEREO (Solar Terrestrial Relations Observatory) spacecraft in the same orbit as Earth and the two Voyager spacecraft more than 100 astronomical units from the Sun, comprised 18 missions (28 spacecraft). Only one mission, the four-spacecraft Magnetospheric Multiscale mission, is in prime phase (see Figure 4.1). Thus, extended missions are an essential component of the ensemble of HSO spacecraft that is monitoring the interconnected system of the solar wind and Earth as well as the outer boundary of the heliosphere. The importance of the HSO is acknowledged in the first research recommendation of the 2013 heliophysics decadal survey (NRC, 2013), which calls for continued support of the complement of spacecraft it comprises.
Other divisions have equally compelling reasons to extend the operation of missions beyond their prime phases. For example, the Cassini mission of the Planetary Science Division has gathered extensive data on Saturn’s small moon, Enceladus, during its extended phases. Only during the extended operations were the properties of the vapor plumes of this small moon established, and in addition, it was shown that Enceladus likely harbors a global-scale ocean beneath its icy surface. Data collected during the mission’s extensions also revealed that the puzzling periodicities of electromagnetic phenomena at Saturn vary in frequency with season. By operating missions into their extended phases, missions in the Earth Sciences Division have monitored the retreat of the Antarctic ice shelf and established the temporal variation of atmospheric gases and other key elements of the coupled atmosphere-ocean system. Astrophysics has also benefitted from missions in their extended phase, including new discoveries made by the Kepler, Spitzer, and Chandra observatories.
Extended missions require resources, which naturally raises the question of how much SMD resources should be allocated for this purpose and whether typical expenditures are the proper amount. The most recent budget figures indicate that SMD is spending approximately 12 percent of its budget on extended missions. NASA officials stated to the committee that, although the fraction of funding going to operating missions in extended phase has fluctuated over time, it has, on average, remained close to the present 12 percent. As demonstrated in Chapter 2 of this report, major scientific discoveries have been made by NASA missions in extended phase. This record of scientific productivity leads the committee to conclude that continuing most NASA missions into extended phase is justified.
Missions in prime or extended phase also utilize communications support including the DSN (Deep Space Network) and NEN (Near Earth Network), which may be stressed by the number of spacecraft requiring their services. As such, the total number of missions and their locations in the sky impact the support infrastructure (although the impact cannot be quantified without a detailed evaluation of mission-specific needs).
Typically for space science missions in different divisions, maintaining balance among small, medium, and large missions, and including a diversity of targets, have been identified as important goals. “Lack of balance” has been generally understood by the scientific community to mean too much emphasis on either a single bandwidth or target (e.g., measurements in a specific range of frequencies or measurements at a particular planet) or support of one costly space mission at the expense of all others. The committee is unaware of any published evaluation of what constitutes the “proper” balance between new and extended phase missions, other than the 2005 National Research Council report Extending the Effective Lifetimes of Earth Observing Research Missions (NRC, 2005). The various decadal surveys consistently have stressed the importance of missions in extended phase, but they have not specifically addressed the balance between extended phase missions and new ones, or even sought to define a desirable balance (see Appendix D).
Extended missions provide a suitable training ground for students and early-career scientists. For graduate students, the predictability of data sources and operations, particularly with respect to the timeline for completing
thesis research, is invaluable and far preferable to delaying graduation or completing a changed project if a prime mission’s launch is delayed or, in a worst case, lost. For other early- or mid-career scientists, the experiences gained in an environment conducive to learning on the job provide valuable payback to the enterprise in the form of much more experienced personnel to perform in the pressure cooker of mission formulation and development. Thus, a robust portfolio of extended missions helps to provide the workforce for future new missions.
The committee considered the issue of appropriate balance between prime and extended phase missions, initially seeking to identify how much NASA currently spends on prime and extended missions in each division. A key question the committee considered was the approximate buying power of the funds that support mission extensions—in other words, if a division canceled all of its extended missions and spent all of that money on new missions, how many new missions could it buy? More specifically, the data show that if the Astrophysics Division canceled and turned off all of its missions currently in extended phase—Hubble, Chandra, Spitzer, NuSTAR, and so on—it could purchase less than one MIDEX (Medium-Class Explorer) mission per year, or approximately one additional flagship mission every decade. Of course, this would come at tremendous cost in scientific productivity—ending data return from eight operating missions in return for adding perhaps two new medium-sized missions every 3 years.
The calculation for the Earth Science Division indicated greater adverse impact: ending all Earth science missions in extended phase—such as Aura, Terra, Aqua—could release funding for approximately one new Earth Systems Science Pathfinder mission every 2-plus years, or one new flagship class mission every 12 years. For the Heliophysics Division, the effects were also disproportionate: ending all current extended missions could provide funds for approximately one new MIDEX mission every 4 to 5 years, or two new Small Explorers (SMEX) every 3 years, or a new flagship class mission every 19 years. The scientific loss to heliophysics, however, would be tremendous. The Heliophysics System Observatory, which relies upon multiple observations at multiple locations, would simply collapse.
The results for the Planetary Sciences Division are similar: canceling all operating extended phase missions—Curiosity, Opportunity, Lunar Reconnaissance Orbiter, Mars Reconnaissance Orbiter, MAVEN, Cassini, and even New Horizons, which will finish its prime phase soon—would result in approximately one new Discovery mission every 2-plus years, or one new flagship class mission every decade (see Table 4.1).
|Division||Total Budget for Fiscal Year 2016 ($millions)a||Approximate Savings ($milions) If All Extended Missions Are Eliminated||Equivalent Number of New Small Science Missions per Yearb||Equivalent Number of New Large Science Missions per Yearc|
|Astrophysics||768 (+JWST: 620)||214||~ 0.6 MIDEX||~ 1/10 flagship mission|
|Earth Science||1,921||180||~ 0.4 ESS Pathfinderd||~1/12 flagship mission|
|Heliophysics||640||78||~ 0.2 MIDEX ~ 0.4 SMEX||~1/19 flagship missione|
|Planetary Science||1,628||216||~ 0.4 Discovery missions||~ 1/10 flagship mission|
NOTE: The table does not account for the normal spending profile for a mission that is not evenly distributed over each year.
a NASA, “NRC Extended Missions Follow up questions, SMD Responses,” submitted to the committee, April 5, 2016.
b The committee assumed launch costs of approximately $150 million. A MIDEX mission costs from approximately $330-$350 million total, including launch costs, and a Discovery mission costs approximately $575 million total, including launch costs. (http://explorers.gsfc.nasa.gov/missions.html and http://discovery.nasa.gov/p_mission.cfml, accessed May 5, 2016).
c This assumes that a typical flagship mission costs $2 billion and launch costs are approximately $250 million, for $2.250 billion total.
d For Earth Science, the committee took the cost of the most recent Earth Systems Science Pathfinder mission, the Orbiting Carbon Observatory 2, which cost approximately $470 million, including launch (http://www.jpl.nasa.gov/news/press_kits/oco2-launch-press-kit.pdf, accessed May 26, 2016).
e For heliophysics, the committee took the current $1.5 billion estimated cost (including launch) for the Solar Probe Plus mission.
Of course, it would be possible to cancel some but not all extended-phase science missions in a division. Criticism of continuing to fund extended science missions (see Chapter 1) is usually formulated as a proposal to spend an undefined “less” on extended missions and to devote the money saved to new mission development. But what Table 4.1 demonstrates is that even drastic cuts to the extended missions budgets would result in very few new science missions. Another way to look at this trade-off is that, because each of the divisions spends approximately 50 percent of its budget on new development and approximately 12 percent on extended missions, ending all extended missions in a division would increase the respective development budget by approximately 25 percent. Thus, even the drastic action of ending all extended missions has a relatively limited effect on both development spending and the number of new missions.
The cost to science of ending all extended science missions, however, would be catastrophic. In some cases, it could create gaps during which no new data are being returned from any mission for a division. Such breaks could destroy some scientific disciplines, particularly Earth science and heliophysics, which require understanding their subjects via multiple observations made by multiple spacecraft over many years. For planetary science, ending extended missions at Mars would not just impact science but could mean shutting off spacecraft that provide data relay for other spacecraft, thus eliminating infrastructure needed to support both prime and extended missions (see Figure 4.2). Astrophysics benefits by using multiple observatories—many in their extended phase—to take data at different wavelengths simultaneously to understand how many astrophysical systems work. Ending missions that have many productive years left would also be tremendously wasteful—the equivalent of throwing away a functioning appliance at the end of its warranty. Finally, eliminating all extended missions would contradict the recommendations in the divisions’ decadal surveys.
Of course, ending many or all extended missions is an extreme example, but it demonstrates the limitations of what can be accomplished even by making major changes to the current balance of spending on extended missions. Although the committee could not establish a clear definition of balance, it was able to conclude that substantial changes in the current balance between new and extended missions would be highly deleterious in terms of scientific return.
Finding: NASA’s extended science missions constitute approximately three-fourths of the missions in flight, but cost a relatively small percentage of the overall SMD budget, on average 12 percent over the last 5 years.
Finding: Eliminating all of the extended missions would
- Increase the funds available for new development only by approximately 25 percent;
- Make it difficult or impossible to achieve many objectives of decadal survey science; and
- Adversely and significantly impact SMD’s overall science return.
Finding: The current balance between prime and extended missions is reasonable.
Recommendation: NASA should continue to provide resources required to promote a balanced portfolio, including a vibrant program of extended missions.
Although the committee did not develop a formal definition or recipe for the ideal balance between prime and extended missions, it found the present mix to be excellent and identified no basis for substantially altering the current balance based upon either scientific or monetary considerations.
NRC (National Research Council). 2005. Extending the Effective Lifetimes of Earth Observing Research Missions. The National Academies Press, Washington, D.C.
NRC. 2013. Solar and Space Physics: A Science for a Technological Society. The National Academies Press, Washington, D.C.