Monday, August 22, 2016

Selecting the Next New Frontiers Mission

NASA’s managers have begun the process for a competition to select a new planetary mission to launch in the mid-2020s that will address one of the most important questions in planetary science.  The winning proposal will be the fourth mission in the agency’s New Frontiers program that sent the New Horizons craft to Pluto, the Juno orbiter to Jupiter, and will launch the OSIRIS-REx mission next month to return a sample from a primitive asteroid. 

Previously selected New Frontiers missions.  Credit: NASA
NASA’s planetary missions fall into three categories of ambition and cost.  At the high end at around $2-2.5 billion are the Flagship missions that use highly capable spacecraft for exploration that addresses a wide range of questions at the target world.  These missions include the Curiosity Mars rover, its 2020 Mars rover sibling in development, and the planned Europa multi-flyby mission. 

At the low end, at around $600 million, are the Discovery missions that conduct highly focused missions.  Teams are free to propose missions to study any solar system body except the Sun and Earth (which are studied through other programs at NASA).  Ten of these planetary missions have flown successfully and have included the MESSENGER spacecraft that orbited Mercury and the DAWN spacecraft that currently orbits the asteroid Ceres.  Next up will be the 2018 InSight geophysical station for Mars to be followed by one or two missions to study either asteroids and/or Venus that will be selected by the end of the year.

At a total cost of somewhere around $1 billion, the New Frontiers missions fit between these two programs in ambition.  The goal for these missions are to address focused high priority science questions.  The scientific community selects the candidate themes through the Decadal Survey in which a long list of scientist-proposed ideas are vetted and prioritized. 

The next New Frontiers mission will be selected from among a list of the six mission themes that the planetary science community identified as as highest priority to answer key questions about our solar system:
  • Comet Surface Sample Return – Enable in-depth laboratory analysis of the most primitive material left form the formation of the solar system
  • Lunar South Pole-Aitken Basin Sample Return – Enable in-depth laboratory analysis of material from our moon to understand the how the bombardment of the inner solar system worlds by comets and asteroids effected their formation
  • Saturn Atmospheric Probe – Determine the composition of Saturn’s atmosphere to help us better understand the formation of the solar system
  • Trojan Asteroid Tour and Rendezvous – Explore a reservoir of remnant bodies from the formation of the solar system to understand how materials from different regions of the early solar system mixed during planetary formation
  • Venus In Situ Explorer – Understand the formation, evolution, and current state of the atmosphere and surface of our sister world that evolved into a hell
  • Ocean Worlds (Titan and/or Enceladus) – Do these two moons of Saturn have the conditions to support life and is life present?
The first five of these themes were selected through the Decadal Survey.  NASA’s managers added the Ocean Worlds theme in response to a Congressional directive and further discoveries by the Cassini mission.  For the next, fifth New Frontiers competition, Jupiter’s moon Io and a lunar geophysical network theme will be added.

NASA’s managers currently expect to select a New Frontiers mission from the list of themes approximately every five years.  At that pace, completing this series of investigations, including the new themes for the next selection, will require around forty years (assuming no changes to the list from future Decadal Surveys).  The pressure on each proposing team to have their proposal selected now rather than waiting decades must be intense.  (If this long time frame seems disheartening, some of the themes may be addressed by other space agencies.  A European team, for example, is proposing a Saturn atmospheric probe to the European Space Agency.  NASA Discovery missions may also partially address some of the themes.  Among the five proposals competing to be among the next Discovery missions are spacecraft that would address several of the atmospheric objectives of the Venus theme and address the Trojan theme through flybys rather than the proposed New Frontiers orbiter plus flybys approach.)

For some of the themes, the Decadal Survey listed (and NASA’s managers have adopted) very specific research goals.  Any team proposing a mission for the Venus in situ explorer, for example, must propose a probe that would descend through the atmosphere and likely land on the surface. Here are the objectives from the draft document announcing NASA’s request for proposals (called an Announcement of Opportunity (AO)):

The Venus In Situ Explorer mission theme is focused on examining the physics and chemistry of Venus’s atmosphere and crust by characterizing variables that cannot be measured from orbit, including the detailed composition of the lower atmosphere, and the elemental and mineralogical composition of surface materials. The science objectives (listed without priority) of this mission theme are:
  • Understand the physics and chemistry of Venus’s atmosphere through measurement of its composition, especially the abundances of sulfur, trace gases, light stable isotopes, and noble-gas isotopes;
  • Constrain the coupling of thermochemical, photochemical, and dynamical processes in Venus’s atmosphere and between the surface and atmosphere to understand radiative balance, climate, dynamics, and chemical cycles;
  • Understand the physics and chemistry of Venus’s crust;
  • Understand the properties of Venus’s atmosphere down to the surface and improve understanding of Venus’s zonal cloud-level winds;
  • Understand the weathering environment of the crust of Venus in the context of the dynamics of the atmosphere of Venus and the composition and texture of its surface materials; and
  • Search for evidence of past hydrological cycles, oceans, and life and constraints on the evolution of Venus’s atmosphere.

This is an ambitious list, and the AO specifically states that proposers can select, but must thoroughly justify their selection, a subset of these goals.

By contrast, the goals for the Trojan asteroid tour and rendezvous reflect the fact that we know very little about these never-visited bodies that share Jupiter’s orbit.  This population of small worlds represent fragments left over from the formation of the planets.  The diversity of the composition of these worlds will allow scientists to select from among competing models for how the solar system formed.  The requirements for this theme are short:

The Trojan Tour and Rendezvous mission theme is intended to examine two or more small bodies sharing the orbit of Jupiter, including one or more flybys followed by an extended rendezvous with a Trojan object. The science objective of this mission theme is:
  • ·    Visit, observe, and characterize multiple Trojan asteroids

The briefness of the requirements for the Trojan theme likely makes life harder for teams proposing a mission to these worlds.  In judging proposals, NASA’s review teams will score proposals on their scientific merit (~40% of score), the feasibility of the specific proposed instruments and measurements (~30%), and overall mission feasibility within the cost cap (~30%).  Scientific merit includes an explanation of the, “Compelling nature and scientific priority of the proposed investigation's science goals and objectives. This factor includes the clarity of the goals and objectives…  Teams proposing for Venus have the benefit of goals developed and specified by the Venus science community while teams proposing for the Trojans have to develop and defend their own list of specific science goals and objectives. 

(At the end of this post, I’ve copied the specific goals for the remaining mission themes from the AO.)

Missions proposed for the next New Frontiers program will need to meet many criteria including these:
  • Total cost for the development of the spacecraft, the instruments, and analysis of the returned data cannot exceed $850 million.  NASA will separately pay for the mission’s launch and operation costs while in flight (likely several tens of millions of dollars per year), which together probably will bring the total cost of the mission to $1 billion or more. 
  • Proposals can include instruments paid for by foreign governments, but the costs of these instruments cannot exceed one-third of the cost of the total instrument compliment.  As one NASA manager put it, NASA invests a great deal of money to develop instrument technologies by American scientists, and it wants to see a return on that investment by having the majority of instruments on the selected mission be American. 
  • Teams can propose the use of radioisotope heaters and radioisotope electrical power generators for their missions.  These units would be useful for missions operating far from the sun (for example, at Saturn).  However, a mission using these units would need to reserve a substantial portion of the core $850 million to cover the cost of these units.  Using just the heaters would incur a cost of $47-79 million (depending on the number) and the electrical power generators would cost $133-195 million (again based on the number of generators used).  These costs could drastically reduce the capabilities of the spacecraft and instruments compared to missions that don’t require these technologies.

I suspect that for many readers of this blog, a mission to return to Enceladus or Titan to continue their exploration with a new generation of spacecraft and instruments would be a personal favorite.  I share that desire, but also recognize the challenges any proposal to these worlds would face.  First, these worlds were just added to the list of candidate themes in the past few months.  The in-depth analysis of objectives for these missions is just getting underway by the scientific community.  Second, the technical maturity of instruments to explore their oceans, determine their habitability, and search for life may be low – NASA has not made major investments in these technologies for these worlds (but plans to begin to do so).  And third, these missions are likely to need radioisotope power generators and their cost would eat significantly into the mission budget, potentially making it less competitive.  (Solar powered missions are possible at Saturn, but appear to be on the edge technically.  This could make a proposal that depends on solar power appear technically risky.)  Balancing these negatives is a heritage of three Discovery-class proposals to these worlds that were not selected but which could form the basis of a New Frontiers-class mission.  Still, I personally doubt that a mission to these moons will be selected this time.  (If I am wrong, given a mid-2020’s launch and a flight that could last 10 years, it could be the mid-2030s before the spacecraft arrives at its target.)

I’ve learned to not try to predict which Discovery or New Frontiers mission is likely to be selected from the list of proposals made.  The scrutiny given these proposals is intense.  Any fault with the details of a proposal can rule it out.  If the review panel decides that a proposed key engineering manager doesn’t have sufficient experience, that could kill a proposal.  If the review panel concludes that a technology proposed to be used for the spacecraft or a key instrument lacks maturity, that could kill a proposal.  If the review panel concludes that the specific set of scientific objectives proposed are not as compelling as for other proposals, that could kill a proposal.  No matter how sexy a proposal might look from the limited information that we in the general public get to see, faults in the details that we never see may rule it out.

However, we need to remember that all the candidate themes for the upcoming selection of the fourth New Frontiers mission represent questions deemed to be among the highest priority for exploring the solar system.  Whichever mission is finally selected will significantly expand our understanding of the solar system.

Schedule for the next New Frontiers competition and launch:
  • Final AO Release Date  -- January 2017 (target)
  • Deadline for Receipt of Proposals -- AO Release + 3 months + 4 days
  • Selection of a subset (historically, two) of proposals for further study -- November 2017 (AO release + 10 months)
  • Final selection -- July 2019 (target)
  • Launch -- December 31, 2024 if solar powered or December 31, 2025 if radioisotope power sources are required
  • Flight time to the target world: Days (the moon), months (Venus), years to a decade or more (comet with Earth return, Saturn, or Trojan asteroids)

Science goals for the remaining mission themes (goals for the Venus and Trojan asteroid themes listed above):

The Comet Surface Sample Return mission theme is focused on acquiring and returning to Earth a macroscopic sample from the surface of a comet nucleus using a sampling technique that preserves organic material in the sample. The mission theme would also use additional instrumentation on the spacecraft to determine the geologic and geomorphologic context of the sampled region. Because of the increasingly blurred distinction between comets and the most primitive asteroids, many important objectives of an asteroid sample return mission could also be accomplished by this mission. The science objectives (listed without priority) of this mission theme are.
• Acquire and return to Earth for laboratory analysis a macroscopic comet nucleus surface sample;
• Characterize the surface region sampled; and
• Preserve sample complex organics.

The Lunar South Pole-Aitken Basin Sample Return mission theme is focused on returning samples from this ancient and deeply excavated impact basin to Earth for characterization and study. In addition to returning samples, this mission would also document the geologic context of the landing site. The science objectives (listed without priority) of this mission theme are:
• Elucidate the nature of the Moon’s lower crust and/or mantle by direct measurements of its composition and of sample ages;
• Determine the chronology of basin-forming impacts and constrain the period of late, heavy bombardment in the inner solar system, and thus, address fundamental questions of inner solar system impact processes and chronology;
• Characterize a large lunar impact basin through “ground truth” validation of global, regional, and local remotely sensed data of the sampled site;
• Elucidate the sources of thorium and other heat-producing elements to understand lunar differentiation and thermal evolution; and
• Determine the age and composition of farside basalts to determine how mantle source regions on the Moon’s farside differ from the basalts from regions sampled by Apollo and Luna

The Ocean Worlds mission theme is focused on the search for signs of extant life and/or characterizing the potential habitability of Titan and/or Enceladus. For Enceladus, the science objectives (listed without priority) of this mission theme are:
• Assess the habitability of Enceladus’ ocean; and
• Search for signs of biosignatures and/or evidence of extant life.

For Titan, the science objectives (listed without priority) of the Ocean Worlds mission theme are:
• Understand the organic and methanogenic cycle on Titan, especially as it relates to prebiotic chemistry; and
• Investigate the subsurface ocean and/or liquid reservoirs, particularly their evolution and possible interaction with the surface.

The Saturn Probe mission theme is intended to deploy one or more probes into Saturn’s atmosphere to directly determine the structure of the atmosphere as well as noble gas abundances and isotopic ratios of hydrogen, carbon, nitrogen, and oxygen. The science objectives (listed without priority) of this mission theme are:
• Determine noble gas abundances and isotopic ratios of hydrogen, carbon, nitrogen, and oxygen in Saturn’s atmosphere; and
• Determine the atmospheric structure at the probe descent location.