NASA has several different space mission classes for exploring our solar system. These are arranged by funding level as well as by how quickly they can respond to new science. Discovery provides the least funding but is meant to respond to discoveries that may not have even been made at this point. The medium class, New Frontiers, consists of a list of exciting destinations set out in the planetary decadal survey, the latest of which was just completed last year. The largest missions are run directly by NASA and respond to deep and meaningful science questions that cannot be addressed under the other classifications.
Image caption: The four members of the New Frontiers family: The New Horizons mission to Pluto and beyond, the Juno mission to Jupiter, the OSIRIS-REx mission to Bennu, and the Dragonfly mission to Titan. In the next few years, they will be joined by a fifth member that currently only exists as an idea on paper. (NASA/JHUAPL/SwRI/GFSC)
By Conor Hayes
As a planetary scientist, proposals for new missions to explore the the Solar System are understandably quite exciting to me, and I’ve recently become interested in understanding how those proposals are prepared and selected. This January, NASA released the draft Announcement of Opportunity (AO) for New Frontiers 5, the first major AO of my time as a graduate student. Although the final AO is not expected to be released until November, this is an excellent opportunity to take a look at what missions we will expect to be proposed over the next year or so.
New Frontiers (NF) is the middle tier of NASA’s three-tier Solar System exploration program, sitting between the low-cost Discovery Program and the flagship Large Strategic Science Missions Program. As the NF5 name suggests, there have been four previous NF missions: three that are ongoing (New Horizons, Juno, and OSIRIS-REx), and one under development for launch in 2027 (Dragonfly). The mission selected in NF5 must be launch-ready by no later than the end of 2034.
The science objectives laid out in the NF5 AO can be traced back to the 2013-22 Planetary Science Decadal Survey. The Decadal Survey is a document created every ten years through a collaborative effort of the planetary science community that outlines the highest-priority goals for the next decade. These priorities inform the selection criteria at all three mission levels, depending on what is felt can be accomplished with those levels’ respective budget caps. NASA aims to release two NF AOs for each Decadal Survey but has fallen short of that cadence in recent years, hence why the NF5 AO was released after the completion of the 2023-32 Decadal Survey despite being the second NF AO of the 2013-22 Decadal Survey.
So what are the science objectives of the NF5 AO? There are six mission themes, each of which has its own list of objectives. To be selected, a mission proposal must address a “proponderance of the science objectives” listed for at least one of the themes. The AO specifies that its use of “proponderance” rather than “majority” is meant to reflect the fact that not all of the listed objectives are of equal importance. A successful proposal could target a small number of high-importance objectives or a large number of low-importance objectives.
The mission themes are as follows, with a brief explanation of some of the key objectives:
Comet Surface Sample Return
In recent years, there have been several missions to return samples from asteroids that were at least partially successful: Hayabusa (Itokawa), Hayabusa2 (Ryugu), and OSIRIS-REx (Bennu). One mission has returned samples from the coma of a comet: Stardust (Wild 2). However, there has not yet been a mission to return samples from the surface of a comet. Cometary surfaces are interesting as they are rich in volatile organic molecules that have been modified during the comet’s journey through the Solar System. It has been suggested that Earth’s organics may have been delivered through cometary impacts, providing even more motivation to return surface samples. Although some in-situ studies of cometary surfaces have been conducted remotely (most notably Rosetta at 67P), these studies have been necessarily limited by weight and cost considerations that would not be present if samples were returned to be examined in labs on Earth. Organic molecules can be fragile, so it is critical that missions in this theme are able to transport the samples to Earth without destroying the samples by subjecting them to conditions that would degrade their constituent molecules. A cometary mission should also be able to provide context information about the site the samples were extracted from.
Io is the most volcanically-active body in the Solar System, and missions in this theme will be focused on understanding why that is the case. From my reading of the objectives, an Io Observer has the widest range of science to choose from, and it is unlikely that any proposal would be able to cover them all. Once the mission proposals are finalized, it will be interesting to compare how different proposals in this theme prioritize the science to be returned. These could include determining what fraction of Io’s mantle is molten versus solid, examining the tidal heating mechanisms that are suspected to drive the volcanism, looking for potential tectonic activity, and studying the interactions between the materials ejected from Io’s volcanos and Jupiter’s extremely strong magnetic field.
Lunar Geophysical Network
The goal of missions in this theme should be to examine the Moon’s interior. This could include studying its minerology and composition, as well as its interior heat flow and the distribution and origins of the radioactive isotopes that create that heat flow. To more fully understand the Moon’s interior structure, I would expect to see proposals similar to the InSight lander, which probed the interior of Mars through seismometry. Although the Moon, like Mars, is geologically dead, it experiences Moonquakes like the Marsquakes measured by InSight. The most interesting part of this theme (at least to me) is the “network” in its name. Although none of the science objectives explicitly require it, the name suggests the deployment of several spacecraft across the Moon’s surface conducting coordinated observations. With multiple stations in place, such a system would not face the same kinds of challenges that InSight did operating as a solo seismometer.
Lunar South Pole-Aiken Basin Sample Return
The South Pole-Aiken Basin is one of the largest impact structures in the Solar System, measuring approximately 2,500 km across and 6-8 km deep. It is also the oldest known lunar basin, having formed less than 500 million years after the Moon itself. Consequently, there is interest in understanding its geology as doing so would contribute to our knowledge of the processes that formed the inner Solar System by constraining the specific timing of the Late Heavy Bombardment. Key objectives include providing in-situ validation of remote sensing data, determining the sources of the radioactive isotopes that contribute to the Moon’s internal heat, and comparing the properties of basaltic rock samples returned from the basin with those returned by the Apollo and Luna missions. In addition to returning samples to Earth, a mission in this theme should also provide geologic documentation of the sampling site.
Ocean Worlds (Enceladus)
Although Jupiter’s moon Europa loves to take all of the media attention for its potential subsurface ocean, it is not the only moon that may be hiding liquid water beneath its thick, icy crust. Saturn’s moon Enceladus is particularly interesting due to the presence of over 100 geysers near its south pole that spray massive plumes of water vapor and other volatiles at velocities sufficient to escape Enceladus’ gravitational influence and enter orbit around Saturn. Exploration of these plumes is a priority because they provide us with an opportunity to examine the contents of the potential subsurface ocean without having to drill through the crust. The goals of such a mission would be to determine if the ocean is potentially habitable and, if it is, whether or not life currently exists within it.
Although Cassini orbited Saturn for over 13 years, it did not include an atmospheric entry probe to perform in-situ measurements of the Saturnian atmosphere like Galileo did at Jupiter. A Saturn probe mission would rectify this by launching at least one probe into the atmosphere with the goal of studying both its physical structure and its elemental composition.
Is there one mission theme that is more likely to succeed than any of the others? At this point, it’s difficult to say, given that no missions have been proposed. Although it may be my own bias as someone who studies the Moon speaking, I would guess that either of the lunar missions might have a slight leg up over the other themes, just considering the fact that you can do a lot more science by sending $900M to the Moon rather than the outer Solar System. The target date for proposal submission is currently April 2024, with initial Step-1 selections announced by the end of 2024, so the shape of the playing field will become much clearer over the next year.