Proposals to Explore the Solar System’s Smallest Worlds

This summer, NASA’s managers are in the delightful position of having a tough choice to make.

For the space agency’s lowest cost planetary missions, known as Discovery missions, NASA’s managers ask the planetary science community to propose the missions they’d like to see fly. Any solar system body except the Sun and Earth (both covered under other programs) can be targeted. There’s a strict cost cap of $450M for the spacecraft and instruments, with the agency separately picking up other costs such as the launch and mission operations. 

The competitions are extremely competitive, with a single mission selected every few years from what’s typically a field of 25 to 30 proposals. Because it’s common to resubmit proposals in the next competition, in the past, proposers often have been reluctant to even give the names of their missions and only a few released any details of the concepts. Why give your competitors ideas for their next proposal?

For the Discovery competition in progress (the thirteenth), that near silence has been broken and most teams have published or presented summaries (and in at least one case, nearly a full description) of their proposed missions. We now know at least the names and destinations of 24 of the 28 proposals. I am delighted with the creativity and quality of the ideas. I suspect that NASA’s managers are equally impressed and find themselves having to make a tough choice this summer among a number of top-notch proposals.

My understanding of the selection process is that each proposal is evaluated twice for the initial round of selection. First, teams of scientists review the scientific merits of the proposals and rank them. Second, teams of engineers review the proposals for technical feasibility and the likelihood of being built within the cost cap. The very few proposals that receive top scores in both evaluations become eligible to be selected as finalists that receive funding for a year to more fully develop their concepts (typically three proposals in recent competitions). Approximately a year later, NASA’s associate administrator for science makes the ultimate selection from among the finalists. Launch of the selected mission is expected by 2021.

The openness of the proposing teams for the current competition allowed James Callahan to publish brief summaries of most of the proposals. I’ve published fuller descriptions for the four proposals for missions to study the outer planets, as well as more detailed descriptions of a proposed mission to a metallic asteroid and a mission to remap Venus with modern instruments. (While the latter focused on a European proposal for their own mission competition, at least two teams reportedly have submitted similar proposals for the current Discovery competition.)

At a meeting of the Small Bodies Assessment Group at the end of June, scientists proposing missions to the solar system’s comets and asteroids gave 10 minute summaries of their proposals. In this post, I’ll report on the concepts presented.

The key to thinking about the small body Discovery proposals is to understand how diverse these worlds are. They lay scattered from the inner solar system to the edge of interstellar space. Some are rich in volatiles and are commonly called comets. Some are almost all rock and are called asteroids. Some (like Ceres where the Dawn spacecraft now orbits) are mixtures of both and we don’t yet have a good name for them. Within these broad classes of objects, they show tremendous diversity in size, shape, and composition. 

To understand the clues these bodies provide on the solar system and its formation, we need to explore a few of them in depth to tease out their subtle details. We also need to observe many more with less detail to build up a statistical understanding of their diversity. Each of the small body Discovery mission proposals plays to one or the other of these strategies.

Comet Missions

Depending on how you define a comet, either three or four teams propose missions to rendezvous with a comet and explore their target in depth. Several spacecraft have made quick observations of comets during the minutes surrounding closest approach during high speed flybys. The Rosetta spacecraft and its lander Philae are currently conducting a lengthy in-depth exploration of comet 67P/Churyumov–Gerasimenko.

All four of the Discovery comet missions would rendezvous and then orbit their comet for long-term studies. Unlike the Rosetta spacecraft, these missions would carry just a few selected instruments for focused studies. The Rosetta mission has cost approximately €1.4 billion Euros (~$1.5B) and carries 11 instruments on the orbiter and another ten on its Philae lander. To fit within the $450M cost cap of a Discovery mission, these missions have to carefully target just one or two questions with as few as two instruments.

Interestingly, two missions propose to study the same comet, the 2.3-kilometer-long Hartley 2 that was previously visited by the Deep Impact EPOXI spacecraft during a brief flyby in 2010. The choice of target may owe partially to the chance alignment of its orbit with Earth around 2020 allowing an easy flight. However, the comet itself is interesting, with highly active jets emitting water vapor from one part of the surface and carbon dioxide and ice from another. Both the CHagall mission and the Primitive Material Explorer would orbit the comet and study the structure and composition of its surface with cameras and an infrared spectrometer. A mass spectrometer would taste the gases jetting from the surface to analyze their composition, including measuring the fractions of key isotopes that provide compositional clues to the formation of the solar system. The CHagall spacecraft would place small explosive charges on the surface to expose fresh subsurface material. The PriME spacecraft would carry an additional ion and electron spectrometer to further analyze the material emitted from the comet. While the primary science questions for the CHagall are to understand the formation and heterogeneity of comets, the primary question for the PriME mission is to determine whether comets such as Hartley 2 could have delivered water to Earth. 

In the last Discovery competition, a mission proposal similar to CHagall, CHOPPER, was the one of the three finalists (but not chosen). PriME, too, competed last time, and while the mission was not a finalist, its MASPEX mass spectrometer was funded for further development. MASPEX was selected for NASA’s mid-2020’s Europa mission and is proposed to be included by several of this Discovery competition’s proposed missions.

The Proteus mission would visit 238P/Read, a small body within the asteroid belt that behaves like a comet (such bodies are known as "main belt comets"). The spacecraft would make slow flybys past this 0.4 km-radius world before entering orbit. This mission would carry just two instruments, a copy of the Dawn spacecraft’s camera and the MASPEX mass spectrometer. Like the PriME mission, this mission would focus on determining whether comets like this could be the source of Earth’s water, as well as seek clues in its composition as to where it formed in the solar system.

The final comet mission would also orbit a comet, this time 10P/Tempel 2 (which should not be confused with the more famous Tempel 1 comet that has had two spacecraft flybys). This mission, though, carries no instruments to measure composition. Its focus is on the structure of the comet from its surface to its center. A camera (another copy of the Dawn instrument) would map the surface morphology, and an infrared imager will study how the surface heats and cools to determine its properties (for example, a hard solid or a fluffy dust pile). The main instrument, as the mission’s name – the Comet Radar Explorer (CORE) – suggests, would be an ice-penetrating radar that would see into the depths of the comet to give it the equivalent of a CAT scan. The data would allow scientists to determine how the comet came together (large chunks or small snow balls) and would map the distribution of ices, rocky material, and voids. (Similar ice-penetrating radars are operating on two spacecraft at Mars, and the JUICE and Europa missions will use them to study Ganymede and Europa next decade.)

Asteroid orbiters

Scientists are proposing four missions that would orbit asteroids ranging from those in near-Earth orbits to asteroids that share an orbit with Jupiter. The Binary Asteroid in-situ Explorer (BASiX) shares similar goals with CORE – understand the structure of a tiny asteroid (1.7 km 1996 FG3) at its tinier (0.5 km) moon. Both bodies are likely aggregates (a nice way to say rubble pile), but scientists are unclear as to how they form, how they are structured, and how they have changed through time. While larger bodies have substantial gravity to hold them together, these are worlds of microgravity. The BASiX spacecraft would image these worlds, measure the surface properties with thermal imaging, and study the interior through gravity studies. Small explosive pods along with geophones would be placed on the surface to study the interior from the seismic waves created by the explosions.

If tiny worlds are rubble piles, the Psyche mission could explore the opposite end of the spectrum – a metal asteroid that may be the solid remnant core of a shattered proto world. The deep core of our world, the other terrestrial worlds, and the larger asteroids remains hidden beneath layers of rock. One or more ancient impacts may have blasted these layers off the surface of the asteroid 16 Psyche. This mission’s spacecraft would image the surface, map its composition, and study the interior through gravity studies. Its scientists may find the intact core of a young world, a core broken into a rubble pile, or a metal world that formed directly through accretion without a rocky surface. One of the primary goals of this mission will be to determine what this world is and what it can say about the formation of the interior of larger rocky worlds.

The Advanced Jovian Asteroid Explorer (AJAX) spacecraft would journey beyond the asteroid belt to the Trojan asteroids that share Lagrangian orbits with Jupiter. Scientists have two theories about how these asteroids formed which, based on their colors, are different than other asteroid populations. The Trojan asteroids may have formed from the same cloud of dust and gas as Jupiter. Or, thanks to the planetary migration in the early solar system, they may have formed from beyond the orbit of Neptune and later been captured into their present orbits. Either way, they could tell us much about conditions in the outer solar system early in its history. AJAX would orbit a 32 km diameter D-type Trojan asteroid (I didn’t catch the name) and map its surface morphology and composition. It would also place a lander with mobility (a hopper? a wheeled rover?) on the surface for more precise composition measurements. The mission also has an option to flyby a second Trojan asteroid.

The mission proposal for which I have the least information is the Dark Asteroid Rendezvous (DARe). No slides were presented at the SBAG meeting. It would orbit multiple asteroids – it’s not clear if these are near-Earth or in the main belt or both – that are the rarer and likely more primitive D- and P-Types. The spacecraft would carry copies of the DAWN cameras (this design is popular), instruments to map composition of the surface, and a radar.

Martian Moons

Mars’ two tiny moons, Phobos and Deimos, are favorites for Discovery competitions (and their European counterpart) for several reasons. First, there is the mystery of their origin. Are they captured asteroids (in which case their color suggests they may be rare primitive bodies)? Are they material left from the formation of Mars? Or are they material blasted into orbit from asteroid strikes on Mars’ surface? Any of these choices makes them interesting scientific targets. A second reason for the interest is that these moons may serve as initial targets for human exploration as we build the skills and technologies to enable missions to the surface of Mars. And third, transits to Mars— and therefore its moons — are relatively easy.

For this competition, three teams are proposing missions to these moons using three distinct strategies. The Phobos and Deimos and Mars Environment (PADME) would be a small spacecraft that would orbit Mars and make 16 flybys of Phobos and 9 of Deimos. The craft would carry a suite of cameras that would take images with resolutions as small at 2.8 centimeters to study fine scale features and the processes that formed them. During the flybys, a neutron spectrometer would remotely measure surface composition while a mass spectrometer would directly measure the composition of surrounding dust particles ejected from the surface. Tracking the radio signal during flybys would provide information on the gravity field and therefore the interior structure of the moons.

The Pandora mission, in contrast, would use a solar electric propulsion system (similar to that used on the Dawn spacecraft currently at the asteroid Ceres) to enable it to orbit each of the moons for extended studies. The Pandora mission would use cameras, a near-infrared spectrometer, and a gamma ray neutron spectrometer to remotely study the surface, as well as radio tracking. Unlike PADME, Pandora wouldn’t carry a mass spectrometer to directly measure the composition of dust ejected from the surface. That type of measurement requires high relative speeds so that the dust vaporizes on impact with the instrument to enable the composition to be determined. PADME’s flybys would provide the relative speed necessary while Pandora’s slower relative motion in orbit likely would not. This is an example of the types of tradeoffs that mission proposers must make – PADME’s simpler mission design enables a valuable measurement while Pandora’s mission design enables longer and more detailed measurements with other instruments. (This mission proposal is in many ways similar to NASA’s possible 2020s Mars orbiter that also would use solar electric propulsion. On its way to its close in orbit around the Red Planet, NASA has discussed that its future Mars orbiter could visit and orbit Deimos and Phobos. The Pandora team would transfer their spacecraft to the Mars program at the end of their mission. However, the Pandora spacecraft would lack the ultra-high resolution camera and atmospheric instruments that the Mars program would want for a dedicated mission.)

The final mission proposed for these moons, the Mars-moons Exploration, Reconnaissance, and Landed Investigation (MERLIN) makes a different set of tradeoffs. Remote composition measurements are less precise than those that a landed spacecraft can make. The MERLIN team proposes to carry just a camera, a simple dust counter, and its radio tracking system for remote studies done during flybys past Deimos and in orbit around Phobos. However, the spacecraft would land twice on Phobos in areas that appear to have different compositions (the so-called blue and red materials). There it would use a small arm, much like the rovers on Mars have, to put its instruments directly in contact with the surface for detailed study of its texture and elemental composition. With this proposal, richer global studies from flybys and orbits are traded off for more detailed studies at two locations on the surface.

Asteroid Surveys

All of the mission discussed above would study just one or two (it’s not clear how many asteroids DARe would orbit) comets or asteroids. Surveying larger populations of asteroids through close flybys would give us a look at a greater variety of these bodies.

The Main-belt Asteroid and NEO Tour with Imaging and Spectroscopy (MANTIS) spacecraft would fly by nine asteroids (two of which are binary systems so two extra bodies are thrown in for free) that orbit near Earth and in the main asteroid belt. The targets were chosen (subject to the laws of astrodynamics and fuel constraints) to sample a range of asteroid sizes and compositions. The spacecraft would carry a narrow-angle camera, a near-infrared imaging spectrometer, a mid-infrared multispectral imager, and a dust instrument (it’s not clear if the last simply counts dust particles thrown off the surface of the asteroids by micro-meteorite strikes or would measure their composition). The proposers emphasize that the capabilities of modern instruments will give us resolutions from flybys today that exceed the resolutions available from asteroid orbiters in the 1990s.

The Lucy mission would explore the “fossils of planet formation” among the Trojan asteroids (with a flyby of one main belt asteroid thrown in). The goal of this mission is to sample all the composition types within the Trojans (C-, D-, and P-Types) and sample both those asteroids that lead and follow Jupiter in their shared orbit. This mission looks to the New Horizons Pluto mission for its instruments with copies of that mission’s LORRI high resolution camera and the Ralph color camera and imaging spectrometer. Another infrared spectrometer would draw on instrument heritage from Mars orbiters and the upcoming OSIRIS-REx asteroid sample return. Tracking of the spacecraft’s radio signal would provide information on each asteroids mass — and therefore density — which provides clues to their composition and to whether they are solid objects or rubble piles.

Telescopes

All but one of the twelve Discovery missions selected to date have sent spacecraft to bodies throughout the solar system. NASA, however, allows teams to propose space telescopes that study planetary bodies from afar—the Kepler telescope, for example, was funded under the Discovery program. These missions would gather limited information about each small body – color or spectra, size, and orbit. However, these measurements would be made for thousands or even millions of bodies. These are the ultimate in survey missions.

The Near-Earth Object Camera (NEOCAM) would follow Earth from inside Earth’s orbit from the stable Lagrange 1 point between Earth and the Sun. From this location, it would look to either side of the Earth for asteroids and comets whose orbits approach the Earth. The proposers expect to find and determine the orbits and physical characteristics of up to millions of objects. The goal is to catalog bodies that might someday hit the Earth, characterize the origins and evolution of these populations, and find new destinations for future exploration. This mission was originally proposed in the previous Discovery competition and awarded funding to further its critical technology development. It has returned, as a more mature concept, for the current competition.

What the NEOCAM mission would do for small bodies in the inner solar system, the Kuiper Space Telescope would do for small bodies in the outer solar system. In addition to the small bodies, this mission would also study the planets of the outer solar system, along with their active moons.  This mission would survey the Trojan asteroids, the Centaur asteroid-comets that orbit between the outer planets, and bodies in the Kuiper belt of which Pluto is just the largest and best known. The goal will be to use the statistics gathered on these worlds to trace the formation and evolution of the outer solar system. (For information on Kuiper’s goals for studying the outer planets and their moons, see this earlier post.)

Both the NEOCAM and Kuiper telescopes would study bodies close enough to the Sun and large enough to be detected through the reflection of the Sun’s light. At the fringes of the solar system lies a large population of small worlds in the Kuiper and Oort belts too small to detect by their reflected sun light. The Whipple telescope would instead stare at the more distant stars to watch as these small bodies randomly pass in front of the stars. From the way the starlight is defracted, scientists will learn the size and the distance of these distant small bodies. The statistics built up from these observations will provide us with our first observations of the hypothesized Oort bodies and clues to the formation and evolution of these fossil populations. Like NEOCAM, the Whipple team received funds in the last Discovery competition to mature their technology.