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.