Because they spent almost their entire history in the deep outer solar system, comets likely preserve ices and organic materials that have undergone little modification over the eons. Scientists are particularly interested in studying the organic material to better understand the processes that created them and to understand what organic material were possibly delivered to the early Earth. Comets are also rich in dust particles that each can tell a story about the materials from which the Sun’s rocky worlds formed. The Stardust mission collected hundreds of dust particles during a high-speed encounter with a comet. However, the collection process necessarily degraded the samples and no ices were collected. To fully understand the history recorded in comets, scientists have made returning minimally-altered samples from a comet their top priority for future comet studies.

The proposed COmet Nucleus Dust and Organics Return (CONDOR) mission would collect up to two samples of material from the surface of comet 67P/Churyumov-Gerasimenko, each of which would represent >50 grams in mass. The samples would be collected during brief and gentle touch-and-grabs that would cause little or no modification to the comet material. To get material that would be as least altered as possible by the Sun’s heat, the samples would include material from up to 15 centimeters below the surface. After collection, the samples would be maintained at less than -20 degrees Celsius during the return trip and refrigerated at -80 degrees Celsius at NASA’s curation facility to prevent alteration of the samples. For materials such as hydrogen cyanide and carbon dioxide that would sublimate at these temperatures, the sample container will trap the gases from these ices for study on Earth.

Many of you likely recognize that the target comet for this mission, P67, is the same one examined in detail by the Rosetta mission. By returning to this richly-studied comet, scientists will be able to understand the samples acquired within the context of the entire body. The CONDOR spacecraft also will use its camera and measurements of the gravity field to study how P67 has changed during its passes through the inner solar system since the end of the Rosetta mission.

A second comet mission, the COmet Rendezvous, Sample Acquisition, Investigation, and Return (CORSAIR) mission would sample a comet, 88P/Howell, that has never been visited by a spacecraft. This creates the opportunity to explore a new comet in-depth. Where the CONDOR spacecraft would carry just two instruments (with its radio system for gravity studies an effective third instrument), the CORSAIR spacecraft would carry five instruments to measure the composition of the gases and dust released by the comet and to remotely image and study the surface (plus the radio system) for 10 months. Two samples from at least 10 cm below the surface would be collected using a harpoon system along with nine collections of dust gathered from the coma. The samples would not be kept refrigerated; instead, “any volatile ices that are collected are sublimated from the samples and chemically characterized before return” by the instruments on the spacecraft’s mass spectrometer instrument.

There is reportedly a third comet sample return proposal led by Stephen Squyres who has also led the Mars Spirit and Opportunity rover missions.

Trojan Tour and Rendezvous

The Trojan asteroids share Jupiter’s orbit around the Sun and are believed to have originated from a range of locations in the early solar system. For this reason, the scientific community has prioritized a mission to study several of these worlds and orbit at least one to learn about their origins and how they shifted position as the solar system formed.

I have not found any information on New Frontiers proposals to study them. The selection of a simpler flyby mission, Lucy, last year to study these worlds may have led potential proposers to conclude that NASA is unlikely to select two Trojan missions back to back.

Saturn Atmospheric Probe

For over a decade, the Cassini spacecraft has observed Saturn’s atmosphere to study its composition and weather. Although remote sensing is invaluable for studying and understanding a planet’s atmosphere, there are certain very key measurements that remote sensing spacecraft are not capable of providing. In particular, noble gases that carry the signature of the epoch, location, and conditions of planetary formation are undetectable from outside the atmosphere. So are the details of many atmospheric processes including the detailed thermal structure and stability of the atmosphere, the deep cloud structures, and the winds.

The Saturn PRobe Interior and aTmosphere Explorer (SPRITE) mission would deliver a probe to Saturn’s atmosphere. As with the proposed Venus missions, scientists have many questions about Saturn’s formation and evolution that can only be answered by directly measuring the precise composition of its atmosphere with a descent probe. Among those questions are where in the early solar system Saturn formed and what role it played in the possible migration of the giant planets following their formation - first inward and then outward to their present locations. Measurements of the helium abundance could resolve a mystery of why Saturn is much warmer today than simple models of its evolution suggest it should be. A rain of helium deep in the deep atmosphere could be the explanation, in which case helium abundance should be depleted in the upper atmosphere where the probe can make its studies. For these measurements, the SPRITE probe, like the proposed VICI Venus probe, would carry both a mass spectrometer and a tunable laser spectrometer.

Another set of questions for the SPRITE probe revolve around the meteorology of the upper atmosphere. During its approximately 90-minute descent, SPRITE would measure thermal structure: temperature vs. pressure, and the change in Saturn’s wind from the cloud tops to the deeper atmosphere. SPRITE would also determine the locations and compositions of Saturn’s different cloud decks. Together these measurements will extend Cassini’s remote measurements of Saturn’s meteorology below the level of the highest cloud tops. Prior to entry, a camera on the SPRITE carrier-relay spacecraft would remotely image the atmosphere – both near the probe entry point and globally - so that the probe’s measurements can be understood in their global context and connected back to prior missions equipped with only remote sensing instruments.