Discovery Finalists

Over the last few months, NASA’s managers have had the tough job of selecting a handful of proposals for new missions from an outstanding set of 27 proposals. Proposed targets ranged from Venus, our Moon, Mars and its moons, the comets and asteroids, Jupiter’s moon Io, Saturn’s moon Enceladus, to space telescopes to observe solar system bodies.

In the end, two Venus and three asteroid missions received the nod to receive $3 million each for further study pending final selections in a year. Launch of the selected mission or missions is likely for the early 2020s.

The current competition is to select the 13^th and possibly the 14^th missions in NASA’s Discovery mission program. In the past, missions in this program have orbited Mercury, orbited the Moon, landed on Mars, flown by comets, orbited three asteroids (landing on one), and searched for exoplanets. This program lets teams of scientists propose and lead the missions (in partnership with a NASA or industry partner for engineering expertise). Neat fact about the current finalists: Four of the five teams are led women.

Costs for the current competition are capped at $500 million for the spacecraft and instruments with NASA separately paying for other costs such as the launch. For comparison, this is more or less half the cost of the New Frontiers Pluto mission, a fifth the cost of the Curiosity Mars rover, and a quarter the cost of NASA’s planned Europa mission.

In the early years of the program from approximately the mid-1990s into the early 2000s, NASA regularly selected Discovery missions every two to three years and would often select two missions at once. Then budgets became squeezed and the last two mission selections were stretched out to every five years with a single selection each. (The GRAIL orbiters selected in 2007 studied the moon for a year beginning in late 2011, and the Mars InSight geophysical mission selected in 2012 will launch next year).

In this current competition, NASA believes it may be able to again select two missions, which would be staggered in their launch and development costs. NASA’s managers haven’t stated why they now hope to select two missions instead of the originally planned one. Their budget forecasts may look rosier than previously expected. It may be because the cost of the Discovery competitions to NASA and the proposal teams is high enough that the agency’s managers want to limit their frequency by delaying the next one and selecting two at once. Or the estimated cost of some of the missions selected as finalists could be implemented under the cost cap.

NASA evaluates the proposals based on two sets of criteria, and the competition is tough. Teams of scientists rank each of the proposals based on its scientific potential to help us understand the solar system. Separately, teams of engineers and budget analysts scrutinize the implementation details to determine whether each proposal likely could be implemented within the budget and schedule. The finalists are selected from among the proposals that rank highest on both sets of criteria.

The DAVINCI (Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging) proposal would drop an instrumented probe into Venus’ atmosphere. During its descent to the surface, the DAVINCI probe would measure the composition of the atmosphere’s gases and image the surface from below the clouds.

The team proposing DAVINCI was one of the quietest during the competition; while many other teams presented their proposals in some detail, not even the name of this proposal leaked. A brief post on the Unmanned Spaceflight message board reports that the instruments would include a mass spectrometer, a tunable laser spectrometer, an atmospheric structure package, and a visible and near-infrared descent camera.

By looking at papers and conference proceedings that include the proposal’s Principal Investigator, Lori Glaze with NASA’s Goddard Spaceflight Center, we can get some ideas about the mission’s scientific questions.

The composition of a planet’s atmosphere can reveal much about the planet’s formation, its evolution, and current geological processes such as surface weathering and volcanic eruptions. A recent conference abstract that included Dr. Glaze stated, “A key issue that remains after more than 50 years of planetary exploration is the formation and evolution of the atmosphere, particularly in the context of the other terrestrial planets. Comparing noble gas mixing ratios and isotopes of Venus, Earth, Mars, Jupiter, and the sun will help determine the timing and extent of atmospheric escape on Venus, a central process in planetary evolution.” Several research papers that include Dr. Glaze also discuss how volcanoes on Venus would release gases such as sulfur dioxide into the atmosphere that would indicate whether or not Venus has currently active volcanism.

According to a blog post on the journal Science’s site, the probe would descend over one of the planet’s tesserae and would image the terrain below as it fell. These crumpled highlands may be remnants of ancient crust on Venus. Images as the probe falls below Venus’ clouds could provide clues about the origins of these mysterious regions and the evolution of the planet’s surface.

DAVINCI is an example of a mission in which a few key measurements focus on selected critical science questions. The entire descent would likely take less than an hour. The data from the atmospheric composition measurements might be just a few megabytes of data. (A study of an atmospheric Saturn probe to study composition listed the total data as less than 2M bytes, less than the size of a high-resolution image from my personal camera.) The images collected by the probe’s camera might be a few megabytes to gigabytes. By comparison, orbiter missions at planets can return terabytes of data.

However, detailed composition measurements of planetary atmospheres are a high priority for planetary research because they can reveal details about the formation of each planet and its subsequent evolution. The Pioneer Venus probe from the 1970s lacked the resolution for key measurements. We have high-resolution measurements of Mars’ atmosphere from various landers and from Jupiter from the Galileo atmospheric probe. Obtaining high-resolution composition measurements from Venus (as well as Saturn, Uranus, and Neptune) has been a high priority for planetary scientists for decades. Each high resolution set of measurements for a new world provides a new piece of the puzzle to help us understand how the solar system formed.

Where the DAVINCI mission would focus on specific scientific questions and gather a small amount of critical data, the other finalist Venus mission takes the opposite approach. The VERITAS (Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy) mission would remap Venus’s surface with radar and conduct the first global mapping of its surface composition. NASA’s Magellan mission previously mapped Venus’s surface in low to moderate resolution in the early 1990s.

The mission’s VISAR (Venus Interferometric Synthetic Aperture Radar) would map the surface in three ways. First, it would create images of the surface at 30 m resolution globally and 15 m in selected regions compared to Magellan’s 280 m to 120 m resolution. Second, it would measure elevations to create a topographic map at 250 m resolution compared to Magellan’s 15 to 27 km resolution. And third, it would make repeat measurements in what’s known as an interferometric mode to spot tiny changes in relative elevations that could indicate surface movement from a seismic event or the swelling of a volcano.

A second instrument, the German Venus Emissivity Mapper (VEM), would study the planet’s thermal emissions for composition studies. The Galileo and Venus Express missions’ instruments discovered narrow spectral windows where thermal emissions can be transmitted through the otherwise opaque global cloud cover. These few windows would give the VEM instrument the ability to map the surface in six spectral bands to identify thermal hotspots that could indicate areas of current volcanic activity, map differences in the surface composition, and detect changes in key atmospheric gases that could indicate the eruption of a volcano. Because Venus’ thick atmosphere would scatter the light, the surface resolution of VEM would be low, perhaps around 50 km. The recently completed Venus Express mission carried out some measurements using this technique, but its instrument wasn’t optimized for measurements using these spectral bands and covered only the southern hemisphere.

While the DAVINCI mission would focus on a few critical measurements and would produce a relatively small data volume, the VERITAS mission would make multiple and repeated measurements over the surface of a large world. A proposal for a similar European mission said that it would return hundreds of terabytes of data; VERITAS likely would do the same. Researchers could use this database to enable hundreds of studies. A poster on the VERITAS mission (unfortunately no longer available on the web) listed a few:

Origin and Evolution: How did Venus diverge from Earth?

• Determine if tesserae are remnants of an earlier wetter past

• Search for past tectonic or cratered surface beneath the plains

Venus as a Terrestrial Planet: What processes shape the planet?

• Determine how and when Venus was resurfaced

• Estimate lithospheric thickness variations with time

• Identify sources and rates of recent and active volcanism

NASA also has the option for a technology demonstration for the VERITAS mission that would partially address the DAVINCI composition measurement goals. If funded, the tiny Cupid’s Arrow CubeSat would be released by the main spacecraft and would skim the edges of the outer atmosphere to reach below the homopause where the atmospheric gases are well mixed. A miniaturized mass spectrometer would measure ratios of key noble gases that provide clues to the formation and evolution of Venus.

These two Venus missions illustrate the different types of missions needed to explore the solar system. The study of Venus requires both and eventually both will fly.

The three finalist proposals to study asteroids provide another example of the complementary types of missions needed study the solar system. Asteroids are remnants of small proto-worlds from the early formation of the solar system and differ in location and composition. Our spacecraft will never visit more than a few of the millions of these bodies believed to orbit the sun. Scientists instead use telescopes to gather a few facts on many bodies to enable statistical studies, make brief flybys of a small number to flesh out the statistics, and make prolonged visits at a very few for in-depth studies (and to return samples from a few).

The Near-Earth Object Camera (NEOCam) mission would launch the first space telescope dedicated to observing asteroids. Its focus would be on the population of asteroids that, as its name states, approach near to our own world. By making measurements in two infrared channels for each of the tens of thousands of near-Earth asteroids, the science team will be able estimate sizes, shapes, composition, orbit about the sun, and rotation for each body. While the information on any one body will be limited, the statistical analysis made possible on a data set of tens of thousands of bodies would enable scientists to explore the dynamics, origins, and fate of these populations. (Past or future observations of many of the same bodies in other wavelengths of light, particularly the visible, will add valuable complimentary information.) During its survey, NEOCam also would observe approximately a million main belt asteroids and discover perhaps a thousand new comets, extending the usefulness of the statistics derived from its data.

However, the scientific study of these asteroids is only a part of the mission’s justification. Some proportion of near-Earth asteroids will eventually strike our world. Finding even one that threatens the Earth in the next few decades would justify the mission by itself. Some of the objects discovered also could become targets of future robotic or human missions.

The Lucy mission would follow the second strategy for asteroid exploration, brief flybys of a number of asteroids. The mission’s proposers have reused the name of one of the most famous fossils from human paleontology to emphasize that the spacecraft would focus on a fossil population of asteroids that may hold the potential to illuminate the ancient history of the solar system. It would study the Trojan asteroids that share Jupiter’s orbit, either preceding (the “Greek” camp in L4 Lagrangian orbits) or trailing (the “Trojan” camp in L5 Lagrangian orbits) the giant planet. Telescope observations suggest these bodies have primitive compositions, several of which don’t appear to be represented in our meteorite collections and that haven’t yet been visited by spacecraft.

The origin of this asteroid population is a mystery, and its solution would tell scientists much about the dynamics of the young solar system. We now believe that the orbits of the giant planets migrated in toward the sun and then out again soon after their formation. In the process, they scattered the tiny asteroids and comets hither and thither. One set of theories holds that the migration brought in groups of asteroids from throughout the outer solar system into Trojan orbits with Jupiter. Another theory suggests that the Trojans originated in the same region as Jupiter and followed it in its movements and are therefore samples of conditions where Jupiter formed. Either way – and it’s possible that the present population represents a mixture of sources – these bodies hold clues to conditions and processes from the infancy of our solar system.

The creativity of the Lucy mission is that its proposers found a set of trajectories that over 11 years allow flybys of two Trojan asteroids in the L4 swarm and a binary Trojan system in the L5 swarm with a bonus flyby of a main belt asteroid. The three Trojan encounters would sample a diversity of compositions, the C-, P-, and D-types.

This mission looks to the New Horizon Pluto mission for two of 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 OSIRIX-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.

The third asteroid mission would make an extended study of a single asteroid. The asteroid16-Psyche is unique among the larger asteroids in having a composition that appears to be largely metallic. Understanding how this world came to be is one of the goals for this mission. Psyche could be an asteroid in which repeated collisions chipped off the crust and mantle, leaving the core a naked body. It could be the remnant of the collision of two protoplanets that shattered and expelled the core of the smaller body to become Psyche. Or Psyche could have formed so close to the early sun that only metals (and some silicates) could have condensed from the nebula; the later migration of the giant planets could have moved it to its present location in the asteroid belt. In either of the first two cases, we’d get our first look at material from one of the most inaccessible locations in the solar system – the deep core of a rocky world. In the third case, we’d see the result of a new class of worlds that formed very close to the sun.

In its implementation, the Psyche mission would be much like the current Dawn mission to the larger asteroids Vesta and Ceres. Solar electric ion engines would slowly propel it to its destination where the spacecraft would orbit the asteroid for long-term studies. A combination of cameras and spectrometers would image the surface and map its composition while radio tracking would reveal its interior structure.

From the original 27 proposals, these five are the ones that NASA’s managers determined have the best combination of scientific appeal and low implementation risk. For the next several months, the proposal teams will be consumed with fleshing out the design of their missions. Then the space agency scrutinize the enhanced proposals to select one or two to fly.

If either DAVINCI or Lucy isn’t selected, scientists interested in their studies will get a quick chance to try again. NASA has another program for scientist-led missions, the New Frontiers program, which flies missions costing approximately twice what Discovery missions cost. For this program, NASA accepts proposals from a list of pre-selected, high priority concepts. One of those concepts is for a Venus atmospheric probe and lander that would replicate the DAVINCI atmospheric studies and also provide measurements studies from the surface. A second concept would be for a mission that would orbit a Trojan asteroid and possibly fly by one or two others. (The other candidate concepts are for a lunar sample return, a comet sample return, and a Saturn atmospheric probe.) The competition to select the next New Frontiers mission is scheduled to begin immediately after the selection of the next Discovery mission(s) next September.

I’m personally hoping that the agency selects both a Venus and an asteroid mission from the current Discovery competition. The greatest strength of the Discovery program has been missions to a diversity of worlds.