[meteorite-list] Discovery Proposals to Explore the Solar System's Smallest Worlds

[Ron Baalke]

http://www.planetary.org/blogs/guest-blogs/van-kane/20150727-proposals-to-explore-the-solar-systems-smallest-worlds.html 

Proposals to Explore the Solar System's Smallest Worlds
by Van Kane
Planetary Society
July 27, 2015

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.
 
Many of the proposed missions to small bodies will show us new worlds.
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 flyby a number of the Trojan asteroids, which follow 
Jupiter's orbit about the sun. The leading group of asteroids are sometimes 
referred to as the "Greeks" and the trailing as the "Trojans."
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.

Kuiper Space Telescope (proposed)

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.