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By Leonard David
Science
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Proposed New Frontiers Missions
NASA’s managers are in the processing selecting the agency's next
planetary mission from a field of twelve competitors. This fourth mission
in the New Frontiers program will follow in the footsteps of the three missions
in this program that have already launched: The New Horizons Pluto spacecraft, the
Juno Jupiter orbiter, and the OSIRIS-REx asteroid sample return
mission.
By law, NASA’s managers cannot reveal
information on the proposals submitted to protect the proprietary ideas of the
proposing teams. However, the proposing teams often will disclose some
information on their proposed missions at meetings and conferences. For
the past year, I’ve been collecting their presentations and abstracts and can
describe nine of the twelve missions in this article. (For two other
proposals, I know only their target, and the twelfth proposal remains a
mystery.)
New Frontiers missions are a key
component of NASA's program to explore the solar system. Like Goldilocks's
bears, NASA's planetary missions come in three sizes. At the low end,
costing $600-700 million, are the more frequent Discovery missions that address
tightly-focused questions on (for planetary exploration) tight budgets.
NASA plans to fly several Discovery missions in the coming decade. At the
high end, typically costing greater than $2 billion, are the Flagship missions
that host a wide range of instruments for in-depth studies. NASA
typically flies just one Flagship mission a decade, although the next decade
will see two launches, the Mars 2020 rover and the Europa Clipper.
However, there are a range of studies
that the scientific community deems essential to understanding the solar system
that can't fit within the Discovery program, but don't require a Flagship
mission. This is the role of the New Frontiers program with missions
costing somewhere around $1 billion with a planned flight rate of two per
decade.
The list of possible missions for the
New Frontiers program are pre-selected by a panel of scientists once a decade
in a process that sets exploration priorities known as the Decadal
Survey. For this current competition, NASA’s managers added two
additional targets, Saturn’s moons Enceladus and Titan.
The New Frontiers missions that have
been publicly described include proposals that emphasize composition
measurements to address the formation and evolution of the solar system
(the CONDOR and CORSAIR comet sample return and the SPRITE Saturn probe
proposals) or the formation and evolution of terrestrial worlds (the VISAGE and
VICI Venus atmospheric probe-landers and the lunar Moonrise sample return
proposals). The ELF Enceladus mission, the Titan Oceanus orbiter, and the
Titan Dragonfly rotocraft would explore their target worlds for habitability
and signs of life. The latter two missions also would continue the
broader exploration of Titan.
The selection of any of these missions
would significantly advance planetary science. NASA will choose among
them based on two criteria: Would the proposed mission likely meet its
scientific goals? How feasible is the mission technically within the
budget cap of $850 million for the spacecraft, instruments, and mission
operations? (Additional costs such as launch would raise the final cost
to around $1 billion.) The review process is thorough and daunting; any
weaknesses on either criterion can disqualify a proposal or cause it to be
ranked poorly compared to its competitors.
NASA expects to select two to three of
these proposed missions this fall for further definition and review. The
winning mission will be chosen from this short list by mid-2019 for launch no
later than 2025. Travel time to the selected destination could be as
short as a few days or as long as twelve years.
 |
| Dates for key events in the proposed New Frontiers missions. *Proposing teams have not given dates for the Moonrise and Dragonfly Titan missions. Dates shown are my best estimates based on the requirement to launch by the end of 2025 and flight times to reach destinations. |
Venus In Situ Explorer
 |
Two of the proposed New Frontiers
missions would return to land on Venus.
Image from the Soviet Venera 13 lander.
Credit: Brown University
/ Vernadsky Institute / Olivier de Goursac
|
Two key questions have led the two
Decadal Surveys (published in 2002 and 2012) to include a mission to directly
sample the atmosphere and surface of Venus.
The first is how terrestrial planets formed and the second is how Venus
evolved from a world that likely had a moderate climate with oceans to its
present hellish conditions. As the 2002
Decadal Survey report notes, “Venus and Earth may have had very similar surface
conditions early in their histories, but Venus’s subsequent evolution was
different from Earth’s, developing an environment unsuitable for life. However,
Venus is still a dynamic world with active geochemical cycles and non-equilibrium
environments in the clouds and near surface that are not understood.”
To meet the goals of this mission, a
spacecraft would need to conduct two sets of measurements. First, it would make detailed measurements of
the atmosphere’s composition during its descent to and on the surface. The mixture of gases in a planet’s
atmosphere, especially the ratios of isotopes of key elements, provides both a
fossil record of that world’s formation and evolution and insights into current
processes such as surface weathering and volcanic activity.
Once on the surface, the craft would measure the composition of the
rocks and soils to allow scientists to reconstruct the processes that initially
formed that location and subsequently modified it.
Soviet and American probes in the
1970s and 1980s performed both sets of measurements, but the then available
technology lacked the precision to answer key questions. The two proposed New Frontiers missions would
each carry the latest generation of instruments that would provide far more
detailed measurements than their predecessors.
In the last New Frontiers competition,
a Venus proposal led by Larry Esposito of the University of Colorado was a
finalist but wasn’t selected to fly (losing out to the OSIRIS-REx mission). Esposito is again leading a team for the
current competition, proposing the Venus In Situ Atmospheric and Geochemical
Explorer (VISAGE) mission. During the
atmospheric descent, the VISAGE craft would use a mass spectrometer to measure atmospheric
composition and a suite of instruments to measure the temperature, pressure,
and wind profiles. Mass spectrometers
are the workhorse instrument for atmospheric composition measurements, and
you’ll notice that several of the proposed New Frontiers missions would carry
one or more. Mass spectrometers “weigh”
the material they sample and the distribution of those weights reveal the
composition of the atmospheric gases.
(Versions also exist that can be used for composition studies of ice and
dust particles.)
Once on the surface, a drill on the
VISAGE lander would retrieve soil samples from “two depths” and deliver them
inside the spacecraft. There, one or
more instruments could operate at moderate temperatures to make detailed
elemental and mineralogical composition measurements. (The X-ray fluorescence
instrument would almost certainly be entirely inside the craft’s pressure
vessel. The mission’s visible and
near-infrared spectrometer likely also would view the samples inside the craft,
but might also examine composition outside the craft through a window.) Cameras to image the surface during the
descent and take panoramas after landing would round out the instrument
package. Because of the extreme
temperatures on the surface, the craft would survive to make measurements for
only several hours.
The Venus In situ Composition Investigations
(VICI) mission led by Lori Glaze of NASA’s Goddard Spaceflight Center would
address the same overalls goals but with a different emphasis. While the VISAGE mission would accept the
complexity of a drill and air lock to bring samples inside the craft for
improved soil measurements, the VICI mission would carry a second instrument (a
tunable laser spectrometer) in addition to the mass spectrometer for improved
atmospheric composition measurements. Tunable
laser spectrometers complement mass spectrometers by being able to precisely
measure specific isotope ratios and the abundance of key gases such as water,
carbon dioxide, and sulfur dioxide.
On the surface, another instrument
uses Raman and laser-induced breakdown spectroscopy to study the composition of
the rocks and soil. This instrument would
fire laser pulses through a window to measure composition at multiple locations
near the craft. At lower power but
higher wavelength, the laser enables Raman spectroscopy for mineralogical
measurements, and at higher power but a lower wavelength the laser melts a tiny
portion of the soil or rock for elemental measurements.
Additional VICI instruments would provide further surface measurements. A gamma-ray
spectrometer would sense the presence of trace amounts of radioactive elements
in the soil through the hull to characterize bulk composition. Like the VISAGE mission, the VICI mission
would image the surface during its descent, but there’s no mention of a camera
to take panoramic images once on the ground.
The conference abstract describing the
VISAGE mission doesn’t mention where on Venus it would land. (In the previous New Frontiers competition,
Esposito’s proposed mission would have landed on what is believed to be relatively
fresh lava flows.) The VICI mission
would target the continent-size tessera highlands that are believed to be the
oldest surviving surfaces on the planet.
And while the VISAGE mission apparently would deliver a single lander,
the VICI mission would deliver two landers.
Lunar South Pole-Aitken Basin Sample
Return
 |
| The Moonrise lander and ascent vehicle concept from the last New Frontiers competition. Credit: JPL and the Moonrise team |
No spacecraft could hope to carry
instruments that can match the capabilities of the best instruments on
Earth. As a result, sample return is
often viewed as the ultimate goal of planetary science after the missions of
exploration have told us enough about a world or class of worlds to ask
detailed, targeted questions. In
addition to the lunar and comet sample return missions proposed for this New
Frontiers completion, there have been three missions launched to return samples
from asteroids, Japan plans to return samples from the Martian moon Phobos, and
both the United States and China are working toward returning samples from Mars.
In the case of the moon, we have
samples from a number of locations from the American Apollo and Soviet unmanned
missions. However, answering many of the
specific questions that remain about the moon require samples from carefully
selected new locations. This is driving
the Chinese lunar sample return mission planned for the next year and the
proposed Moonrise New Frontiers mission.
Over four billion years ago, an impact
on the early moon excavated the largest crater in the solar system creating the
South Pole-Aitken Basin. The impact exposed
buried layers within the crust and appears to have dug into the mantle below. Later volcanic eruptions occurred over
portions of the basin creating small mare.
Subsequent impacts are believed to have spread material that resulted
from these events across the basin. A kilogram
or two of soil (technically, regolith) from this basin should include samples
from across this basin that would record how the moon formed and the role of
cataclysmic impacts in the history of the terrestrial planets.
The Moonrise lander would touch down
within the basin. Its robotic arm then would
sieve the surface to collect thousands of tiny (3-20 millimeters) rock fragments because those are what are needed to determine
ages and to identify deep-seated materials excavated by the impact. A small bulk sample of the regolith
would also be collected. Each of these rock fragments carries
the story of its creation in its composition.
An ascent
vehicle would return these samples to Earth where each rock fragment can be
analyzed in detail. Scientists would be
able to place the revealed history of each fragment in the context of the
evolution of the moon and the evolution of the Solar System in its first 500
million years using the extensive remote sensing data collected by lunar
orbiters and samples from other sites.
In the last two New Frontiers
competitions, the Moonrise mission was a finalist for both.
Comet Surface Sample Return
 |
| The CONDOR mission would return to comet 67P/Churyumov-Gerasimenko (shown here) to collect two samples for study in terrestrial laboratories. The CORSAIR mission would return studies from a comet, 88P/Howell, that has never been visited by a spacecraft. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; processing by Giuseppe Conzo |
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 at least 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 depth 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.
 |
| Mission timeline for the proposed SPRITE mission to deliver a probe into the atmosphere of Saturn. Credit: SPRITE team/JPL/NASA. |
Ocean
Worlds
Saturn’s moons Enceladus and Titan
have two proposed missions between them, although for one Enceladus mission, I
have found only its name, Enceladus Life Signatures and Habitability (ELSAH). The
proposers for the other mission, the Enceladus Life Finder (ELF), on the other
hand, have provided considerable information their concept. The Cassini mission discovered that this moon
is venting material from its internal ocean.
Extensive studies of the plumes suggest that this world may have the
conditions needed to host life.
Cassini’s now-vintage 1990s era instruments, though, left key questions
unanswered. Because the Cassini
spacecraft performed extensive studies of Enceladus, the ELF spacecraft could
focus its studies on what is perhaps the biggest question of all, is there life
elsewhere in the universe?
The ELF spacecraft would enter Saturn
orbit and repeatedly fly through this moon’s plumes to determine whether its
oceans have the conditions needed to be habitable and whether there are complex
organic molecules suggesting pre-biotic chemistry or possibly life. The spacecraft would carry three mass
spectrometers to study the composition of the gases and ice and dust particles
in the plumes. The measurements from
these instruments would be far more sensitive than their counterparts on the
Cassini spacecraft allowing the mission to address subtle questions of
habitability and life. A camera will
also take what the mission’s principle investigator, Jonathan Lunine of Cornell
University, says will be spectacular images.
 |
| The ELF mission would make ten passes through the plumes of Enceladus to investigate that moon’s habitability and search for signs of life. Credit: ELF team/JPL/NASA. |
The two proposed Titan missions would
have more wide-ranging goals than any of the other New Frontiers
proposals. Both seek to explore Titan as
a diverse world.
The Oceanus mission would target the
ocean world Titan first through a series of flybys and then from two years in
orbit. The spacecraft would explore the creation of complex organic
molecules by skimming through the upper atmosphere and measuring the
composition using a mass spectrometer that has an order of magnitude greater
sensitivity, mass resolution, and mass range than the one the Cassini
spacecraft carries. The principle investigator for this proposal,
Christophe Sotin with the Jet Propulsion Laboratory, wrote me, “What I find
fascinating is that both early Earth and Titan have methane in the upper
atmosphere. We have very little information about what happened on Earth as
life was emerging. Observing Titan is a window to early Earth to understand
what kind of organic chemistry happened there and whether it played a key role
in the evolution of life 3.5 billion years ago.” These molecules high in the atmosphere that
the Oceanus spacecraft would measure eventually drift to the surface to create
organic-rich landscapes.
The spacecraft also would use a camera
that observes the surface through the hazy methane-rich atmosphere in three
infrared bandwidths to map the distribution of the organics that fall to the
surface to understand where they accumulate, how they are transported across
the surface, and where they have been eroded. Scientists also would use
the camera to map the variability of water ice exposed on the surface. A
radar instrument and gravity studies would be used together to understand the
structure of the crust and deep interior including the characteristics of the
salty water ocean that lies beneath the crust.
Scientists have two key questions
regarding that ocean. Is it in contact with the rocky core of the
moon? If so, then key silicate elements needed to support life would be
available, as they are believed to be at Europa and Enceladus. And, are
the rich deposits of organics and the liquid water brought into contact through
processes such as water volcanism or deep impact cratering? If complex
organics, water, and silicates are found to mix, the case for Titan as a
possible abode for life would be strong.
As a side note, both the ELF and
Oceanus spacecraft would use solar arrays to generate their power. Just a
few years ago, it was assumed that any mission to the Saturn system would
require radioisotope generators for power. Advances in using solar cells
together with designs for very large arrays reduces both the cost and
complexity of exploring these moons from orbit.
 |
| The Oceanus spacecraft would map large portions of Titan’s surface in three infrared bands at much higher resolution than the Cassini spacecraft. From its orbit about the moon, the spacecraft will be able to image the surface at 25 meter resolution, about four times higher resolution than in the image from the Huygens probe (bottom left). Credit: Oceanus team/JPL/NASA |
The Dragonfly science would be split
between flight and landing. While
flying, it would remotely study the surface below, sample the atmospheric
composition, and profile the vertical structure of the air. The craft could not continuously fly – it
needs to recharge batteries from a radioisotope generator during landings to
store enough energy for the next flight.
At each landing site, it would study
the composition of the organics and ices using both a mass spectrometer and a
neutron-activated gamma-ray spectrometer.
Titan's day is 16 Earth-days long, so the operations timescale is
relaxed and there's plenty of time to make measurements as well as recharging.
Most of the time is spent on the ground, and in addition to the
compositional measurements Dragonfly would use meteorological and seismic
instruments to study Titan's atmosphere and interior.
 |
| The Dragonfly rotocraft could visit a diversity of terrains event within a single region of Titan. From both its remote and surface observations, the mission would be able to study a variety of geological settings including sites with exposed water ice. Credit: Dragonfly team/John Hopkins Applied Physics Laboratory |
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