I'm finding the professor gig is keeping me pretty busy, and posts
here have become scarce. I still do expect to post irregularly here.
You can find many posts of future planetary missions at the
NASASpaceflight.com site, where I post fairly regularly. Two
recommended forums:
Space science coverage
Mars missions
Future Planetary Exploration
Friday, December 21, 2018
Discovery 2019 Competition
NASA recently posted its draft Announcement of Opportunity for its next Discovery competition that will select two missions, one to fly in the mid-2020s and the other in the late 2020s. I am starting to see in scientific conferences missions likely to be proposed (several of which were also proposed but not selected for the last competition that selected the Lucy and Psyche asteroid missions). NASA expects to select up to five finalists for further definition with the final selections from this candidate list.
Mission proposals can target any solar system study, except those that would study the sun or Earth, that, “Ascertain the content, origin, and evolution of the solar system and the potential for life elsewhere.”
Mission costs for the spacecraft, instruments, flight software, and ground systems (the Principal Investigator costs) are capped at $500 million in FY2019 dollars. NASA provides management, the launch, and covers mission operations in addition to the PI costs. These other costs can be substantial. The projected full cost of the Lucy mission is $914-984 million and the Psyche mission is $907-957 million when the PI costs were capped at $450 million in FY2015 dollars.
The PI budget can be adjusted upward by up to $20 million by adding a technology demonstration (see below) or up or down by $10-20M by selecting a less capable or a more capable launch vehicle than the baseline. Instruments, up to approximately one-third of the payload, can be contributed by foreign space agencies or governments outside of the PI cost cap.
The target schedule for the selection and launches of the Discovery missions is:
AO Release Date (target)................February 28, 2019
Proposals due.................................May 31, 2019
Selection of up to 5 finalists............December 20, 2019
Selection of up to 2 final missions...March 31, 2021
1st launch period............................July 1, 2025, through Dec. 31, 2026
2nd launch period...........................July 1, 2028, through Dec. 31, 2029
NASA will select the winning proposals based on three criteria:
• Scientific merit of the proposed investigation (40%)
• Scientific implementation merit and feasibility of the proposed investigation (30%)
• Technical, management, and cost feasibility of the proposed mission implementation (30%)
Proposed missions can use the plutonium-238 MMRTG power systems. However, including these will cost the PI budget $54 million for the first MMRTG and an additional $15 million for the second.
In what I believe is a new opportunity for Discovery proposals, PI’s can propose (or NASA may add), “An Enhancing Technology Demonstration Opportunity (TDO) consists of either PI-team-developed or NASA-developed technologies that may have a TRL of less than 6 [approaching but not yet ready for flight] when proposed, is not required to achieve the Baseline or the Threshold Science Mission, but could enhance the scientific return or reduce cost and complexity of the proposed mission and/or future missions. An Enhancing TDO may be an instrument, investigation, new technology, hardware, or software demonstrated on either the flight system.”
Missions are also expected to, “provide active research opportunities for current or aspiring graduate
or undergraduate students, including advanced high schoolers. SCs may involve students in multiple aspects of a mission spanning scientific formulation; mission planning; systems engineering; design and development of flight hardware; qualification, test and integration; and mission operations and data analysis.”
Sunday, April 29, 2018
A Comet or Titan: The Next New Frontiers Mission
The two
concepts contending to be selected as NASA’s next planetary mission have two
things in common.
Both
would do compelling science.
Both
would not begin their scientific missions until the mid-2030s.
Otherwise
the two missions could not be more different.
The Comet Astrobiology Exploration
Sample Return (CAESAR) mission would return to the comet 67P/Churyumov-Gerasimenko
that the Rosetta spacecraft and its lander Philae previously studied. The CAESAR spacecraft essentially would be a high-tech
truck that would grab a small sample from the surface and then return it to
Earth. Except for some ancillary observations
performed by its engineering cameras, the science would begin when the samples
are delivered to the exquisitely sophisticated instruments in terrestrial
laboratories where they would be analyzed grain-by-grain and
isotope-by-isotope.
The
Dragonfly mission would return to the surface of Saturn’s moon Titan that is at
once both an Earth-like and an utterly alien world. The craft would alternate between flights to
new locations and much longer periods as a stationary surface station. Every flight would be a voyage of discovery
above a barely known surface. Every
landing could provide a new site to explore in depth. Every minute of its potentially many year
sojourn at Titan would be spent doing science.
Both
missions are finalists in the competition to be NASA’s fourth New Frontiers
mission. (Previous selections were the
New Horizon’s Pluto-Kuiper belt spacecraft, the Juno Jupiter orbiter, and the
OSIRIS-REx asteroid sample return mission.)
These are the mid-class missions of NASA’s solar system exploration
program that are launched once to twice a decade. NASA’s managers selected these two concepts
from a field
of twelve proposals (and here).
The
CAESAR and Dragonfly missions differ so much because they represent different stages
in the progression of exploration.
Comets have already had several flyby missions and one orbiter-lander
mission. As a result, the top priority
among comet specialists is to progress to the next step, sample return. Further exploration isn’t the priority. The lowest risk mission that grabs and brings
back the sample is the best mission.
Titan, on
the other hand, was studied by the Cassini spacecraft through a multitude of
flybys that cumulatively mapped the surface at only low to moderate resolution. (Titan’s atmosphere was also explored by the
Huygens probe that descended to the surface, but it carried out only the
simplest measurements of surface’s physical properties once it landed.) The next step is to land to study the surface
in detail. Titan offers a unique
opportunity in the solar system where a thick atmosphere coupled with low
gravity allows a science station to relocate itself by flying to new
sites. For this world, exploration is
the priority.
CAESAR
Comets
are deep frozen remnants from the birth of our solar system. They contain relatively unaltered ices and
dust from the interstellar cloud from which our solar system formed as well as
the processed material that resulted from the earliest steps in the formation
of comet-sized bodies. By returning
samples to Earth, scientists can determine what materials were available to
build the planets. They also can study
the earliest chemical pathways that led to creating more complex molecules and
the physical processes that accumulated the interstellar material into larger
bodies. As one recent scientific paper
put it, “comets record chemical evolution in the protoplanetary disk and allow
us to unveil the formation history of the organics and volatiles,” while
another in the same issue notes that the Rosetta spacecraft found that comet
67P, “revealed a greater variety of molecules than previously identified and indicated
that 67P contained both primitive and processed organic entities.”
The CAESAR mission would return to comet 67P/Churyumov-Gerasimenko. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; processing by Giuseppe Conzo |
However, instruments
that can fit the tight mass, size, and power budgets to be flown on spacecraft are
limited in their ability to explore the composition of these materials. The far more sophisticated instruments in
Earth-based laboratories – which can be larger than entire spacecraft or even
launch vehicles – can study samples grain-by-grain and isotope-by-isotope. The ultimate exploration goal, where
technically feasible, for each world is to bring samples back to our world to
explore their formation and evolution in depth.
The team proposing the CAESAR mission
has laid out a series of hypotheses that would be enabled by examining samples
in terrestrial laboratories. Arranged in
three themes, the hypotheses address different periods in the formation of the
solar system starting with the transition from the interstellar cloud from
which the solar system formed. The
hypotheses for this theme address questions on what types of unaltered interstellar
material were preserved (or not) in 67P.
The next theme addresses the nature of the material in the
protoplanetary disk and hence the material that would have been available to
build the earliest small bodies. Here
the key questions focus on whether 67P is a direct descendant of the
interstellar medium or also contains high temperature silicates and organics
that would have formed close to the early sun and then been blown out to the
outer solar system and trapped within the material that formed 67P. The final theme addresses how the current
body that is 67P formed from the initial grains of ice and rock available in
the earliest solar system, and what role bodies like it played in delivering
materials, especially volatiles such as water, to the planets.
The
CAESAR mission would be a companion mission to the previously selected, and now
in flight, OSIRIS-REx New Frontiers mission.
While comets are the oldest frozen remains of the solar system’s
formation, several classes of asteroids are considered primitive relics of the
earliest formation of solid bodies closer to the sun. The OSIRIS-REx spacecraft will arrive at one
of these, 101955 Bennu, later this year.
Following a thorough examination of this tiny (~245-meter diameter) body
by a battery of remote sensing instruments, the spacecraft will descend, and a
robotic arm will grab a sample from the surface to be returned to Earth.
Collecting
samples from very small bodies is an engineering challenge because the gravity
is too low to enable conventional landings and surface operations. Like the Japanese Hayabusa and OSIRIS-REx
asteroid sample return missions, the CAESAR spacecraft would use a touch-and-go
sampling mechanism. The spacecraft would
slowly descend to the surface with a sample collection system at the end of a
long arm below the craft. During the
five seconds that the collection system would be in contact with the surface,
nitrogen gas would be used to blow the comet’s surface material into a
containment system. Because comets
contain ices that can freeze into a solid surface, the CAESAR collection system
(unlike the OSIRIS-REx system where ices aren’t expected) would have spring
loaded ripper tines to break up the surface.
The mission’s goal is to collect at least 80 grams of dust and ices,
although the collection system can gather several times that if enough loose
material is available. After that brief
contact, the arm acts as a pogo stick pushing the spacecraft away from the
surface and back into space.
The CAESAR spacecraft with its extended arm and sampling mechanism. Credit NASA. |
Again, like
OSIRIS-REx, following confirmation from instruments that a sample was
successfully collected, the containment system with its sample would be placed
within a sample return capsule that ultimately would deliver the sample to
Earth following a fiery plunge through the atmosphere. However, this capsule and its embedded sample
containment system have two major innovations not needed by the OSIRIS-REx
mission.
For the
CAESAR mission, a key goal is to return both the dusty component and minimally
modified samples of the icy volatile material.
Once in the sample return capsule, the sample would be gently heated to
approximately -30 degrees Celsius to sublimate the volatiles out of the
sample. (If -30 sounds chilly, remember
that comets are deep frozen relics even following heating by the sun during
their passages through the inner solar system.)
The sublimated gases would flow to a gas containment system that would
be kept at -60 degrees Celsius where they would either refreeze (for example,
water) or remain as gases (for example, methane and carbon dioxide) depending
on their freezing points. Removing the
volatiles from the sample ensures that they don’t chemically modify the solid
sample during the return flight to Earth.
Keeping them chilled in the gas containment system also minimizes
chemical interactions among the volatiles.
The need
to preserve a frigid sample system drives a second difference from the
OSIRIS-REx return system. During the
plunge through the Earth’s atmosphere at the end of the mission, the heat
shield would absorb considerable heat that would eventually raise the
temperature of the sample. For the rocky
material that the OSIRIS-REx mission would return, this isn’t a concern. However, the CAESAR mission would drop the
heat shield from the return capsule as soon as the parachute deploys,
minimizing any heating of the sample.
(This is a technology developed by the Japanese for their Hayabusa
asteroid sample return missions, and their space agency would provide the
CAESAR return capsule.)
There’s
another important difference between the OSIRIS-REx and CAESAR missions. To intelligently select and later interpret
samples, scientists need to understand the parent body: What is its shape and
interior structure? What processes have
shaped it surface? How does the composition vary across the body? The OSIRIS-REx mission is going to a body
that has never been visited before. As a
result, that spacecraft carries a rich payload of cameras and spectrometers to
map the physical surface and its composition.
If the mission only completes its survey of the asteroid Bennu but for
some reason fails to return its sample to Earth, it would have told us a great
deal about how this tiny primitive asteroid came to be.
The
CAESAR mission’s target, comet 67P, was previously studied by the Rosetta
mission and its partially successful Philae lander. As a result, the CASEAR spacecraft wouldn’t
need to carry its own scientific instruments.
It does carry narrow- and wide-angle engineering cameras that would be
used to remap 67P’s surface, which is likely to have changed during its
subsequent perihelion passages from solar heating. (Identifying the changes would be a useful
scientific byproduct.) From these
images, the science and engineering teams would map the surface in terms of how
safely the spacecraft can approach the surface, whether the surface material
has physical characteristics compatible with the sampling mechanism, whether
the arm can deliver the sample mechanism to the surface, and the intrinsic
science value.
Based on
maps derived from the Rosetta images, the CAESAR team is currently expecting to
take its sample from one of the smooth terrains that appear to consist of
material that has slumped there from the rugged terrains that would be unsafe
to sample directly. The smooth terrains also
likely contain material delivered from the jets that spew a cloud of volatiles
and dust in the space around 67P each time it comes close to the sun. Because the rocky and organic materials
appear to be nearly uniformly distributed across surface, the key deciding
factor on the final sample location likely would be made based on the richness
of the volatiles present.
Once the
CAESAR spacecraft acquires its sample, it would quickly move away from the
comet to wait for the start of its journey back to Earth. If all goes well, the sample would be
delivered to the Earth’s surface in the midmorning of November 20, 2038,
fourteen years after the mission’s launch.
Following the recovery of the sample capsule, the returned gases, ices,
and solids would be carefully curated and preserved. The initial analyses on a portion of the
sample would be conducted using laboratory instruments that would be 20 years
more advanced than those available today.
It’s expected that the samples would continue to be analyzed for decades
following their arrival on Earth as scientists develop more refined questions
and yet more advanced instruments.
Dragonfly
While
comet P67 is a well explored world, the surface of Saturn’s moon Titan largely remains
a blurry enigma. Thanks to the Cassini
orbiter’s many flybys of the moon, we have low- to moderate-resolution maps of
the surface that show seas of liquid methane and ethane, extensive dune fields,
mountains, a paucity of craters, and river systems. We similarly have low resolution maps of
surface composition differences that are frequently ambiguous as to the actual material
present. We know that sunlight
interacting with the atmosphere creates complex hydrocarbons and nitriles that
continuously rain down from the upper atmosphere to the surface. Cassini’s instruments have observed changes
in the surface following rains of liquid methane. There’s an interior water ocean that may be
in contact with the silicate core that could, like the interior oceans of
Europa and Enceladus, be an abode for life.
The Dragonfly rotorcraft shown in its initial landing in a flat area between Titan’s equatorial dunes. Credit: NASA |
What we
know about the surface becomes the foundation for the fundamental questions
that require a return visit. What processes have reshaped the surface,
erasing, except in a few terrains, the many craters that should have accumulated? Why hasn’t the rain of hydrocarbons buried
the surface over the age of the solar system, and where does new methane come
from to replace that lost to the formation of the hydrocarbons? Are there cryovolcanoes that bring liquid water
to the surface? What types of pre-biotic
chemistries occur on a world rich in hydrocarbons? And is there evidence of life, either that
formed on the surface from the interaction of water and the hydrocarbons or
carried up from the deep subsurface global ocean? Or is there life based on non-water
chemistries?
These
questions cannot be answered by landing at any single location on Titan. Fortunately, this moon’s thick atmosphere and
low gravity make this the easiest world in the solar system for flight. The team proposing the Dragonfly mission have
designed a rotorcraft lander that can fly several tens of kilometers in a
single flight to a new site. In an hour
or so of flying, the craft might travel further than the Opportunity rover has
crawled across the surface of Mars in fourteen years. Over the course of several years at Titan,
the craft could explore a variety of terrains hundreds of kilometers apart.
The
Dragonfly proposal is possible because of the recent advances in multirotor
technology to enable steady flight (think of all the quadcopter hobby drones as
inexpensive examples) and autonomous flight.
Cassini’s maps of the surface enable scientists to identify interesting
locations to visit but are too low resolution (the smallest details are
hundreds of meters across) to be useful to for inflight navigation. On each flight, Dragonfly would be given a
target location to fly to. During the
flight, a combination an inertial measurement unit, radar, and optical
navigation would be used to track progress.
Once at a landing area, a laser system (lidar for those familiar with
the technology) would map the site to find a safe touch down site.
On arrival
at Titan, the Dragonfly craft, safely enclosed in its entry capsule, would
directly enter the atmosphere. Once the
initial descent is completed, the Dragonfly rotorcraft would separate from the
capsule and guide itself to its first landing.
Much of Titan’s equatorial regions are covered by dunes that crest at 50
to 200 meters separated by flat plains two to four kilometers wide. The craft would use its sensors to guide
itself to one of these interdune areas to enable that crucial safe first
landing.
The sands
themselves are known to be composed of frozen organic particles that likely are
material blown in from large areas of the moon’s surface. With its first landings in the dune sea, the
Dragonfly lander can explore how far organic chemistry has proceeded on this
hydrocarbon rich world. The interdune
plains may also contain exposed water ice that is the bedrock of Titan’s
surface. Sampling the primordial water
ice crust is a high priority to explore the chemistry present when Titan formed
and the surface and atmosphere may have been much different than they are
today.
Because
Titan’s surface is likely rugged at small scales, the Dragonfly team won’t send
the craft to land at an unseen location (except for that first landing). Instead, on its first flight following the
initial landing, the craft would be sent to scout for a next safe landing site
and then would return to the initial (and known to be safe) landing site. On the second flight, the Dragonfly craft would
fly past the second known safe landing site to scout a third, and then return
to the second site. In this way, the
mission would leapfrog from one known safe site to the next, using its sensors
to make up for the lack of high resolution orbital imagery.
Beyond
the dune seas, Titan has a rich variety of terrains to explore. Life on the Earth is based on organic
chemistry enabled by liquid water. The
surface of Titan is too cold for liquid water – the water ice is as hard as
rock – but impacts would melt the ice, mixing the hydrocarbons with liquid H2O
for as long as tens of thousands of years.
Where liquid water and the organic material on the surface mixed, complex
pre-biotic chemistry likely occurred, making Titan a natural laboratory to
explore the possible origins of life. These
sites would provide natural laboratories to explore the resulting pre-biotic
chemistry and explore avenues for the origins of life.
Cassini’s
data hint at features that may be extensive cryovolcanic flows where water from
the interior ocean reached the surface. If
the liquid water originated in the deep interior ocean, sampling these areas
may allow the Dragonfly instruments to determine the composition of that ocean,
including searching for signs of any life that may exist there.
While
liquid water on the surface has likely been a rare event, liquid methane plays
the role on Titan that water does on Earth.
The methane falls as rain, flows through river systems, and ends in
lakes and seas. Close to the equator
where the Dragonfly craft would roam, it appears that the seas have largely
dried up. The evaporation of the seas
could have concentrated chemical reactions, and the present dry-to-moist
surface would preserve the resulting material for Dragonfly’s instruments to
sample. Several areas near the equator
have been identified as possible dry sea beds.
(The currently-filled methane
seas in the north polar region would be in darkness and out of sight of Earth
for telecommunication during the Dragonfly mission. Their exploration would need to wait for
another mission a decade or so later).
The
Dragonfly’s explorations would be tempered by the need to carefully manage its
power use. The craft would carry a
radioisotope power source that at Titan would supply a constant ~70 Watts of
electrical power. (And as importantly, it
provides plentiful supply of waste heat to keep the craft warm on the -180-degree
Celsius (-290-degree Fahrenheit) surface.)
That supply is too low for the major power-hungry activities – flight
and returning data to Earth – but is enough to recharge a large battery. Operations would proceed at the pace of each
Titan day (around 16 Earth days). During
the night, the battery recharges, followed by a daytime flight and a series of
science activities and data downlinks. During
the long night when Earth is out of sight, the craft would power down many
systems to keep its meteorology and seismic instruments powered and permit
battery charging. (The Curiosity rover
on Mars is similarly power limited. Its
radioisotope power source charges a battery that powers driving and many operations. The difference is that days on Mars have a
similar length to our own.)
The
public information on how far the Dragonfly craft might go in a single flight
is somewhat vague – the design is not yet complete and there’s a tradeoff
between larger batteries and weight and volume.
One article discusses an example battery that could allow up to 60
kilometers flight. It then states that
flights would be kept substantially shorter to provide a safety margin. If the average distance between landing sites
is 20 kilometers when the goal is to transit distances, then over a five Earth
year mission, the craft might fly 1500 to 2000 kilometers. Not all Titan days, though, are likely to be
spent with the goal of putting kilometers on the odometer. If the craft finds a cryovolcanic volcano or
flow, for example, it may spend many months doing short flights to different
locations within the flow. Some Titan
days may be spent doing aerial reconnaissance to map the feature. Other Titan days might be spent doing bunny
hops between features in a single landing area.
Rather
than thinking of the Dragonfly craft as a global trekker, it seems more apt to
think of it as performing the equivalent of the missions of several rovers on
Mars. It would be able to transport
itself between and within several key landforms within a region of the
moon. Choose that region carefully,
though, and there’s a rich diversity to explore. Within the black box on the map above lie
seas of sand dunes, several features that may be cryovolcanic, possible dried
sea beds, and what may be one of the oldest exposed terrains on the surface in
the Xanadu highlands with its mountains, river channels, and possible impact
craters.
During
its flights, a set of cameras would look forward and downward during flights to
map the. The Huygens probe showed that
surface features can be distinguished in monochrome aerial images, and Dragonfly
would have some color imaging capability to discriminate differences in
composition. The mission’s operators,
though, would need to balance the number of images returned with other demands
on the mission’s power supply. The
strategy would be to return thumbnails of images, from which the mission team
would select a subset to be returned at higher resolution.
After
each landing, panoramic cameras would examine the local area while microscopic
cameras take close-up images. (Depending
on the wavelengths that the cameras can detect, Saturn may be visible in the
images in one of the spectral windows in which the atmosphere is
transparent.) Once on the surface, a
gamma-ray and neutron spectrometer would measure the bulk surface composition
to discriminate between, for example, pure water ice, ammonia-rich water ice,
and a layer of hydrocarbons.
Several
instruments would study meteorology both in flight and on the surface including
temperature, wind, and the methane humidity.
Two instruments would listen for seismic activity and measure variations
in the extremely low frequency electromagnetic field to study the subsurface to
the depth of the interior water ocean.
Other sensors would record the thermal and electrical properties of the
surface immediately around the landed craft.
The
mission’s star instrument, however, would be its mass spectrometer. This instrument, derived from similar
instruments for the Mars Curiosity and the European ExoMars rovers, would be
able to detect the presence of different ices and hydrocarbons at minute
levels. By measuring the precise
chemistry of ices from different locations, scientists can understand the
processes that created them.
This
instrument’s sensitivity would make Dragonfly an exciting astrobiology mission. It could, for example, measure tiny amounts
of key organic compounds such as amino acids, lipids, and sugars, as well as
the chirality of organic molecules. At
the minimum, Titan should have a treasure trove of pre-biotic material that
scientists would use to explore the chemical pathways leading to the complexity
of life. And Titan is a possible abode
for life, either as-we-know-it based on water in the deep ocean or novel forms
of life based on liquid methane. Proving
that life exists at the cellular level likely would be hard, but Dragonfly’s
measurements could be the bridge to future, even more sophisticated missions.
-----------------
Both CAESAR
and Dragonfly would be compelling missions.
It is unfortunate that only one will be selected and the next
competition is likely to be several years in the future.
Launch
for the CEASAR mission would occur in the summer of 2024 and arrive at comet 67P
in December 2028. The spacecraft would
depart from the comet in late 2033 and return to Earth five years later.
Launch
for the Dragonfly mission would occur in 2025 with arrival at Titan in the mid-2030s. Absent a crash or equipment failure, the
length of the mission would be dictated by the slow decay of power delivered
from its power supply. Eventually it would
take two, then three, and then more Titan days to recharge the battery. At some point, power would be too low to
enable any flight. Then the craft would
become a stationary science station as power gradually fades until it becomes
silent likely sometime in the late 2030s or 2040s.
The
decision between these two excellent but fundamentally different missions will
be made in mid-2019 by NASA’s Associate Administrator for Science. His decision will determine whether the
2030s brings a flowering of science for comets or Titan.
I would like to thank Steve Squyres, principal investigator for the CAESAR proposal, and Elizabeth (Zibi) Turtle and Jason Barnes, PI and deputy PI for the Dragonfly proposal, for their comments on an earlier draft of this story.
I would like to thank Steve Squyres, principal investigator for the CAESAR proposal, and Elizabeth (Zibi) Turtle and Jason Barnes, PI and deputy PI for the Dragonfly proposal, for their comments on an earlier draft of this story.
Sunday, October 1, 2017
Venus Origins Explorer New Frontiers Proposal
When
I wrote my blog post
on the New Frontiers mission proposals, I could identify eleven of the
twelve proposals. One, though, then had
no public trace. Shortly after my piece
was published, several people emailed me to tell me that the twelfth proposal
was a third Venus mission called the Venus Origins eXplorer (VOX). However, they offered no details on its
implementation.
Now
the team proposing the mission will present its implementation at an upcoming
scientific conference. Their abstract
for the talk reveals key facts about the proposal, and it’s a gutsy break with
the missions that have been assumed for New Frontiers Venus missions.
To
understand the strategy the VOX team is following, it’s useful to look at the
history of the Juno Jupiter orbiter mission.
Every ten years, the planetary scientific community establishes its
priorities for the exploration of the solar system in process known as the
Decadal Survey. The 2003 Decadal Survey
report called for a Jupiter Polar Orbiter with Probes mission. The orbiter was to study global patterns and
the magnetosphere while three atmospheric probes were to examine the
composition and deep structure of the atmosphere. Unfortunately, since the Galileo mission with
its shallow atmospheric probe, the United States has lost the technology to
build probes capable of entering Jupiter’s atmosphere. The team that proposed the Juno mission focused
on the desired scientific goals and demonstrated that they could be met with
just an orbiter with the right instruments.
Based on that well-presented argument, the mission was selected and is
now orbiting Jupiter.
The
goals for a New Frontiers mission for Venus have assumed an atmospheric probe
that would also conduct surface measurements once it landed. The scientific requirements NASA’s managers
established for a mission appear tailored for that kind of a mission based on
the goals set out by the more recent 2013 Decadal Survey report. For example, the ratio of key gasses and
their isotopes can only be made by a probe that enters and directly samples the
atmosphere. With those measurements,
other questions such as past hydrologic cycles and the existence of now lost
oceans can be addressed. During its
descent the probe could measure the properties of the atmosphere, its clouds,
and its winds at different altitudes.
Measurements of the atmosphere at the surface could address the
weathering environment of the crust.
Chemical analysis of the soils and rocks at the landing site could
provide insight into the physics of and chemistry of the crust.
Two
of the proposed New Frontiers missions would fly either one or two entry
probe-landers to address these questions.
(You can see summaries of these at this blog
post.)
The
VOX proposal takes an entirely different approach. As the abstract states, “At the time of the Decadal Survey the ability
to map mineralogy from orbit and present-day radar techniques to detect active
[surface] deformation were not fully appreciated. VOX leverages these methods
and in-situ noble gases to answer [the key] New Frontiers science objectives.”
The VOX mission would
deploy a small, simple atmospheric probe to make the measurements of the key
gases and isotopes. The rest of the
measurements would be made from instruments on an orbiter, much as the Juno
mission uses orbital measurements to replace measurements from atmospheric
probes. (The composition measurements for
Jupiter that could only be made from within an atmosphere had already been made
by the Galileo mission’s atmospheric probe.)
The
key elements of the VOX proposal closely resemble the VERITAS mission that was
a finalist for the last Discovery program mission but not selected (losing out
to the Lucy and Psyche asteroid missions).
The principal investigator for VERITAS and VOX are the same, Suzanne
Smrekar at the Jet Propulsion laboratory.
The core of the VERITAS proposal was an orbiter that would carry a
modern radar instrument to remap Venus and a spectrometer to measure the surface
composition and study key aspects of the atmosphere. If selected, the VERITAS mission team was
considering proposing a small atmospheric probe call Cupid’s Arrow to toe-dip
into the atmosphere to make key composition measurements. A version of that probe now called the
Atmospheric Sample Vehicle (ASV) would be included in the VOX mission.
The
VOX orbiter would use three instruments to globally study Venus:
A
radar instrument would image the surface at 15- to 30-meter resolution, map
elevations to refine topography measurements, and search for minute changes in
surface elevations to look for evidence of current volcanic or tectonic
activity. The improvements in resolution
for each of these would be one to two orders of magnitude better than those
made by the Magellan orbiter in the 1990s.
The
orbiter’s radio signal would be used to map the gravity field at much higher
resolution than the Magellan mission to study the interior structure of the
planet
The
Venus Emissivity Mapper (VEM) would use spectral bands in five near-infrared wavelengths
where the atmosphere is transparent to map the surface composition. Measurements in additional bands would study
cloud structure and the presence of water vapor in the lowermost atmosphere.
The
ASV would be a small probe released from the orbiter. The probe would enter and briefly traverse
the upper atmosphere where it would collect a small sample of the gasses
present before its momentum carries it back into space. Once back above the atmosphere, a
miniaturized mass spectrometer would measure the composition of key gasses and
their isotopes. By only toe-dipping into
the atmosphere, the probe doesn’t need protection from the crushing pressures
and extremely high heat on the surface that the atmospheric probes of the other
New Frontiers Venus proposals require.
The VOX mission's Atmospheric Sample Vehicle would be based on the Cupid Arrow probe concept. Credit: NASA/JPL. |
While
the VOX mission is in competition with the two Venus entry probe-lander
proposals (and nine other proposals for other solar system destinations), in
many ways it is a compliment to those two missions. Within the column of air in which they
descend and in a square meter or so at their landing sites, the two proposed
entry probe-lander missions would make much more detailed measurements than
VOX’s radar and VEM instruments would from orbit for the same location.
The
VOX spacecraft’s instruments, however, would make those measurements over the
entire planet over at least three years (and possibly much longer in an
extended mission). The VEM instrument,
for example, could look at the surface of every volcano on Venus searching for
the presence of minerals suggesting recent lava flows. The radar images could show whether the
features on the surface are consistent with new flows. Radar interferometric measurements could look
for small surface deformations that could reflect surface movement from
earthquakes associated with eruptions or swelling from underground lava
movement. VEM measurements of the
atmosphere could look for changes in water vapor associated with volcanic
outgassing. And the gravity field
measurements would tie each volcano to the interior structure below the
surface.
The
VOX mission would make comparable studies for all the surface types on Venus as
well as monitor the lower atmosphere.
Perhaps most important of these would be observations of the tessera, which are continent-sized highlands
that likely are the oldest surviving surfaces on the planet. For these, the synergy of composition
measurements, surface imaging, and interior structure measurements could reveal
key aspects of the earliest history of Venus including the role of liquid water
and other volatiles in the then cooler times.
In
one set of measurements, the VOX and the two Venus entry probe-lander missions
are in direct competition. In much the
same way as strata of rock record geologic history, the presence and abundance
of key gasses and their isotopes record the origin and evolution of the
atmosphere. Because the Venusian atmosphere
is in contact with the surface, it also records key geological events. Measurements of xenon and its isotopes, which
have yet to be made at Venus, for example, would resolve key questions about
the origins of the atmosphere. Xenon
measurements also would reveal the total amount of degassing from the planet’s
interior, which reflects the cumulative volcanic activity. Similarly, a long-lost ocean at Venus would be
reflected in the ratio of hydrogen isotopes.
The
US Pioneer Venus and Soviet Venera entry probe and landers carried
now-antiquated mass spectrometers that could not make key composition
measurements such as the isotope ratios of xenon. The VOX mission’s atmospheric probe would
deliver a modern mass spectrometer to Venus that would answer the key first
order atmospheric composition questions.
The other two proposed Venus missions would carry much more sophisticated
instruments that could also address second and third order questions. By descending all the way to the surface, the
probes on these other missions would also study changes in composition at
different layers of the atmosphere and look for compounds that reflect the
current weathering at the surface.
(As
a side note, I’ve been following proposals for missions to explore the solar
system for several decades now. Cupid’s
Arrow, and its incarnation as VOX’s ASV, is in my opinion one of the cleverest
solutions I’ve seen to addressing a key set of planetary science questions at a
fraction of the cost of much more capable entry probes like those that would be
the core of the other two Venus proposals.)
To
understand Venus, eventually it’s not a question of a VOX-like mission versus
entry probe-lander missions. Their
capabilities are complimentary – in depth at one or a few locations versus
broader measurements across the entire globe across a number of years. The interpretations of measurements from orbit,
especially those of composition, would greatly benefit from ground truth
measurements on the surface. Eventually,
we need both types of missions to fly. (And
there’s also an entire branch of Venus science focused on global atmospheric
studies from orbit that neither of these types of mission addresses.)
In
the last New Frontiers competition, a Venus entry probe-lander was a finalist
but lost out the OSIRIS-REx mission now inflight to return a sample from the
asteroid Bennu. For the current
Frontiers competition, NASA will select just one mission from among three proposals
for Venus, one to return samples from the moon, three to return samples from a
comet, and four to study Saturn or its moons.
It’s a tough field. By the end of
this year, we’ll learn whether VOX is selected as a finalist in the current
competition with the final selection planned for 2019.
Appendix:
Here is the full list of studies proposed for the VOX mission from the
conference abstract:
1. Atmospheric physics/chemistry:
noble gases and isotopes to constrain atmospheric sources, escape processes,
and integrated volcanic outgassing; global search for current volcanically
outgassed water.
2. Past hydrological cycles: global tessera composition to determine the role of volatiles in crustal formation.
3. Crustal physics/chemistry: global crustal mineralogy/chemistry, tectonic processes, heat flow, resolve the catastrophic vs. equilibrium resurfacing debate, active geologic processes and possible crustal recycling.
4. Crustal weathering: surface-atmosphere weathering reactions from redox state and the chemical equilibrium of the near-surface atmosphere.
5. Atmospheric properties/winds: map cloud particle modes and their temporal variations, and track cloud-level winds in the polar vortices.
6. Surface-atmosphere interactions: chemical reactions from mineralogy; weathering state between new, recent and older flows; possible volcanically outgassed water.
2. Past hydrological cycles: global tessera composition to determine the role of volatiles in crustal formation.
3. Crustal physics/chemistry: global crustal mineralogy/chemistry, tectonic processes, heat flow, resolve the catastrophic vs. equilibrium resurfacing debate, active geologic processes and possible crustal recycling.
4. Crustal weathering: surface-atmosphere weathering reactions from redox state and the chemical equilibrium of the near-surface atmosphere.
5. Atmospheric properties/winds: map cloud particle modes and their temporal variations, and track cloud-level winds in the polar vortices.
6. Surface-atmosphere interactions: chemical reactions from mineralogy; weathering state between new, recent and older flows; possible volcanically outgassed water.
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