Saturday, May 2, 2015

Mars Plans Advance (and occasionally fade)



Mars has the twin attributes of being close by (at least by solar system standards) and retaining a record of its earliest epoch (lost on Earth) when life might have formed.   These have made it a popular destination with five orbiters currently operating around it and two rovers driving across its sands.  At least as many new missions are in various stages of development or are proposal, ranging from hardware headed for the launch pad in a few months to some that eventually may prove to be no more than vaporware.

In the last two months, there have been significant news about the European-Russian 2018 mission and about NASA’s 2020 rover.  NASA also has announced that it would like to send a new orbiter to the Red Planet in the early 2020’s.  These announcements will be the meat of this blog post, but first I’ll quickly run through the status of other planned and proposed missions.

Assembly of the 2016 Trace Gas Orbiter and Schiaparelli demonstration lander.  Credit:ESA
Six craft to launch as four missions are firmly in development and have fully funded budgets.  Europe’s Trace Gas Orbiter and its Schiaparelli technology demonstration lander are in assembly and on track to launch next January.  NASA’s InSight geophysical lander also is in assembly for its launch next March, although the mission’s principal investigator reports that the schedule is tight.  Design of the 2018 ExoMars European rover and Russian lander are on track as is NASA’s 2020 rover

Icebreaker concept.  Credit: McKay/NASA

It’s likely that another mission to return to the Martian northern polar plains has been proposed for the NASA Discovery program.  The Phoenix lander explored these regions, but was frustrated by clumpy soils that made it difficult to deliver samples to its instruments.  What the Phoenix spacecraft did find was a layer of ice just below the surface dust that could provide a habitat for life.  The proposed Icebreaker mission would follow up on the Phoenix mission with a sampling system that would drill well into the ice and is designed to work with the clumpy soil.  The lander, which would be a near copy of the Phoenix and InSight landers, would carry new instruments that would search for signs of life.  While this proposal has received considerable publicity, I haven’t heard whether it was actually proposed.  Sometimes, proposers learn as they develop their plans that their missions would not fit within the tight budgets of Discovery missions.  (I’ve heard of one proposal that I was excited about that wasn’t submitted for the current Discovery selection for this reason.)  If the Icebreaker mission was proposed and is selected (beating out 27 other proposals), it would launch in 2021.

China announced plans a few months ago for its own Martian rover mission to launch in 2020.  More recently, a Chinese official stated that the budget for this mission was unlikely to be approved in time for a 2020 launch.

There have also been press accounts that India is considering a second Mars mission that might be an orbiter and/or a lander.  I haven’t heard whether the budget for a follow on mission has been approved or not.

And now on to the major announcements of the last couple of months.

The 2018 ExoMars mission will use a Russian landing system and platform to deliver a European rover to the surface.  Russia has planned to use the landing platform as a scientific station after the rover rolls off it.  Until recently, I’ve been unable to find any details about the planned experiments.  Now an announcement of opportunity has been issued for European scientists to contribute to Russian-led instruments and to propose their own additions (see here). 

The Russian landing stage and long term science station with the European rover on top prior to its deployment.  Credit: Russian Academy of Science Space Research Institute.

The documents state that the priorities for the stations are:

“Priority 1:
• Context imaging;
• Long-term climate monitoring and atmospheric investigations.
Priority 2:
• Studies of subsurface water distribution at the landing site;
• Atmosphere/surface exchange;
• Monitoring of the radiation environment
• Geophysical investigations of Mars’ internal structure.”

The documents lists the names only for an ambitious suite of instruments, although it’s not always clear what instruments are already firmly planned versus those that might be added by European scientists.  The instruments break down into several groups:

Camera

Meteorology and atmospheric science: Meteorological package, multi-channel Laser Spectrometer, IR Fourier spectrometer, atmospheric dust particle instrument, and a gas chromatograph-mass spectrometer to study composition.

Ground and shallow below ground: Active neutron spectrometer and dosimeter, radio thermometer for soil temperatures

Geophysics: Magnetometer and seismometer

This suite would be a highly capable science station.  For example, the station will monitor both the physical state of the atmosphere (temperature, pressure, dust load, etc.) as well has changes in its composition (presumably with a focus on changes in trace gases to provide ground truth measurements for the 2016 Trace Gas Orbiter).  The listed target weight for the seismometer suggests a simpler instrument than the InSight lander will carry.  Having a second seismometer would help geophysicists narrow down the source locations of Mars quakes.  The sensitivity of this new seismometer may be limited if there isn’t a way to lower it to the ground to isolate it from the vibrations within the station.

What I am surprised by is that the call for instruments includes requests for significant pieces of hardware to be supplied by European scientists for Russian-led instruments.  In terms of instrument and spacecraft development, 2018 is practically around the corner.  I will be interested to see how the Russians and Europeans manage the selection, development, testing, and integration of these instruments in this short time frame.  Perhaps considerable work has already been done or there are flight-ready designs already available.

Two years after the ExoMars station and rover arrive, NASA will land its 2020 rover.  The rover itself will be a near copy of the Curiosity rover currently on Mars, but with a next generation instrument suite.  A major new goal will be to select and cache a suite of samples that a later mission might collect and return to Earth. 

2020 NASA Mars rover concept drawing.  Credit: JPL/NASA

Each sample will be about the size of a stubby pencil.  Previously, NASA had planned to put each sample into a canister as it was collected.  This canister would then be placed on the surface for later collection after it was full.  NASA has announced a major change in how these samples will be cached (see here).

The original plan had two key limitations.  First, as the canister acquired more and more samples, it would become an increasingly precious resource.  This would lead the mission’s operators to become increasingly conservative in their operation of the rover.  Should they, for example, explore an interesting looking ridge, but one where if the rover fails the rock face would prevent a future mission from being able to reach the canister?  Second, there was no good way to remove samples once they were in the canister.  What if the canister was full and then scientists find the one sample they absolutely want to collect for return to Earth?

In the new plan, dubbed the Adaptable Cache, the rover would still drill out samples and put them into sample tubes.  Then instead of putting the tubes into a canister, the rover would place them on the surface and then move on.  A future sample return mission would carry a rover that would pick up the samples and place them into a canister it carries.  This way the 2020 rover can cache more samples than could be returned and scientists would send the subsequent rover to pick up only the most important ones.  Even with the old scheme where the 2020 rover carried the canister, the follow on mission would still need a rover to fetch the canister.  Now that follow on rover would need a more capable arm to pick up tubes lying on the surface and place them into its own canister.

The new rover will also have an upgrade to its engineering cameras.  On Curiosity, the navcam/hazcam cameras used to operate the rover take black and white images.  The 2020 rover will carry color cameras that will take higher resolution images.  Curiosity carried just one camera to record its descent and landing, placed on the bottom of the rover to look down.  The 2020 rover will carry additional cameras that will look up at the descent stage that carries the descent rockets, a camera on the descent stage looking down at the rover, and a final camera on the backshell to image the parachute opening. 

With these new cameras, being an armchair explorer of Mars will get, as they say, a whole lot better.

In one other item of Mars 2020 rover news, the current cost estimates for the mission appear to be in the $2.14 – $2.35 billion range instead of the previously quoted $1.5 billion.  A reasonable portion of this increase likely comes from the new figures representing inflation through launch and operations, while the original cost estimates were, I’m told, were in 2015 dollars.  At the new figures, the 2020 mission, given inflation, still will be considerably cheaper than the Curiosity mission on which much of the design will be based.

The final major news for Mars exploration was NASA’s announcement that it would like to fly a new orbiter to Mars in the early 2020s (see here).  NASA will need a new orbiter to act as a communications relay for future lander missions (such as a sample return fetch mission).  The agency could fly a fairly simple orbiter to do just this task.  Instead the agency is considering flying a highly capable orbiter that would use solar electric propulsion (SEP).

NASA is considering a range of options for an early 2020s orbiter to replace the Mars Reconnaissance Orbiter (MRO) currently at Mars.  At a minimum, the new orbiter would act as a communications relay for future landed missions.  In the most expansive scenario, the new orbiter would carry a much larger payload than any spacecraft has done in the past to Mars.  Credit: NASA.

All previous Mars missions have used rockets to enter Mars orbit.  Solar electric engines, such as those used by the Dawn and Hayabusa-2 missions, provide a great deal more cumulative thrust.  By using SEP, the new orbiter could spiral into Martian orbit.  At it lowers its orbit, it could rendezvous with each of Mars’ tiny moons for in-depth studies.  Then the orbiter could switch from a near equatorial orbit (where the moons are) to a polar orbit to allow it to study the entire Martian surface. 

NASA’s Mars program manager stated that the agency would like to have the orbiter carry a substantial scientific payload (one chart lists a capability to host up to 300 kg of instruments, which would be a very substantial payload).  The agency has not stated a preference for what types of instruments – a future scientific definition team would make those recommendations.  However, we can do some informed speculation.

In the 2000s, two scientific definition teams looked at science that then future orbiters could make.   The highest priority measurements would be to study the upper atmosphere and trace gases in the atmosphere.  Time has moved on, and the MAVEN orbiter is at Mars studying the upper atmosphere and the 2016 European-Russian orbiter will study trace gases.

The panels in the 2000s did recommend that future orbiters carry high resolution cameras to image possible landing sites and carry out scientific imaging.  Since the mid-2000s, the HiRise camera on the Mars Reconnaissance orbiter has been imaging the planet at 25 to 32 cm pixel resolution.  The HiRise team described a possible future instrument that would use the same optics, but would provide color imaging across the entire image.  (See here.) (The current HiRise camera has color only for the center fifth of each image.)  A future camera also could add imaging in spectral bands in the near infrared that would allow studies of surface composition at high resolution.  This future camera could also acquire stereo images to allow 3D analysis of each scene.

Another concept for a future high resolution camera comes from Malin Space Science Systems that has built cameras for several Mars missions.  (See here.) This camera would carry a bigger telescope than HiRise camera, and the orbiter would fly closer to the planet -- skimming just above the top of the atmosphere at perihelion --  to acquire images at 5 to 10 cm pixel resolution.  This finer resolution would allow more detailed scientific studies of surface features, such as the fine sedimentary bands that are often almost  visible in current HiRise images.  (The published abstract for this proposal doesn’t discuss whether the camera would image in multiple color bands.  It also doesn’t say how narrow the image strips would be.  HiRise’s lower resolution likely would provide wider image strips.)

Another proposal suggested that a future mission might carry a suite of radar instruments and laser to map the surface and subsurface in detail.  (See here.) Ground penetrating radar instruments are already at Mars mapping the subsurface stratigraphy.  However, their capabilities are limited by the power and mass available to them within the overall suite of instruments the orbiters carried.  The proposal suggests that a future orbiter carry one radar optimized for subsurface stratigraphy and a second for surface mapping that would be able to penetrate the sand and dust that covers much of the planet to image the rock structures below. The proposal also recommended flying a new generation Laser Ranging and Detection (LiDAR) instrument that would remap the altimetry of the surface at much higher resolution.  A new orbiter such as the one NASA is discussing would have the power and payload mass to optimize instruments such as these along with a high resolution camera.

Another key capability of the proposed orbiter is that it would use laser optical communication to return data to Earth as well as newer generation radio systems (Ka band).  The limit on how much data past and current orbiters have been able to return has not been the instruments, but instead the bottleneck of the communications system.  High resolution cameras and radar instruments want to be data hogs, and a new generation orbiter with advanced communication could be an enabling technology to map much larger areas of the planet at high resolution.

This orbiter has just been discussed publicly as a concept for the first time in the last couple of months.  None of NASA’s scientific panels have looked into missions past 2020.  They may recommend another mission instead.  It’s also not clear where the money for the mission would come from.  NASA’s planetary program will be funding the development of the $2 billion-ish Europa mission in the early 2020s.  If the agency also wants to continue developing a mixture of the smaller Discovery and New Frontiers missions in the same time frame, a major new Mars orbiter may stretch the budget.  A next generation Mars orbiter would provide new instrument eyes to study the fourth planet.  We will need to wait and see whether the programmatic priorities and budgets line up to enable it to fly. 

Saturday, April 11, 2015

NASA's Mission to Europa May Get More Interesting Still

SpaceFlightNow.com has published an article reporting that NASA officials have asked their European counterparts if they would like to propose contributing a small probe to NASA's Europa mission planned for the mid-2020s.  

If the Europeans are interested, the probe could carry out any of several possible missions.   Two mentioned in the article would be a small lander or a craft that could fly through and study any plumes found erupting from the moon's surface.  


Europa as imaged by NASA's Galileo spacecraft in the 1990s.  Credit: NASA
Another possibility, not discussed by the article might be a small spacecraft that could repeatedly flyby the moon to make magnetic field and gravity measurements.  Both studies are key to understanding the size of this moon's ocean and the structure of its rocky core.  The main Europa Clipper flyby craft will make these measurements during each of its flybys.  Both studies, however, improve with the number of passes by the moon, so a second craft would add substantially to these measurements.  I'm told that one or more of the CubeSat studies NASA is separately funding as possible augments to the mission would do this.

British scientists and engineers have studied small penetrator landers that would be shaped roughly like a cannon shell that would use the force of impact to dive a meter or two into the icy surface.  Penetrators have been used on the Earth to deploy probes from planes and have been proposed several times for planetary missions.  Penetrators avoid many of the complications involved in a soft landing. The penetrator would need an avionics module to keep its orientation and likely would need a retro rocket to reduce the approach speed, both of which add considerable complexity to the design.  (Penetrators have flown on two missions, the Russian Mars 96 spacecraft that failed to leave Earth orbit and the two failed DeepSpace 2 Mars penetrators that were never heard from after their release from their mothercraft.)

Penetrators would not be the only possible small lander design.  All approaches, though, would involve small probe that would carry one to a few small instruments such as a seismometer/geophone and one or more chemistry experiments.  Even simple measurements could provide information that would complement the measurements made by the main spacecraft.  (As a side note, the same would be true of a small lander for Ganymede added to Europe's JUICE mission that will orbit this moon of Jupiter in the early 2030's.  I don't know if that design would still be open to adding a small lander that might be a near duplicate of one carried on NASA's Europa craft.)

You can read more about possible small lander designs in this blog post, this Science News article, or from this presentation.

If the European Space Agency's managers decide they might like to participate, then European scientists could propose specific probe plans as part of a future Medium Class mission competition, where it would compete against other planetary and astrophysics proposals.

Note: My thanks to RL for informing that the Mars 96 mission also carried penetrators; the text has been corrected.  However, there has been no successful use of penetrators on any planetary mission to date.


Monday, March 23, 2015

Four Ideas to Bust the Floor on Outer Planet Mission Costs

In 1972 and 1973, NASA launched two simple spacecraft, Pioneer 10 and 11, to Jupiter.  They were charged with scouting the way for the more sophisticated spacecraft that would follow.  Since then, the outer solar system has been the realm of large, expensive missions: the Voyagers, Galileo, and Cassini.

Large missions costing well over $1B have proven very successful, but they are launched at the rate of approximately one per decade and outer solar system destinations have had to vie with Mars for these rare slots.

At last month’s Outer Planet Analysis Group, scientists and engineers presented three proposals in competition for selection in NASA’s cheapest category of planetary missions, the Discovery program.  One other proposal may eventually vie for a slot in NASA’s mid-range New Frontiers program. 

The road to lower costs outer planet missions has been paved by NASA’s first two New Frontiers missions, the $700M New Horizons mission en route to Pluto and the $1.1B Juno mission en route to Jupiter.  But can the cost of a mission to the outer solar system be cut to $450M, the limit for a Discovery mission?

The three Discovery proposals take very different approaches.




The Enceladus Life Finder (ELF) team proposes to take the tightly focused, minimalistic path. 

We now suspect that many of the icy worlds in the outer solar system harbor oceans beneath their icy crusts.  Among those bodies, Europa and Enceladus are special because their oceans appear to rest atop their rocky cores, providing access to elements and minerals believed essential to life.  These rocky surfaces are also are believed to have hot hydrothermal springs that could provide the energy needed for the complex chemistry needed to support life.  For other icy worlds, the oceans are sandwiched between layers of ice and not in contact with their rocky cores, making Europa and Enceladus priorities for exploring potentially habitable worlds.

Enceladus so far is unique in having plumes of water that steadily jet from its surface, spilling the contents of its ocean into space where they can be easily sampled by a passing spacecraft.  (Hubble Telescope observations suggest that Europa may also have plumes, but repeat observations have failed to confirm the initial sighting.  Any plumes may be episodic, repeating only every few years.) 

The Cassini spacecraft currently at Saturn has already sampled the plumes, but its instruments were designed in the 1990s, and weren't designed to study the highly complex molecules that could indicate life.  The ELF spacecraft would narrowly focus on sampling these plumes with modern, highly sensitive instruments. 

The mission would enter orbit around Saturn and then make ten flights through Enceladus’ plumes. ELF’s instrument payload would consist of two mass spectrometers that “weigh” atoms and molecules to measure composition.  The spectrometers would analyze particles in the plume that are a mixture of frozen ocean water and particles from the seafloor. One of the mass spectrometers would be optimized to study the liquids that originate from the ocean and the other the solid particles that likely originate from the rocky core.  A third instrument might be included that would test whether any amino acids found have predominately left- or right-handed structures.  (Life on Earth predominately creates left-handed forms, and it’s suspected that life that originates elsewhere will similarly favor one form over the other instead of a random mixture likely from abiotic chemistry.)  The spacecraft’s navigation camera also would image the plumes to judge their activity at the time of each flyby.

To appreciate the simplicity of the ELF proposal, you need to consider previous concepts for exploring Enceladus following Cassini’s last flyby late this year.  When Decadal Survey considered this decade’s priorities for exploring the solar system, several Enceladus options were considered.  The simplest would have been a multi-flyby spacecraft like ELF, but that would also have included two cameras and an ice penetrating radar in addition to the mass spectrometers.  The preferred mission would have studied Enceladus both during flybys and from orbit about the moon with a somewhat different, but equally rich instrument compliment.  These concepts would have generated volumes of data that would have investigated the structure of Enceladus, its ocean, and ice shell along with the chemistry of its internal ocean.  Unfortunately, the estimated costs of these missions were over $1.5B. 

(Quoted mission costs often vary based on the specifics of what’s included; in this post, I’ve tried to quote the costs for the items included the widely-quoted $450M cap for Discovery missions.  NASA’s final cost for a full mission including launch and operations would be higher than the costs I’m including here.)




The Io Volcano Observer would take a different tact than the ELF mission.  Like the proposed Enceladus mission, the IVO spacecraft would observe Jupiter’s volcano-rich moon during several flybys.  Unlike the Enceladus mission, the Io mission would carry four instruments – a two camera suite, a thermal imager, a magnetometer, a mass spectrometer – along with a student-built instrument to map volcanic hot spots.  The spacecraft’s radio system would do double duty by also allow precise tracking of the spacecraft’s speed during flybys to study the distribution of matter within this moon.  This would be a data rich mission, and the spacecraft would carry an experimental high data rate optical communications system in addition to the traditional radio system. 

While ELF would focus on one investigation – the chemistry of the ocean – IVO would perform an integrated series of studies to understand Io as a world and a member of the Jovian system.  The mission’s goals are divided between understanding the sources and extent of its intense volcanic activity, the effects of the injection of its volcanic plume material into the wider Jovian system, and long term monitoring of Europa for plumes and Jupiter’s atmosphere.  The nominal mission would last 22 months, but an extended mission might carry on for an additional 6 years to monitor Io through time.  If the extended mission occurred, then we might have three spacecraft simultaneously studying the Jovian system in the early 2030’s: IVO in a polar Jovian orbit with periodic flybys of Io, Europe’s JUICE mission with broad studies of the Jovian system and a focus on the icy moon Ganymede, and NASA’s Europa mission.




The third Discovery proposal takes an entirely different approach to exploring the outer solar system on a budget.  The Kuiper mission would launch a space telescope dedicated to studying the outer solar system.

The Kuiper proposal addresses two problems.  First, we cannot afford to have spacecraft at each of the major outer planets to observe their weather, their magnetospheres, and their moons.  This dedicated outer solar telescope would be able to examine each of these worlds multiple times each day to study these worlds as dynamic systems.  Scientists will be able to observe how storm systems in their atmospheres exchange energy, how variations in auroral activity provides clues to the state of their magnetospheres, and how volcanic and plume activities on the moons Io, Europa, and Enceladus vary over time. 

The second problem is that we don’t understand key questions about the formation of the outer solar system.  We suspect that the outer planets migrated during the early ages of the solar system, but there are competing theories as to whether that migration was smooth or more chaotic.  As the planets’ orbits shifted, they would have flung smaller bodies about.  The Kuiper telescope would analyze the spectra of thousands of small bodies ranging from Jupiter’s orbit to the distant Kuiper belt to analyze their compositions.  The mixture of compositions at different distances from the sun would allow astronomers to distinguish between the competing theories.

Telescopic observations have always played a crucial role in studying the outer solar system.  Earth-based telescopes, however, have key limitations – any solar system target is visible for only a few hours each day and our atmosphere blurs vision and blocks key wavelengths of light.  The science proposed for the Kuiper mission could be done by the Hubble Space Telescope, but its observing time is precious and little is allocated to solar system studies. 

The Kuiper mission would be smaller than Hubble (a 1.2 meter primary mirror versus the Hubble’s 2.4 meter mirror) but would be dedicated to observing the outer solar system.  The spacecraft would be parked in an orbit around the L2 Lagrange point beyond the moon where it could observe the sky without Earth occultations and would be beyond stray Earth light. 



In addition to the three Discovery mission proposals, a fourth mission concept was proposed, the LIFE Enceladus Sample Return.  A previous incarnation of this mission was proposed for the last Discovery competition but wasn't selected.  Like the ELF mission, the LIFE mission would make multiple flybys through Enceladus’ plumes and would use a mass spectrometer to study their chemistry.  Unlike ELF, the LIFE spacecraft would collect dust plumes in a fashion similar to that done by the Stardust spacecraft that collected comet dust samples in the mid-2000s.  The samples would later be returned to Earth where far more sensitive measurements would be made than could ever be done by instruments on a spacecraft.  The cost estimates presented by the LIFE team puts the mission outside the current scope of the Discovery program, and the team is building support to add an Enceladus sample return mission to the New Frontiers candidate mission list for the 2020s. 

After 2017, there are no plans to have a spacecraft operating in the outer solar system until the late 2020s at the earliest.  That decade gap will exist because outer planet missions in the past have had to be infrequent because of their costs.  The four proposals presented at last month’s meeting represent the planetary community’s attempt to find a new class of much lower cost missions that could fly more frequently.  The Kuiper telescope would be an entirely new approach to the problem that could begin providing data in the early 2020s.  The missions to Io and Enceladus face a tougher challenge because they propose to do missions for half the cost of any previous outer planet mission orbiter. 

Missions to both Enceladus and Io have been studied before, and the costs were two to four times that of the cost cap for Discovery missions ($450M).  Teams that propose missions are generally fairly open about the great science their missions would do if they are selected to fly.  These teams tend to be much more reluctant, however, to discuss the specifics of how they would accomplish their goals within the tight cost caps of a Discovery competition – that is their secret sauce.  The teams proposing ELF and IVO are seasoned veterans and their credibility gives me hope that the outer solar system may open to low cost missions.  We can assume that they have had a laser focus on finding ways to reduce costs to a fraction of what previous studies have assumed.  Estimating development cost, however, is always part art.  NASA will perform its own assessment of mission risks and costs, and its reviewers may be more risk adverse and conservative than the mission proposers in assessing likely costs and risks. 

For this Discovery competition, NASA’s managers have changed the rules in a key way that will help outer planet proposals.  In previous competitions, the costs of mission operations had to be included in the mission cap.  A mission to Mars with an operations lifetime of two to three years had an inherent advantage over an outer planets mission that might take five to seven years to reach its target and then require another year or two of operations.  Now NASA has excluded “reasonable” mission operations costs from the cost cap (which means it picks up those costs separately).  This goes a long way to leveling the playing field between inner and outer solar system Discovery proposals.

In the last Discovery competition, a mission to land on a lake in the north polar region of Saturn’s moon Titan made it to the list of finalists.  (The Mars InSight geophysical lander was the winner.)  If either ELF or IVO is selected this time, then outer solar system will have been opened to exploration by a new, low cost class of missions.  If neither mission is selected, then the experience learned from these proposals will become part of the community experience that is likely to sharpen future Discovery proposals for the outer solar system.  I believe that eventually an outer planets Discovery proposal will find the right formula for selection; I hope that this happens sooner rather than later.

You can read the original presentations for these proposals as well as the other presentations from the OPAG meeting here.



Saturday, March 7, 2015

Understanding why our most Earth-like neighbor, Venus, is so different

When I first learned about the solar system a few decades ago, the scientific consensus held that the structure of our solar system resulted from standard processes of stellar evolution.  The arrangement of planets with small rocky worlds closer to the star and gas and ice giants further out would be the normal arrangement of planetary systems.  Then we began to find planets around other stars and we learned that our solar system is – if not an oddity – by no means typical.  With just one example, it is easy to be led astray.

In our solar system, there are only two large rocky worlds, Venus and Earth.  Mercury and Mars are small enough that both lost most of their internal heat billions of years ago and they have largely ceased to further evolve.  (The ancient, preserved, surface of Mars is what makes it so attractive to explore for the types of habitable environments that were long ago erased from the Earth’s surface.)  Both Venus and the Earth, however, retain substantial heat in their cores.  That heat drives plate tectonics on our world and appears to have caused the near global resurfacing of Venus in the last few hundred millions of years (which counts for recent when compared to the age of the solar system).

While Venus and Earth have similar sizes and are solar system neighbors, they have evolved very differently.  Venus today lacks oceans, appears to lack plate tectonics, and has a massive carbon dioxide atmosphere that creates a greenhouse effect that makes the surface a hot hell.  Understanding why Venus and Earth became so different will help us understand why Earth evolved as it has and what the range of conditions for similarly sized worlds around other stars may be.  Venus provides the contrast to the Earth that can help us both better understand the origins of our world’s characteristics and the range of possibilities for similar sized planets orbiting other stars.

Cover page for the EnVision proposal.

Today, our knowledge of Venus’ surface and its interior is similar to our knowledge of Mars in the 1970s following the Viking mission.   The Soviet Union placed several probes on the surface that made simple measurements in the hour or so before the surface heat fried their electronics.  NASA’s Magellan spacecraft mapped the surface with radar in the early 1990s at about 120 m resolution globally.  We know, however, from our experiences mapping the moon and Mars’ surfaces that teasing out the details of geologic processes requires mapping surfaces with resolutions less than 50 m resolution with smaller areas mapped at a few meters resolution.

Mapping Venus’ surface (with one exception we’ll return to later) requires using imaging radars that can penetrate its thick cloud cover.  The technology in the early 1990s when Magellan flew was relatively new and crude by today’s standards.  Now imaging radars are widely used to study the earth both from airplanes and from satellites.  The technology is mature and relatively low cost. 

As a result, something of a cottage industry has grown up proposing new missions to map Venus either through the European Space Agency’s Medium Class program or through NASA’s Discovery program.  The different accounting rules applied by the two agencies make direct cost comparisons difficult, but these missions cost in the neighborhood of $500M to $600M.  A Venus radar mapping mission has been proposed for the current ESA Medium Class competition, and I hear that up to three missions are in competition for selection through the NASA program.

The European selection process tends to be more open than the U.S. process, and the EnVision team led by Dr. Richard Ghail at Imperial College London shared a copy of their proposal to ESA with me. 

Unfortunately, the EnVision mission will not move forward for the M4 competition.  From the team’s Facebook page: “ESA announced this morning that EnVision has been evaluated as 'incompatible with the technical and/or programmatic boundary conditions for the M4 Call'.  Essentially this means that ESA believe we would over-run on cost and/or schedule. We await further feedback which will inform our proposal to the M5 Call expected later this year.”

While EnVision is out for this current M4 contest, reviewing its proposal can still let us see what type of Venus mapping missions are being proposed.  The proposals to NASA’s current Discovery program will have differences from EnVision and the cost assumptions are different.  Also, ESA is expected to begin the competition for its 5th Medium Class mission later this year and there may be a larger mission budget.  The EnVision team hopes to propose this mission, perhaps with modifications, in the next competition.

To get a mission selected for Venus requires playing the long game.  Each competition that a team doesn’t win gives them feedback on how to improve their proposal. 

So let’s look at what a Venus mapping mission might look like using the EnVision proposal as are guide.

The EnVision mission would address several key questions:

  • The average age of Venus’ surface is just a few hundred million years old, a tiny fraction of the age of the surfaces of most rocky and icy moons in the solar system.  What processes resurfaced the planet?  Did they occur in the same time period or have they been spread over time?
  • Is Venus currently geologically active and therefore continuing to remake its surface and release new gases into the atmosphere?
  • What processes modify rocks once they are delivered to the surface?  Venus’ atmosphere is so thick that its surface in many ways is similar in terms of pressure to what is found at the bottom of our oceans.  This should lead to complex weathering and erosion, which is consistent with what we saw from the pictures taken on the surface by the Soviet Union’s Venera landers.
  • What is the internal structure of Venus like?  This is the part of a planet we can never see, but scientists can study it indirectly through the combination of Venus’s gravity field and surface topography.  Both were mapped by Magellan, but at too crude of resolutions to answer key questions.


To address these questions, the EnVision spacecraft would carry four instruments. 

These images, derived from radar imaging of Hawaii, show the improvement in resolution possible with modern radar systems compared to the data returned by the Magellan mission.  This image is from a poster describing a 2012 JPL VERITAS mission that was proposed for the NASA Discovery program.  Credit: JPL

The EnVision spacecraft’s primary instrument would be its VenSAR synthetic aperture radar.  Operating in its primary mode, VenSAR would map almost the entire planet in stereo at a resolution of 27 m.  The radar would produce both images (the equivalent of images from a camera) as well as high resolution measurements of the absolute elevation of the surface to map its topography.  VenSAR would have a number of special modes that would enable other forms of mapping.  Small splotches of the surface would be mapped with resolutions as fine as 1 to 2 meters, which would allow, for example, the instrument to spot the Venera landers on the surface.  An interferometric mode would enable EnVision to spot tiny changes in relative elevation in a location that could indicate movement from a seismic event or the swelling of a volcano.  By using different polarizations of the radar beam, the spacecraft will be able to map differences in texture across the surface to distinguish, say, a plain covered with rocks too small to image directly versus a plain covered in sandy material.

These figures show the improvement in topographic resolution possible with a modern radar instrument using data from Iceland.  The top image shows the equivalent Magellan resolution and the bottom image shows simulated resolution possible from a new Venus radar mission.  Credit: Decadal Survey White Paper, NASA


Example of the ability to detect geologic changes in vertical heights as small as a few centimeters.  This example is from an Earth-orbiting radar mission for changes following a terrestrial earthquake in 1992.  Credit: EnVision proposal, Fialko 2004; Peltzer et al. 1998.



Mapping mode
Resolution
Prime mission
Extended mission
Stereo imaging
27 m
91.5%
99.9%
High resolution imaging
7.3 m
0.9%
11.1%
Spotlight imaging
1.15 m
Selected locations
Selected locations
Interferometry
27 m
40.3%
40.3%
Polarimetry
63 m
6.2%
32.6%

Expected mapping coverage of Venus by the VenSAR instrument for the proposed prime and possible extended missions.  The Spotlight mode would be used for only selected areas such as the locations of the Venera landers.

The VenSAR instrument cannot see below the very top of the surface of Venus.  EnVision would carry a low frequency radar sounder whose beams would penetrate several hundred meters below the surface.  The result is a radargram that looks a bit like a sonogram or x-ray of the stratigraphy of the upper surface.  Two similar instruments are at Mars now, where they have examined the distribution of soils and ice at that world.  At Venus, this instrument would study the depths of lava flows and sedimentary rock layers and search for faults and folds that indicate past tectonic activity.

Example of subsurface stratigraphy revealed by a low frequency radar sounder for the Martian northern polar cap.  Credit:  NASA/ESA/JPL-Caltech/ASI/University of Rome/University of Washington St. Louis

The third instrument, the Venus Emission Mapper (VEM) would study Venus in an entirely different way than the radar instruments.  The Galileo and Venus Express spacecraft’s instruments discovered narrow spectral windows where thermal emissions can be transmitted through the otherwise opaque clouds.  These few windows would give the VEM instrument the ability to map thermal hotspots that would indicate areas of current volcanic activity, map differences in the composition of the surface, and detect changes in key atmospheric gases that could indicate the eruption of a gas-spewing volcano.  Because Venus’ thick atmosphere would scatter the light, the surface resolution of VEM would be low, around 50 km.  It could make, however, ground breaking measurements of the surface variation.  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.  The VEM instrument would provide much more sensitive measurements.


The Venus Express mission carried out the first mapping of Venus’ surface using thermal emissions. Red-orange colors in this Venus Express image of Venus’ Idunn Mons volcanic peak indicate warmer areas suggesting different surface compositions and younger ages than surrounding material.  Credit: ESA

The EnVision radio system would be its fourth instrument.  By tracking minute differences in radio frequency caused by the spacecraft speeding up or slowing down as the mass of the planet below it varies, the spacecraft can study variations structure deep below the surface.

EnVision has been proposed for ESA’s fourth Medium-class scientific mission.  The ESA competitions pit proposals from across space science against each other, so the EnVision proposal will be judged against both other solar system missions as well as those that would study astrophysics and the Earth’s magnetosphere.  The next key milestone will come in the next month or two when ESA’s managers select several finalists for more detailed analysis.  If the EnVision mission is the final selection, it would launch in December 2024 and would arrive at Venus a few months later.  The prime science mission would take approximately two and a half years, although the team hopes that the mission would be extended for a second 18 month observing campaign. 

The Envision team expects to hear the result of the technical review of their proposal in a week or two (it’s a straight pass or fail). If they pass on the technical review, the proposal then goes through a science review stage with ESA expected to make the final selection of M4 candidates in May/June.  You can find more information on the proposal at their website:  http://www.envisionm4.net/

When I read about proposals for most small-scale planetary missions (ESA’s Medium class, NASA’s Discovery class) their goals are often narrowly focused so that they can be met within a constrained budget.  In the last Discovery mission selection, several Venus mapping missions were proposed.  I’ve heard that each focused its science questions in different somewhat narrow ways that lead review panels to be confused about what the actual priority would be.  For me, the true value of a mission like EnVision is the breadth of its data.  Scientists could mine the data from a mission like this for many years and use the data to develop and then answer questions that we don’t yet know enough to ask.