Sunday, November 22, 2015

Justin Atchison has been kind enough to provide background on the proposed NASA Double Asteroid Redirection Test (DART) mission.  As Justin states, the NASA mission may be paired with a European orbiter that would also deliver a lander and several CubeSats to explore both members of this binary asteroid pair.  ESA has a nice webpage describing their proposed mission: Asteroid Impact Mission if you would like additional information.

Preface: Greetings, I’m Justin Atchison, an aerospace engineer at the Johns Hopkins University Applied Physics Laboratory. I’m excited to be guest-writing an article about the Double Asteroid Redirection Test (DART). I recently presented some research about DART at the International Astronautical Congress, which I was able to attend thanks to a travel fellowship through the Future Space Leaders Foundation (FSLF). I’d strongly encourage any student or young-professional (under 35) to apply for this grant next year. It’s a great opportunity to attend this premier conference and interact with a variety of leaders in the aerospace field. FSLF also hosts the Future Space Event on Capitol Hill each summer, which offers engagement with US Congress and aerospace executives on the latest and most relevant space-related topics.


The Asteroid Impact Deflection Assessment (AIDA) mission is a proposed joint program between NASA and ESA to study and demonstrate kinetic asteroid deflection as a means of Earth impact mitigation [[1]]. Basically, we want to understand what’s involved with using a high speed impacting spacecraft to change the orbit of an asteroid that is (hypothetically) threatening Earth.  DART is the proposed NASA component of AIDA. Its role is to hit a small asteroid at high enough velocity to create a measurable deflection. The proposed ESA component of AIDA is called the Asteroid Impact Mission (AIM), and its role is to rendezvous in advance of DART and study the pre- and post-impact asteroid conditions in-situ. The final component of AIDA is Earth-based observations.

AIDA consists of DART, AIM, and Earth Based Observations. AIM will rendezvous with the binary asteroid Didymos and observe as DART impacts the smaller secondary asteroid.

What makes this concept unique is that the target is a binary asteroid system, specifically Didymos (1996 GT). Didymos was one of the first Near Earth binary asteroids to be discovered; we now estimate that roughly 15% of all Near Earth Asteroids (NEAs) are binary systems [[2]]. The primary body of the Didymos system is about 800 m in diameter, and the secondary body (playfully called “Didymoon” on the mission) is only 170 m in diameter [[3]]. We want to impact Didymoon using the DART spacecraft and measure the change in orbit about the primary. 

Here’s the point—the binary system represents a sensitive experimental setup. The DART impactor is about 300 kg and will impact the asteroid at nearly 7 km/s. That is a lot of linear momentum, but we estimate that it will only impart ~1 mm/s to Didymoon. For a single asteroid, the heliocentric velocity is ~30 km/s. Measuring 1 mm/s out of 30 km/s (1 part in 10,000,000) would be exceedingly difficult. You could pick an even smaller target, so that you impart a higher amount of momentum, but then you have the challenge of trying to observe a very small body at long ranges. However, in the case of a binary, the velocity of Didymoon with respect to the primary is only about 10 cm/s. The effective signal-to-background is now just 1 in 1000. (Ok…the order-of-magnitude argument isn’t entirely accurate here, but it communicates the point.) Further, Didymoon is observable at long ranges as a signature in the system’s light-curve. In fact, that’s how it was first confirmed to be a binary system.

Why Didymos?

Of the known binary asteroids, Didymos makes an excellent candidate target for the experiment for a few reasons:

      Accessibility – Didymos falls in a class of asteroids that requires less total V with which to rendezvous than the Moon. That means that AIM can be kept relatively small and affordable. Likewise, there are very feasible trajectories for DART to impact with high relative velocities.

Orbits of Earth and Didymos from above the ecliptic (top), and from the “side” along the ecliptic y-axis [[4]]. The dashed line shows the line where the two orbit planes intersect. Didymos is inclined about 3.4 degrees relative to Earth’s orbit plane.

Conjunction – In October of 2022, Didymos will make a rare (every ~25 years) conjunction with Earth. If we time the encounter to occur during that conjunction, Earth-based instruments, including radar, can participate in the observations and add a great deal of science. 

Earth-Didymos range between 2000 and 2050. The 2022 conjunction is especially close.

The Experiment

Although I described the kinetic impact measurement in terms of velocity, we’re really going to be measuring the change in orbit period of Didymoon. This approach has the added advantage that the measurement sensitivity “integrates” with time. After enough time, the phase of Didymoon will be clearly different from what one would predict without the impact. Within practical limits, the longer we watch, the more clear the effect becomes. (The practical limits are associated with the native acceleration environment and the quality of the pre-impact characterization.) 

At this point, if you’re like me, you’re probably wondering why there’s any uncertainty in the imparted momentum. (In fact, there’s a Reddit thread dedicated to this exact question). After all, didn’t high school physics pretty clearly describe this type of event using inelastic collisions and conservation of linear momentum? The one word answer is: hypervelocity. That’s meant to say that the velocity of the impact, 7 km/s, is significantly higher than the speed-of-sound in an asteroid. Bear in mind that a bullet travels at something like 1 km/s.  When DART impacts the surface, its energy will propagate into the asteroid where it will generate a new crater. Some fraction of the material from the crater will be ejected off the asteroid at high speed. This ejected material acts like a thruster, imparting additional momentum to Didymoon. Total momentum is conserved…but Didymoon gets more than DART’s momentum added to it because it’s ejecting some of its material. 

The amount of momentum amplification is called β and it depends principally on the speed of impact, the asteroid composition, and the asteroid structure (porosity and cohesion) [1]:

(mS and vS are the mass and velocity of the spacecraft. mA is the mass of the bulk asteroid. ∆v is the change in the asteroid’s velocity.)

Estimates suggest that β can be anywhere between 1 and 5 for the types of conditions you’d expect for kinetic deflection. (Yes, it’s hypothetically possible to get cases where β < 1, in the event that material is ejected from the asteroid’s side opposite the impact.)

There are roughly three different cases of expected impact momentum exchange. The low-speed case (at top) matches simple expectations for an inelastic collision (β = 1). The middle hypervelocity case is more likely; a crater is formed at the impact site and debris is ejected away. This ejected material imparts additional momentum to the remaining asteroid, giving an effective “amplification” to the momentum exchange (β > 1). The bottom hypervelocity case is unlikely, but shows a scenario where the impact’s shock wave crosses the asteroid and ejects material on the far side of the asteroid. In this case, some of the spacecraft’s momentum is transferred to the ejected material and the bulk asteroid receives a reduced change in momentum (β < 1).

Right now, the DART team is parametrically modeling the encounter using really impressive numerical simulations. The codes were developed from explosives and weapons modeling, which are some of the most complex computer models to create. These simulations require massive supercomputing resources and help us understand what happens to different kinds of materials as they undergo complex physical events such as high-velocity impacts and explosionsSpecifically, the region around the impact is divided up into a grid consisting of millions of little asteroid “cells”. The simulation evaluates the propagation of energy and momentum through each cell, determining the interaction of each cell with its neighbors. The fraction of cells containing material that are ejected from the surface represents the amount of material that affects the value of β

The simulations are very useful, but it’s quite difficult to validate them in 1-g.  Scientists conduct scaled experiments with high energy mechanical contraptions like air-guns and catapults, but it’s hard to accurately reproduce the expected structural properties of the target--a gravitationally bound “rubble pile”. With that in mind, DART will represent a chance to validate our hypervelocity impact modeling and enable us to then  more-accurately extrapolate to other asteroid types.

Various computational hydrocode results for DART impacts at differing geometries [[5]].

Program Status

Together, the two proposed missions form a coherent experiment. However, they’re being developed such that they aren’t codependent: That is, AIM will be the first spacecraft mission to a binary asteroid, which would be pretty exciting with or without DART.  AIM carries a suite of instruments including a visible imager, thermal infrared imager, two radar systems, and a landed package. It also will be carrying a set of 2-6 cubesats [[6]].

Should only DART launch, the change in orbit period can be observed without AIM by using Earth-based observatories. Amateur and professional astronomers worldwide will want to study the event to try to characterize the post-impact environment. (Think Shoemaker-Levy 9’s impact at Jupiter in 1994.) Between light curves and radar, we intend to measure the orbit period change to better than 10%. If AIM is there, the results are obviously much clearer and more accurate, but nonetheless the experiment can be conducted using DART only.

DART itself is a relatively simple spacecraft with only a single instrument, DRACO, which is an imager derived from the narrow-field-of-view telescope on New Horizons. DART has the challenge of reliably targeting and impacting a slight 170 m diameter target. To successfully achieve this, we’ve been working hard to develop and prove out algorithms for autonomous optical guidance, navigation, and control (GNC). The GNC software must distinguish between the two bodies at Didymos and then drive the spacecraft towards the image centroid corresponding to Didymoon, all within a matter of hours. For this complex problem, we’re leveraging decades of missile guidance algorithms, namely something developed in the 1970’s called Proportional Navigation. I can’t help but call to mind a quote from the opening of the Arthur C. Clarke book Rendezvous with Rama (1973):

“A hundred years earlier, a much poorer world, with far feebler resources had squandered its wealth attempting to destroy weapons launched suicidally by mankind against itself. The effort had never been successful, but the skills acquired then had not been forgotten. Now, they could be used for a far nobler purpose, and on an infinitely vaster stage. No meteorite large enough to cause catastrophe would ever again be allowed to breach the defenses of Earth.”

Lofty quotes aside… AIM is currently a Phase A/B1 study within ESA, with two companies developing conceptual designs. DART is also a Phase A study, managed by the NASA Planetary Defense Coordination Office, within the Science Mission Directorate at NASA Headquarters. Both projects will proceed over the next year towards their respective key decision points. 

I hope you’ll agree that this is a compelling concept. To me, it seems to answer that old criticism, 

“Everyone complains about the weather asteroid-impact-threat, but no one does anything about it.”

Let’s do something about it.


[1] A. F. Cheng, J. A. Atchison, B. Kantsiper, A. S. Rivkin, A. M. Stickle, C. Reed, A. Galvez, I. Carnelli, and P. Michel, “Asteroid Impact and Deflection Assessment Mission," Acta Astronautica, vol. 115, pp. 262-269, 2015.
[2] Bottke W. and H. J. Melosh, Binary Asteroids and the Formation of Doublet Craters, Icarus 124: 372–391 (1996)
[3] Scheirich, P., and P. Pravec, 2009, Modeling of lightcurves of binary asteroids, Icarus, 200:531-547
[4] J. A. Atchison, M. T. Ozimek, B. Kantsiper, and A. F. Cheng, “Trajectory Options for the DART Mission,” International Astronautical Congress, Jerusalem, Israel, IAC-15-C1.1.31080.
[5] A. M. Stickle, J. A. Atchison, O. S. Barnouin, A. F. Cheng, D. A. Crawford, C. M. Ernst, Z. Fletcher, and A. S. Rivkin, “Modeling momentum transfer from kinetic impacts: Implications for redirecting asteroids," 13th Hypervelocity Impact Symposium, 2014.
[6] I. Carnelli, A. Galvez, K. Mellab, M. Kueppers, “Industrial Design of ESA Asteroid Impact Mission,” International Astronautical Congress, IAC-15-A3.4.9.x30901, 2015.

Monday, October 26, 2015

Links for ESA Europa Mission Studies

A reader pointed out that I forgot to provide links to the European Space Agency studies of possible contributions to NASA's Europa mission.  You can find a press article on NASA's original offer to ESA here and a link to the studies here.

I've also put a poll in the upper right corner of the blog page where you can show your preference for which of NASA's Discovery missions you would like to see selected.

Saturday, October 24, 2015

A European Spacecraft to Accompany NASA’s Europa Spacecraft?

Last year, NASA’s managers invited the European Space Agency (ESA) to propose a small spacecraft to explore the Jovian system.  The small craft would be carried to Jupiter by NASA’s own, large Europa multi-flyby spacecraft.  This daughter mission could add to the exploration of Europa or study another target within the Jovian system.

ESA has recently posted the results of studies for two possible spacecraft that might be carried by NASA’s Europa spacecraft to the Jupiter system.  One would land on Europa and the other would fly by the volcanic moon Io.  While these were concept studies and not an actual proposal from ESA to NASA, they give an idea of the possible capabilities and limitations on an ESA contribution. 

In the coming year, Europe’s scientists can make actual proposals for spacecraft to be added to NASA’s mission.  They can do so through ESA’s competition to select it’s fifth medium sized (~550M Euros or somewhat more in dollars at the current exchange rate) science mission.  Proposals for the Europa mission will be pitted against other planetary and astrophysics missions.  However, because NASA would cover the costs of launch and delivery to the Jupiter system, proposals for the Europa mission could have a leg up in the competition.

A small ESA spacecraft would need to find a scientific angle not already taken by its larger cousins.  Next year, NASA’s Juno spacecraft will study the Jupiter itself from just a few thousand kilometers above the top of the cloud deck.  ESA’s JUICE spacecraft will arrive in the late 2020s to study Jupiter from afar, flyby Europa and Callisto multiple times, and then orbit Ganymede.  NASA Europa Mission (apparently no longer called the Europa Clipper) will flyby Europa 45 times as well as flyby Ganymede and Callisto.  These missions will carry suites of extensive and highly capable instruments.

Under NASA’s proposal, the American space agency is reserving space and 250 kg of mass to host the European spacecraft.  (NASA is also separately reserving mass for the equipment to connect the two spacecraft.)

One concept for an ESA daughter probe (in blue) shown attached the NASA’s Europa Mission spacecraft.  Credit: ESA

So if you had a ride to the Jupiter system for 250 kg, what could you do with it?

Two obvious possibilities were mentioned at the time that the offer was announced: build a small lander for Europa or a small spacecraft that would fly through any plumes erupting from Europa’s surface.  The ESA team looked at both.

For a lander, the European study group considered a type of hard lander known as a penetrator.  Shaped roughly like a cannon shell, penetrators smack into the surface and then travel a few meters into it before coming to a stop.  (Think of a bullet shot into the ground.  The friction between the bullet’s surface with the soil slows and eventually stops the bullet.)

Concept illustration of a Europa penetrator (blue) embedded in the icy surface.  An after body (green) remains on the surface with the antenna to communicate with NASA’s Europa mission spacecraft.  A cable would connect the two sections.  Credit: ESA

Penetrators have been used on the Earth to deploy sensors from planes into remote locations such as ice shelves.  Penetrators have the advantage of not requiring expensive landing systems – the penetration into the surface supplies the braking.  A unique advantage on Europa is that once buried, the surrounding ice protects the probe from the radiation fields around Europa.

The concept studied would use the penetrator to deliver two sets of instruments in to the ice.  The first set would study the chemistry of the ice and materials within it.  A habitability package would use chemical reactions to look for conditions such as pH consistent with possible life, a mass spectrometer would analysis the composition, and a microscope would image the sample.  A seismometer would record Europa-quakes to study the level of activity within the icy crust and to gather clues about its structure.

The penetrator with its deployment stage.  Credit: ESA

The penetrator would be delivered by a deployment stage that would essentially be a small spacecraft with a substantial retrorocket.  The main NASA spacecraft would target the daughter craft to pass just 35 km above the landing zone.  Just before that distance, the European spacecraft would fire its rocket, reducing its speed to zero relative to the Europan surface below.  The delivery spacecraft with the attached penetrator then begin their free fall to the surface.  The delivery craft would have 231 seconds to reorient itself so that the penetrator points straight down and then release it.  NASA’s Europa spacecraft passing overhead would have a short few minutes to listen for a radio confirmation that the landing succeeded.

The penetrator deployment.  SRM burn is the rocket burn that removes all velocity between the penetrator and the surface of Europa.  The diagram uses the older name ‘Clipper’ for the NASA spacecraft.  Credit: ESA

Approximately ten days later, the NASA spacecraft would return to Europa and receive the data collected by the penetrator’s instruments.

While the penetrator concept is exciting, the devils are in the details.  Planetary penetrator missions have been studied for many potential missions.  Russia launched two on a mission intended for Mars but which never left Earth orbit.  NASA delivered two tiny penetrators to Mars, but they were never heard from after they were released by their carrier spacecraft.  Japan spent years developing penetrators for the moon but eventually cancelled the project because of development problems. 

A key problem with penetrators is that they need relatively flat landing sites for successful landings.  Europa’s surface is covered in slopes and rough terrain.  Also, Penetrators are built to tolerate high vertical velocities, but any lateral velocity can destroy the payload inside.  (Put another way, engineers can design for high G’s in one direction, but it’s hard to design for all directions.)  This means that the retro rocket must successfully kill all but the smallest lateral movement so the penetrator moves only vertically during its descent.

Another problem with penetrators is that the space inside is small and any instruments must be built to withstand high impact forces.  As a result, there’s usually significant instrument development required.  The penetrator concept report states that the instruments to study the chemistry of the ice are at a low state of technical readiness for use in a penetrator.

Issues such as these have kept penetrators as a great idea that has never been matured enough to become a reliable tool for planetary exploration.

My take on the report descripting the penetrator concept is that delivering a penetrator for Europa appears to be a high risk possibility both for completing the development in time for a launch and for actual delivery.  Another significant problem is that the concept craft would have a mass greater than 300 kg, well above the 250 kg NASA is offering.

The other concept studied by ESA’s engineers would be a daughter spacecraft that would be a straightforward use of existing technologies.  The original idea was for a small spacecraft that could fly through plumes erupting from Europa.  This idea seems to have lost its appeal.  First, diligent searches have failed to confirm the original observation of a possible plume (which was made at the limits of detectability).  Second, NASA’s Europa spacecraft is highly capable with cutting edge instruments, and it could fly through any plumes itself.

Between the Juno, JUICE, and Europa missions, almost all of the Jupiter system is already targeted for detailed study.  An exception, though, is the extremely volcanic moon Io that sits deep within Jupiter’s radiation field.  That moon became the target for the second study.

In this concept, NASA’s craft would release the European orbiter shortly after the two jointly enter Jovian orbit.  The ESA craft then would fire its own engine to lower the perijove of its orbit to encounter Io at least twice.

The Io flyby spacecraft would be so small that this top view engineering diagram uses millimeters for its length units.  The triangular main spacecraft body would be below the center main antenna dish.  Credit ESA.

The Io flyby spacecraft would carry four instruments.  A multi-color camera would image the surface at resolutions ranging from 2.2 km to 18 m per pixel, a thermal mapper would identify and measure the temperature of hotspots at resolutions ranging from 30 km to 50 m per pixel, a mass spectrometer would measure the composition of ions and particles ejected from Io, and a magnetometer would study the magnetic field around Io.

The trajectory for each of the two Io flybys (second flyby shown here) would be chosen so that the ground tracks pass over several areas of known volcanic activity so that these areas could be studied in high resolution.  The first flyby would pass 500 km above the surface at closest approach, the second flyby 100 km.  Credit ESA

Jupiter’s radiation is strongest in the plane of the equator where its major moons, including Io, orbit.  Since the ESA craft would be released from the NASA craft in an equatorial orbit, the ESA mission would receive the full radiation baking.  ESA’s spacecraft would have to traverse this radiation on both the inbound and outbound legs of its passes.  (A larger, fully dedicated Io mission such as the proposed Io Volcano Observer proposed for NASA’s Discovery program, would use a polar Jovian orbit instead, limiting its radiation exposure.)  The tiny ESA spacecraft potentially could perform more than two flybys if the harsh radiation close to Jupiter degrades the craft’s electronics more slowly than expected. 

The Io spacecraft study report does suggest that if the idea of an Io spacecraft is pursued, that the option of releasing it before the Jupiter orbit insertion burn is done should be considered.  That way, the Io spacecraft could do its own insertion burn and enter a Jovian polar orbit to reduce radiation exposure.

The Io flyby concept studies would come in on the heavy side by missing the 250 kg target mass by 17 kg.  To get that close, the concept design had accept a “more risky operational scheme”, that is, reduce backup systems and capabilities to minimize weight.

Either of these missions would be an exciting addition to the already planned JUICE and Europa missions.  The Io flyby craft seems to be less risky both to design and to fly.

When NASA first announced that it would offer room and mass for an ESA probe, ESA’s managers said they would let scientific teams propose which concepts would be considered as part of the next Medium science mission competition.  These two proposals are proof of concept studies that the proposing teams can use to inform their proposals.  We may be seeing even more interesting proposals from the science teams.  I can think of several alternative probes, and I’m sure the professionals can think of even more creative ones.

If ESA does contribute a small probe to NASA’s mission, the exploration of the Jupiter system may be more interesting than it already promises to be.

Saturday, October 3, 2015

Finalists for the Next NASA Discovery Mission

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

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

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

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

In the early years of the program from approximately the mid-1990s into the early2000s, NASA regularly selected Discovery missions every two to three years and would often select two missions at once.  Then budgets became squeezed and the last two mission selections were stretched out to every five years with a single selection each.  (The GRAIL orbiters selected in 2007 studied the moon for a year beginning in late 2011, and the Mars InSight geophysical mission selected in 2012 will launch next year).

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

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

The primary goal for the DAVINCI mission would be to make high priority measurements of the composition of Venus’ atmosphere.  Credit: NASA

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

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

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

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

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

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

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

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

The VERITAS mission would map with radar and infrared spectroscopy.  Credit: NASA/JPL-CALTECH

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

VERTITAS could greatly improve the resolution of images of Venus surface (top) and topography (bottom) as shown by these simulations based on terrestrial data (Hawaii and Iceland; the topographic image is from a paper discussing general improvements possible by a new mission and VERITAS’s actual resolution may be different).  Credit: NASA/JPL-CALTECH; Decadal Survey White Paper, NASA 

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

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

Origin and Evolution: How did Venus diverge from Earth?
• Determine if tesserae are remnants of an earlier wetter past
• Search for past tectonic or cratered surface beneath the plains

Venus as a Terrestrial Planet: What processes shape the planet?
• Determine how and when Venus was resurfaced
• Estimate lithospheric thickness variations with time
• Identify sources and rates of recent and active volcanism

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

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

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

The NEOCam space telescope.  Credit: NASA/JPL-CALTECH

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

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

Summary of the Lucy mission from the proposal’s factsheet.  Credit: SwRI

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

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

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

This mission looks to the New Horizon Pluto mission for two of its instruments with copies of that mission’s LORRI high resolution camera and the RALPH color camera and imaging spectrometer.  Another infrared spectrometer would draw on instrument heritage from Mars orbiters and the upcoming OSIRIX-REx asteroid sample return.  Tracking of the spacecraft’s radio signal would provide information on each asteroids mass and therefore density which provides clues to their composition and to whether they are solid objects or rubble piles.

The Psyche spacecraft above an artist’s concept of what the surface of a metallic asteroid might look like.

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

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

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

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

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