Centauri Dreams
Imagining and Planning Interstellar Exploration
The Value of an Exo-Venus
Looking back at science fiction’s treatment of Venus, you can see a complete reversal by the 1960s, at which time we had learned enough about the planet to render earlier depictions invalid, and even quaint. Think back to the inundated surface of Venus in Bradbury’s “Death by Rain” (1950) or Henry Kuttner and C. L. Moore’s Clash by Night (1943), where humans live under water and the land surfaces are carpeted with jungle. Heinlein’s Space Cadet is another example of a fecund Venus much like an Earthly rain forest.
But by 1965, Larry Niven would be writing “Becalmed in Hell,” about a nightmare Venus based on our insights into its intolerable surface. I should also mention a prescient tale by a writer who is a personal favorite of mine, James Gunn. It’s “The Naked Sky” (1955), which shows us a desert Venus with hydrochloric acid clouds and huge atmospheric pressure, a land Gunn described as “embalmed at birth.” As far as I know, this was the first SF tale that began to get Venus accurately, though at the time Gunn wrote, spacecraft had yet to confirm the hypothesis.
By 1962, we would have Mariner 2 readings of surface temperatures on Venus, while Soviet Venera probes would begin their work in 1967 with the first landing of a man-made object (Venera 4) on another planet. The program would continue for 16 launches, with 10 successful landings on the planet. Venera found a surface hot enough to melt lead, and survival times were short, with even the final iterations lasting no longer than about two hours.
Image: An image from the Soviet Venera 13 spacecraft. This was the first of the Venera missions to include a color TV camera and the first to succeed in obtaining pictures since Venera 10. Venera 13 lander touched down on 3 March 1982.
Now we’re finding planets much like Venus — or with the potential of being so — around other stars. With all the current interest in identifying an Earth 2.0, why be interested in Venus analogues? Elisa Quintana (NASA GSFC/SETI Institute), who works with the Kepler team, is one of the discoverers of such a world, Kepler 1649b. And here’s her take:
“Many people are hung up on finding other Earths. But Venus analogs are just as important. Since new telescopes coming down the pike will allow us to probe atmospheres, focusing on both Earth and Venus analogs may help decipher why, in our Solar System, one planet allows life to thrive, and one does not, despite having similar masses, comparable densities, etc.”
It’s a telling point, because the reason we got all those science fiction tales about a jungle Venus is that in so many ways it seemed to be a twin of the Earth, albeit one that was 40 percent closer to the Sun. It was easy to transfer tropical traits to the place, as if a journey there were somewhat like heading into unexplored equatorial terrain on our own planet.
Although Kepler 1649b circles a red dwarf star about 220 light years away, it receives about 2.3 times the stellar flux that the Earth does. The number for Venus is 1.9 times the terrestrial value. Looking into what makes Venus the nightmare it is can help us understand habitable zone boundaries around M-dwarfs that much better as we examine the tidal effects and stellar activity through flares and stellar wind that distinguish red dwarf planets from the Sun’s.
Follow-up spectroscopy and imaging of Kepler 1649 indicated to lead author Isabel Angelo (SETI Institute) and team that the parameters of the star had to be adjusted. It turned out to be considerably hotter and larger than had been thought, the adjusted figures affecting the planet observed in transit. We now know that Kepler 1649b is just slightly larger than Earth. This is another case where Kepler planets have been re-characterized because of revisions to the properties of the host star, and a reminder of the importance of such follow-ups.
We learn from all this that the planet is on a 9-day orbit, but it is too small to produce solid radial velocity data that would help determine its mass, leading the authors to make no conclusions about mass or composition. The likelihood, though, considering that Kepler 1649b is 1.08 times the radius of the Earth, is that we are dealing with a rocky world. The paper compares the planet to Kepler-186f, an Earth-sized exoplanet thought to orbit in the habitable zone of another M-dwarf. Flares and coronal mass ejections are factors around such stars, while tidal locking and heating could affect the geological activity on both. The authors consider the two to be good candidates for Earth- and Venus-analog studies.
From the paper:
The discovery of Kepler-1649b is part of a larger movement toward confirmation and characterization of a variety of Earth-sized exoplanets, with the ultimate goal of understanding what factors place constraints on habitability. Most of these planets have orbital periods measured to high precision, allowing us to calculate the flux received by the planet from its host star. As a result, determining the correlation between incident flux and atmospheric compositions would be highly useful in assessing the habitability of known exoplanets. More specifically, determining the compositions and atmospheres of planets like Kepler-1649b and Kepler-186f, two planets that together span a wide range of distances within the habitable zones of M-dwarfs, will be useful in understanding the nature of habitable zone boundaries for such star types. Future missions like K2, TESS, and JWST… will make these studies possible and therefore lend themselves to a better understanding of conditions required for exoplanet habitability.
Confirming what surface conditions are actually like on either world will demand spectroscopic analysis of their atmospheres, and when it comes to Venus analogues, this is a tricky proposition because of their opacity. But the paper adds that there are distinguishing features in the high clouds of Venus like carbon dioxide absorption and an upper haze layer with sulfuric acid, that could make detection possible. The thick atmosphere dominated by clouds also produces scattering and reflection effects that lead to high albedo, which the authors see as another piece of evidence that could link an atmosphere to a runaway greenhouse.
All such studies rely upon our confidence in the properties of the planets we find, and that means accurate information about their host stars. Missions like Gaia are designed to measure the distances to nearby exoplanet systems like Kepler-1649 through the most precise parallax measurements ever obtained, which should further tighten the parameters on systems like this one. We need to know more about the factors that can take two worlds of similar mass and density in our system and render one habitable while turning the other into a furnace.
The paper is Angelo et al., “Kepler-1649b: An Exo-Venus in the Solar Neighborhood,” Astronomical Journal Vol. 153, No. 4, published online 17 March 2017 (abstract and full text).
Atmosphere Detected around Super-Earth GJ 1132b
There’s interesting news this morning about planets around M-dwarfs. A team of astronomers led by John Southworth (Keele University, UK) has detected an atmosphere around the transiting super-Earth GJ 1132b. While we’ve examined the atmospheres of gas giants and have detected atmospheres on the super-Earths 55 Cancri e and GJ 3470 b, GJ 1132b is the smallest world yet where we’ve detected one. 39 light years from Earth in the constellation Vela, the transiting planet is 1.4 Earth radii in size, with a mass 1.6 times that of our world.
We’re continuing to move, in other words, into the realm of lower-mass planets when we study planetary atmospheres, an investigation that will be crucial as we look for biosignatures in distant solar systems. With GJ 1132b, we’re dealing with a planet too close to its star to be habitable (it receives 19 times more stellar radiation than the Earth does, and has an equilibrium temperature of 650 K, or 377° C). But finding a thick atmosphere here is encouraging given the level of flare and stellar wind activity on M-dwarfs.
Such activity could strip a planet of its atmosphere in some scenarios, so the survival of atmospheres on planets in the habitable zone of similar stars remains in play. In GJ 1132b, we have a planet whose atmosphere has evidently persisted for billions of years.
The GJ 1132b work was done with the GROND imager attached to the 2.2 m ESO/MPG telescope at La Silla. GROND (Gamma-ray Burst Optical/Near-infrared Detector) is normally used to study Gamma Ray Burst afterglows at seven different wavelengths from the optical to near-infrared, allowing rapid follow-up spectroscopic observations at other telescopes, but it can also be used to study exoplanets as well as optical, X-ray and radio transients.
Using GROND, the researchers could measure the decrease in brightness as the planet’s atmosphere absorbed some of the starlight while passing in front of the star in transit. The team’s intention was to determine the radius of the planet in each of the seven passbands (filters) for which it could obtain transit lightcurves, analyzing the significance of variations between the passbands in terms of atmospheric composition.
The result: The planet appeared larger at some wavelengths than others, an indication of an atmosphere opaque to specific wavelengths while transparent otherwise. The average radius of the planet could be separated out into a surface radius of 1.375 Earth radius overlaid by this atmosphere, which increases the observed radius at the wavelengths mentioned.
Simulating different atmospheres through follow-up work from team members at the University of Cambridge and the Max Planck Institute for Astronomy, Southworth and company found that an atmosphere rich in water and methane fit their observations. As to what the planet’s surface composition might be, two possibilities emerge. From the paper:
We find that the mass and radius are consistent with two broad compositional regimes. Firstly, an exactly Earth-like composition, with 33% iron, 67% silicates and no volatile layer, is inconsistent with the data within the 1σ uncertainties. But, a composition with higher silicate-to-iron fraction, including a pure silicate planet, is ostensibly consistent with the data, albeit marginally.
So perhaps a rocky world, or perhaps not:
On the other hand, the data are also consistent with a large range of H2O mass fractions between 0% and 100% in our models. In principle, consideration of temperature-dependent internal structure models would lead to larger model radii for the same composition… and therefore could lower the upper limit on the water mass fraction. Nevertheless, the mass and radius of GJ 1132 b allow for a degenerate set of solutions ranging between a purely silicate bare-rock planet and an ocean planet with a substantial H2O envelope.
Image: Artist’s impression of the exoplanet GJ 1132 b, which orbits the red dwarf star GJ 1132. Credit: MPIA.
The authors advocate extensive follow-up work on this planet with instruments like the Hubble Space Telescope, ESO’s Very Large Telescope, and the James Webb Space Telescope. In particular, we can begin to delve into the atmosphere here to look for its constituents:
Intermediate-band photometry at 900 nm or bluer than 500 nm would enable finer distinctions to be made between competing model spectra and a clearer understanding of the chemical composition of the planetary atmosphere. The planet’s mean density measurement is also hindered by the weak detection of the velocity motion of the host star, an issue which could be ameliorated with further radial velocity measurements using large telescopes. Finally, infrared transit photometry and spectroscopy should allow the detection of a range of molecules via the absorption features they imprint on the spectrum of the planet’s atmosphere as backlit by its host star.
We’re getting close to the day when improved space- and ground-based installations will allow us to use transmission spectroscopy to look for biosignatures in the atmospheres of planets in the habitable zone of nearby red dwarfs, markers like oxygen, ozone, methane and carbon dioxide in a simultaneous presence that would indicate replenishment by living systems. We’re not there yet, but what we have here is a demonstration that a planet with 1.6 Earth’s mass in a tight orbit of its red dwarf host is capable of holding on to an extensive atmosphere.
The paper is Southwork et al., “Detection of the atmosphere of the 1.6 Earth mass exoplanet GJ 1132B,” Astronomical Journal Vol. 153, No. 4 (31 March 2017). Abstract / preprint.
New Horizons: Star Fields Beyond
The attitude you bring to a star field changes everything. When I was a kid trying to figure out how to use a small telescope, I scanned the usual suspects — the Moon, Saturn and its rings, the Galilean satellites of Jupiter — all the while planning to branch out into major wonders like M31 or the Ring Nebula in Lyra. But when I turned to deep sky objects, what I discovered was that I could see little more than faint smudges — I was using no more than a 3-inch reflector. It was a disappointment for a while, until I accepted the limitations of my equipment.
And then I became a cataloger of faint smudges, as avidly tracking down celestial objects as any stamp collector sorting through new finds. A patient uncle showed me how to look slightly away from the object I sought, to pick it up in peripheral vision. I began keeping notebooks listing my first glimpses of various nebulae and clusters. So many celestial objects were out of reach, but somehow a field of stars became wondrous not only for what I was seeing but for what I knew I might see with a larger instrument.
The image below recalled those days precisely because of what we cannot see in it. This is actually drawn from a series of images taken by New Horizons’ LORRI instrument showing the area toward which the craft is heading. We’re looking toward a close approach and flyby of the Kuiper Belt object MU69 at 0200 Eastern US time (0700 UTC) on New Year’s Day of 2019. Right now we’re still far enough way that the target isn’t visible even to LORRI, but to me this image is freighted with the raw excitement of exploration as we push ever deeper into the Solar System.
Image: In preparation for the New Horizons flyby of 2014 MU69 on Jan. 1, 2019, the spacecraft’s Long Range Reconnaissance Imager (LORRI) took a series of 10-second exposures of the background star field near the location of its target Kuiper Belt object (KBO). This composite image is made from 45 of these 10-second exposures taken on Jan. 28, 2017. The yellow diamond marks the predicted location of MU69 on approach, but the KBO itself was too far from the spacecraft (877 million kilometers) even for LORRI’s telescopic “eye” to detect. New Horizons expects to start seeing MU69 with LORRI in September of 2018 – and the team will use these newly acquired images of the background field to help prepare for that search on approach. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
New Horizons is going into hibernation this week for a 157 day period, and I hadn’t realized until getting this JHU/APL update that the craft had been in full operational mode for almost two and a half years now, which of course dates back to the Pluto encounter and the long period of data return (16 months). Along the way New Horizons has continued to study the dust and charged particle environment of the Kuiper Belt as well as hydrogen in the heliosphere.
We’re now halfway between Pluto and MU69, having reached this point — a distance of 782.45 million kilometers from Pluto and MU69 — early on April 3 (UTC). The gravitational pull of the Sun continues to slow the craft, so it won’t be until tomorrow (April 7) that we reach the halfway point in terms of time between the two close approaches. Remember that New Horizons left Earth orbit traveling faster than any vehicle ever launched, but nine years of climbing out of the gravity well have slowed it to 14 kilometers per second at the Pluto/Charon flyby, significantly below the 17 kilometers per second-plus that Voyager 1 has attained.
The good news is that the mission includes further exploration beyond MU69. Hal Weaver is a New Horizons project scientist from the Applied Physics Laboratory (Laurel, MD):
“The January 2019 MU69 flyby is the next big event for us, but New Horizons is truly a mission to more broadly explore the Kuiper Belt. In addition to MU69, we plan to study more than two-dozen other KBOs in the distance and measure the charged particle and dust environment all the way across the Kuiper Belt.”
Looking ahead once we’re past MU69, there will be so many things we cannot see in the star field ahead. So much to discover for the deep space missions beyond New Horizons. When will a true interstellar probe — a mission designed from the start to examine the local interstellar medium — be launched? Without an answer, we can only keep pushing for exploration, an innate characteristic of our species, and one unlikely to be limited by our Solar System.
‘Blue Binaries’ Argue for Smooth Neptune Migration
We’re getting a few clues about the nature of planet migration in the early Solar System thanks to a class of objects being described as ‘blue binaries.’ Cold Classical Kuiper Belt Objects (CCKBOs) are generally reddish, but a population of widely separated binaries has now been identified that is thought to have originated in the inner edges of the Kuiper Belt.
The paper reporting on this work argues that these objects were pushed out a distance of over 4 AU to their present location among the CCKBOs as the result of gravitational interactions with Neptune billions of years ago, a movement induced by the migration of the planet from 20 to 30 AU. If so, we can draw some conclusions about that migration, and we’re reminded in the process of how rich the Kuiper Belt is in objects of different origins.
Led by Wes Fraser (Queen’s University, Belfast), the study used data drawn from the Gemini North instrument and the Canada-France-Hawaii Telescope, both on Mauna Kea, as part of a project called the Colors of the Outer Solar System Origins Survey. The team went to work on spectral data in a range of wavelengths from the ultraviolet to the optical and near-infrared. Its conclusion, reached by checking against models of Neptune’s migration, is that the blue binary pairs moved outward slowly enough that they were not disrupted into single objects.
Image: Artist’s conception of a loosely tethered binary planetoid pair like those studied by Fraser et al. in this work, which led to the conclusion that Neptune’s shepherding of them to the Kuiper Belt was gradual and gentle in nature. Credit: Gemini Observatory/AURA, artwork by Joy Pollard.
Let me quote from the paper re the team’s simulations:
The idea that the blue CCKBO binaries are contaminants that were pushed out into cold classical orbits during Neptune’s migration agrees well with the hypothesis that the blue–red bifurcation of the excited KBO colour distribution is a result of an object’s heliocentric formation distance. In our simulations, surviving binaries were pushed out by no more than 6 au, originating in the ?38–40 au range. Similar simulations suggest that the red-coloured, widely separated binary 2007 TY430, which now resides in the 3:2 MMR [mean motion resonance], plausibly originated at ?37–39 au, but could have originated even further out . This would require that the distance inside which blue binaries originated was only a handful of au inside the current inner edge of the cold classical region.
The Kuiper Belt contains a heterogeneous population indeed. Work by Alex Parker (University of Victoria, BC) and J.J. Kavelaars (Herzberg Institute of Astrophysics, Canada) tells us that the Cold Classical Kuiper Belt Objects are the only part of the Kuiper Belt population that formed in place. These objects share a reddish color, low inclinations and low eccentricities, a contrast to a population of dynamically excited objects that come in a range of colors.
Whereas this latter group is comprised of only about 10 percent binaries, the Cold Classical Kuiper Belt Objects exist as binary pairs about 30 percent of the time. The newly discovered blue binaries, though, distinguish themselves from both camps. In an email this morning, Dr. Fraser told me that while a few blue objects were previously known to exist in this region, his team had realized that the blue objects they were studying all came in binary pairs: “It was the discovery of blue+binary together that showed us we were looking at something special.”
Moreover, the binary nature of the objects as the products of Neptune’s ‘push out’ demands that they originally formed as multiple objects. From the paper again:
This push-out scenario would have the startling implication that virtually all planetesimals that formed in the region from which the blue binaries originated must either have formed as binaries or higher multiplicity systems, or attained high multiplicity before push-out occurred. This is required by the fact that all but one of the blue CCKBOs are found in binary pairs.
Thus we can see the blue binaries, originating at about 38 AU, as contaminants that in the paper’s language “…could provide a unique probe of the formative conditions in a region now nearly devoid of objects.” The paper explores various formation scenarios for these binaries that include pebble accretion and later binary production as well as binary formation from the collapse of a cloud of gas and solids that produces bound systems of two or more objects.
The paper is Fraser et al., “All planetesimals born near the Kuiper Belt formed as binaries,” published online by Nature Astronomy 4 April 2017 (abstract). The paper by Parker and Kavelaars cited above is “Destruction of binary minor planets during Neptune scattering,” Astrophysical Journal 722, L204–L208 (2010) (full text).
New Options for Locating Fast Radio Bursts
Our catalog of distant, highly energetic events continues to grow. On the Fast Radio Burst (FRB) front, we have the welcome news that the Molonglo radio telescope some 40 kilometers from Canberra, Australia has undergone extensive re-engineering, a project that is paying off with the detection of three new FRBs. The telescope’s collecting area of 18,000 square meters and an eight square degree field of view make it ideal for such work.
Image: Artist’s impression shows three bright red flashes depicting Fast Radio Bursts far beyond the Milky Way, appearing in the constellations Puppis and Hydra. Credit: James Josephides/Mike Dalley.
You’ll recall that Fast Radio Bursts are millisecond long, intense pulses that can appear out of nowhere with a luminosity a billion times greater than anything we have observed in the Milky Way. The phenomenon was noted for the first time a decade ago at the Parkes radio telescope in New South Wales. The sources remain enigmatic, but Manisha Caleb, a PhD candidate at Australian National University, has been developing software to examine the 1000 TB of data that is being produced daily at the Molonglo site. The new FRBs are the result.
“Conventional single dish radio telescopes have difficulty establishing that transmissions originate beyond the Earth’s atmosphere,” says Swinburne University’s Dr Chris Flynn.
But Molonglo (the Molonglo Observatory Synthesis Telescope, to give its full name, usually shortened to MOST) is a parabolic, cylindrical antenna consisting of 88 bays, each of these made up of four identical modules, to produce 352 independent antennae. The upgrade to the Molonglo instrument is known as UTMOST. The researchers used the array in a 180-day survey of the Southern sky, carrying out up to 100 hours of follow-up for each FRB, though no repeating bursts were seen. A repeating burst at UTMOST, or an FRB simultaneously detected at Parkes and UTMOST, would allow a localization of a few arcseconds.
From the paper (note that ” is the symbol for arcseconds):
In this paper we present the first interferometric detections of FRBs, found during 180 days on sky at UTMOST. The events are beyond the ≈ 104 km near-field limit of the telescope, ruling out local (terrestrial) sources of interference as a possible origin. We demonstrate with pulsars that a repeating FRB seen at UTMOST has the potential to be localised to ≈ 15″ diameter error circle, an exciting prospect for identifying the host.
Image: View of the MOST at the end of the East arm looking West. Two arms, each 800 meters long, together have a collecting area of circa 18,000 square meters. MOST is the largest radio telescope in the Southern hemisphere. Credit: Swinburne University/UTMOST.
The recent discoveries point to a new ability to locate FRBs in the sky, allowing us to link them to specific galaxies, a feat that has been accomplished only once so far (see Pinpointing a Fast Radio Burst). The paper is Caleb et al., “The first interferometric detections of Fast Radio Bursts,” accepted at Monthly Notices of the Royal Astronomical Society (preprint). For more, see this UTMOST news release.
Building the Tools for Icy Moons
With my own memories of the July 4, 1997 landing of Mars Pathfinder at Chryse Planitia as fresh as yesterday, it’s hard to believe that we are looking at the 20th anniversary of rover operations on the planet. But as the Curiosity rover continues its travels and we look toward the Mars 2020 rover mission, we’re also taking a much longer look ahead at the worlds where life may be more likely to be found, the icy moons of Jupiter and Saturn.
Ponder this: Testing at the Jet Propulsion Laboratory is showing that ice grains in conditions like we will encounter on places like Enceladus or Europa can behave like sand dunes. That means fine grains that could stall an improperly designed rover, leading NASA engineers to begin rethinking designs harking back to the early days of Moon exploration, lightweight commercial wheels attached to a flexible chassis, a system that has worked for a variety of missions but will need adjustment for future work. The rovers that use these systems will, in the case of moons like Europa, need serious attention to radiation hardening.
Image: Artist’s concept of Europa’s frozen surface. Credit: JPL-Caltech.
The project looking into extreme environments for these next generation rovers is called the Ocean Worlds Mobility and Sensing study, which is developing prototypes for sample acquisition on icy moons. Working tricky environments like Enceladus means coping with fissures that blow jets of material out into space. And Europa’s challenge goes far beyond Jupiter’s cascading radiation given the need to get through surface ice to sample the subsurface ocean. On Titan, we prepare for a possibly mushy ethane-rich surface.
“In the future, we want to answer the question of whether there’s life on the moons of the outer planets — on Europa, Enceladus and Titan,” says Tom Cwik, who leads JPL’s Space Technology Program. “We’re working with NASA Headquarters to identify the specific systems we need to build now, so that in 10 or 15 years, they could be ready for a spacecraft.”
The Europa work is particularly interesting given the question of astrobiology in the Europan ocean. The biggest problem for designers right now is that we have no certainty about the thickness of the surface ice, which could conceivably extend as deep as 20 kilometers. Richard Greenberg has argued in Unmasking Europa (Copernicus 2008) that the evidence of surface melt-through of Europan ice points to ice as thin as five kilometers. Whatever the case, so-called ‘melt probe’ designs are being scaled up for the challenge.
The JPL work on melt probes is led by Brian Wilcox, who has concluded that the thickness of Europan ice, in comparison with the Earth ices that conventional melt probes have coped with, demands an emphasis on heat efficiency. Wilcox is examining a vacuum-insulated capsule that uses a heat source like plutonium to keep its energy as it sinks into the ice. At one end, a rotating sawblade turns and cuts ever deeper, while pushing ice chips into the body of the probe, where they are melted and pumped out behind it.
The idea is that the water would be sampled in the midst of this process, with some being returned to the surface lander through pneumatic tubing. There it would be checked for biosignatures. And given concerns about possible contamination by Earthly life, Wilcox would heat the probe to almost 500° C during the cruise phase of the flight, thus killing off any residual organisms that may be hitching a ride from Earth.
“We think there are glacier-like ice flows deep within Europa’s frozen crust,” Wilcox said. “Those flows churn up material from the ocean down below. As this probe tunnels into the crust, it could be sampling waters that may contain biosignatures, if any exist.”
This JPL news release offers more, including work on a new generation of robotic arms that will out-reach our Mars rovers, which have never extended beyond about 2.5 meters from the base unit. JPL is looking at a robotic arm with a 10 meter extension, and is also considering a projectile launcher that can fire a sampling mechanism to distances of 50 meters. Having obtained the sample, the device would be reeled in by a tether.
These technologies are designed to be used along with an ice-gripping claw which could have a coring drill attached to it for taking surface samples at locations of interest. The drill would be able to reach about 20 centimeters below the surface ice. The ice claw itself would be deployed from a boom arm and would anchor itself with heated prongs that melt into the ice to secure the unit. It would then penetrate the ice with its drill bit to collect the needed sample.
Image: A robotic claw, one of several innovative tools developed at JPL for exploring icy, ocean worlds like Europa. Credit: NASA/JPL-Caltech.
The Ocean Worlds Mobility and Sensing site is sparse at the moment but it’s one you might want to keep an eye on as the effort continues. NASA is considering a second phase of the study that would be tapped as we work out the overall design of any icy moons lander.
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