Centauri Dreams
Imagining and Planning Interstellar Exploration
Changing Conditions on 55 Cancri e
Roughly twice the radius and eight times as massive as Earth, 55 Cancri e is a ‘super-Earth’ in the interesting five-planet system some 41 light years away in the constellation Cancer. No habitable conditions here, at least not for anything remotely like the kind of life we understand: 55 Cancri e orbits its G-class primary every 18 hours (55 Cancri is actually a binary, accompanied by a small red dwarf at a separation of 1000 AU). The closest super-Earth we’ve yet found, this is a tidally locked world that, helpfully for our purposes, transits its host.
What we find in a just announced study of the planet’s thermal emissions out of the University of Cambridge is an almost threefold change in temperature over a two year period. Although we’ve done it before with gas giant atmospheres, this is the first time any variability in atmosphere has been observed on a rocky planet outside our own Solar System. No other super-Earth has yet given us signs of possible surface activity, and Cambridge’s Nikku Madhusudhan, a co-author of the study, calls the changes in detected light ‘drastic.’ They imply a huge temperature swing, from 1000 degrees to 2700 degrees Celsius (?1300 – 3000 K) on the star-side of this tidally locked world. Brice-Olivier Demory is lead author of the paper on these observations:
“We saw a 300 percent change in the signal coming from this planet, which is the first time we’ve seen such a huge level of variability in an exoplanet. While we can’t be entirely sure, we think a likely explanation for this variability is large-scale surface activity, possibly volcanism, on the surface is spewing out massive volumes of gas and dust, which sometimes blanket the thermal emission from the planet so it is not seen from Earth.”
Image: Artist’s impression of super-Earth 55 Cancri e, showing a hot partially-molten surface of the planet before and after possible volcanic activity on the day side. Credit: NASA/JPL-Caltech/R. Hurt.
The 55 Cancri e work was performed with data from the space-based Spitzer instrument. To understand the results, the authors look at the entire category of ultra-short period (USP) planet candidates found by Kepler, of which there are more than 100. Most of these have radii less than twice that of Earth, and some may be undergoing periods of intense erosion. The paper notes that the planet KIC12557548b shows changes in transit depth and shape that are consistent with what it calls a ‘cometary-like environment.’ The supposition here is that KIC12557548b is sub-Mercury in size but giving off an opaque cloud of dust, perhaps driven by surface volcanism.
From such scenarios the authors derive the idea that 55 Cancri e, one of the largest known USP planets, is likely subject to volcanism, with possible magma oceans on the day side. But in comparison to KIC12557548b, this world is large enough to contain its volcanic outgassing. From the paper:
…whereas extremely small planets (nearly mercury-size) subject to intense irradiation can undergo substantial mass loss through thermal winds, super-Earths are unlikely to undergo such mass-loss due to their significantly deeper potential wells… Thus, ejecta from volcanic eruptions on even the most irradiated super-Earths such as 55 Cnc e are unlikely to escape the planet and would instead display plume behaviour characteristic to the solar system. The extent and dynamics of the plumes if large enough can cause temporal variations in the planetary sizes and brightness temperatures and hence in the transit and occultation depths.
The larger picture is that we have begun to probe atmospheric conditions on worlds as small as two-Earth radii. Theories vary as to the composition of 55 Cancri e, with some observations suggesting a carbon-rich world while others point to a silicate-rich interior with a dense atmosphere. The variability found in this study calls the earlier models into question. But as we learn more about the material surrounding 55 Cancri e, we’ll be conducting what the authors call ‘a direct probe of the planet surface composition’ that may help us understand other USP planets.
The paper is Demory et al., “Variability in the super-Earth 55 Cnc e,” submitted to Monthly Notices of the Royal Astronomical Society (preprint). A University of Cambridge news release is available.
A Stagecoach to the Stars
Imagine the kind of spaceship we’ll need as we begin to expand the human presence into the nearby Solar System. We’d like something completely reusable, a vessel able to carry people in relative comfort everywhere from Mars to Venus, and perhaps as far out as the asteroid belt, where tempting Ceres awaits. Capable of refueling using in situ resources, these are ships not crafted for a single, specific mission but able to operate on demand without entering a planetary atmosphere. Brian McConnell, working with Centauri Dreams regular Alex Tolley, has been thinking about just such a ship for some time now. A software/electrical engineer, pilot and technology entrepreneur based in San Francisco, Brian here explains the concept he and Alex have come up with, one that Alex treated in a previous entry in these pages. The advantages of their ‘spacecoach’ are legion and Brian also offers a sound way to begin testing the concept. The author can be reached at bsmcconnell@gmail.com.
by Brian McConnell
“What if a spacecraft, like a cell, was made mostly of water?”
That’s what Alexander Tolley and I asked when we were working on our paper for the Journal of the British Interplanetary Society, “A Reference Design For A Simple, Durable and Refuelable Interplanetary Spacecraft” [1]. The paper explored the idea of a crewed spacecraft that used water as propellant in combination with solar electric propulsion. We dubbed them spacecoaches, as a nod to the stagecoaches of the Old West. Alex also gave the concept an excellent fictional treatment in Spaceward Ho!, also published here on Centauri Dreams. We are currently finishing a book about spacecoaches, to be published by Springer this fall. Subscribe to spacecoach.org for updates about the book and spacecoaches in general.
The idea of crewed solar electric spacecraft is hardly new. In 1954, Ernst Stuhlinger proposed a “sun-ship” powered by solar steam turbines and cesium ion drives [2,3]. Since then solar electric propulsion has been used in a wide variety of uncrewed craft. Meanwhile, the convergence of several technologies will make crewed solar electric vehicles feasible in the near future.
The core idea behind the spacecoach architecture is the use of water, and potentially waste streams, as propellant in electric engines. Water, life support and consumables are critical elements in a long duration mission, and in a conventional ship, are dead weight that must be pushed around by propellant that cannot be used for other purposes. Water in a spacecoach, on the other hand, can be used for many things before it is reclaimed and sent to the engines, and it can be treated as working mass. This, combined with the increased propellant efficiency of electric engines, leads to a virtuous cycle that results in dramatic cost reductions compared to conventional ships while increasing mission capabilities. Cost reductions of one or two orders of magnitude, which would make travel to destinations throughout the inner solar system routine, are possible with this approach.
Water is, for example, an excellent radiation shielding material, comparable to lead on a per kilogram basis, except you can’t drink lead. It is an excellent thermal battery, and can simply be circulated in reservoirs wrapped around the ship to balance hot and cold zones (this same reservoir doubles as the radiation shield). When frozen into fibrous material to form pykrete, it forms a material as tough as concrete, which can potentially be used for debris shielding or for momentum wheels, and if positioned correctly, can double as a supplemental radiation shield. If mixed with dilute hydrogen peroxide, which is safely stored at low concentrations, oxygen can be generated by passing it through a catalyst, similar to a contact lens cleaner. Dilute H2O2 is also a potent disinfectant, and can also be used to process human waste, as is done in terrestrial wastewater treatment plants. Anything the crew eats or drinks can be counted as propellant, as the water can be reclaimed and used for propulsion. This greatly simplifies planning for long missions because the longer the mission is, the more propellant you have in the form of consumables. This will also provide excellent safety margins and enable crews to survive an Apollo 13 scenario in deep space.
A spaceship that is mostly water will be more like a cell than a conventional rocket plus capsule architecture. Space agriculture, or even aquaculture, becomes practical when water is abundant. Creature comforts that would be unthinkable in a conventional ship (hot baths anyone?) will be feasible in a spacecoach. Meanwhile, inflatable structures will eventually enable the construction of large, complex habitats that will be more like miniature O’Neill colonies than a conventional spaceship [4].
In the book, Alex and I present a reference design that combines inflatable structures and thin film PV arrays to form a kite-like structure that both has a large PV array area, and can be rotated to provide artificial gravity in the outer areas [5]. The ability to generate artificial gravity while providing ample radiation protection solves two of the thorniest problems in long duration spaceflight. Alex wrote an excellent fictional treatment of the concept for Centauri Dreams called Spaceward Ho! This is intended as a straw man design to kickstart design competitions. We envision a series of design competitions for water compatible electric propulsion technologies, large scale solar arrays, and overall ship designs. Much of the reference design can be validated in ground based competitions and experiments, followed by uncrewed test vehicles (similar to what Bigelow Aerospace did by flying its Genesis I and II habitats in low earth orbit).
Spacecoaches are possible not because of any one insight or breakthrough, but because of the convergence of improvements in component technologies, specifically thin film photovoltaics, electric propulsion, and inflatable structures. The combination of the three, particularly when you add water for propulsion, leads to one or two order of magnitude improvements in mission economics.
Thin film solar photovoltaics, which enable the construction of large area PV sails, will enable ships to generate hundreds of kilowatts to several megawatts of electrical power (thin film PV material coincidentally is much more resistant to radiation than conventional silicon PV material) [6]. While thin film solar is not as efficient as silicon in terms of power per unit area, from a power density (watts/kilogram) standpoint, it offers multiple order of magnitude improvements, and will continue to improve for decades due to dematerialization in manufacturing processes.
SEP (solar electric propulsion) is a well understood, flight ready technology. Engines that function with water or gasified waste will be well suited to the spacecoach architecture. We simply need to test existing SEP technologies with water and waste streams to pin down performance and efficiency numbers, which can be done via an X-Prize style engineering competition. Scaling them to propel a large (40 tonne) ship will be done by clustering them in arrays, so there will be no need to build a single high power engine when an array of many 10-20 kilowatt units will do just fine, while also adding redundancy. One interesting discovery we made while doing our analysis is that ultra high specific impulse engines, such as VASIMR, are neither necessary nor desirable. Engines that operate at the low end of the electric propulsion envelope still yield excellent economics due to the synergies created by using water as propellant, while also being able to operate with less electrical power per unit of thrust, which reduces PV array size and mass.
Inflatable/expandable structures are just now beginning to be recognized as a flight ready technology, with Bigelow Aerospace’s BEAM unit due to fly on the ISS later this year. Bigelow already has two uncrewed inflatable habitats in low earth orbit. The basic idea with inflatable structures is to replace a rigid metal hull with a flexible high strength Kevlar type material and utilize pressurization to inflate and deploy the structure. This also enables a large habitable space to be compacted into a standard cargo fairing, thus requiring a minimal number of surface launches for initial delivery to orbit. We expect this technology to improve, both in terms of mass per unit of habitable space (currently about 60 kg per m3), and in terms of the types of shapes that can be created. [7]
Spacecoaches will not be mission specific ships. Even the first generation ships will be able to travel to many destinations within the inner solar system. They will be fully reusable, travelling from a high earth orbit or a Lagrange point to and from their destinations, without ever entering a planetary atmosphere. Spacecoaches will be able to travel to cislunar space, Mars, Venus, NEOs and maybe even Ceres and the Asteroid Belt. They can also be dispatched for asteroid interception and deflection missions on short notice. This is a huge departure from conventional spacecraft which are purpose built for a specific mission, usually Mars, that is planned decades in advance. Mars is certainly an interesting destination, but Ceres, with its abundant water resources and shallow gravity well, may turn out to be an even more interesting destination for human exploration and settlement.
The amount of water required for propellant on any given route will vary depending on the delta-v needed, and also the specific impulse of the engines on board, but water is easy to handle and store. Need to add an extra two kilometers per second to your delta-v budget? Just add water! (or replace the electric engines with slightly more efficient models). Because water is so easy to handle compared to conventional propellants, this will also simplify the construction and operation of orbiting fuel depots, which will be little more than orbiting water tanks.
Simplicity and upgradability is another key design element of the spacecoach. We assume that component technologies will continue to improve for decades. So instead of designing spacecoaches to fly only with today’s technology, they will be designed more like personal computers were in the 1980s. The original PCs were built around a common electrical and communication bus, the ISA bus, which allowed memory, CPUs and peripherals from many manufacturers to be combined. If you wanted to, you could buy the component parts from catalogs and build your own PC from scratch.
We envision something similar for the spacecoach, for the electrical system and engines in particular, which will have standard electrical and fluid interconnects, and uniform form factor requirements. The engines will also be mounted in a sealable compartment that can be pressurized so the crew can replace or upgrade engines without doing an EVA. This will not only make spacecoaches field upgradable, but will also reduce the need to design engines for extreme reliability. If a few units fail, crews would replace them in an operation not much different than replacing a rack mounted server. Upgrading engines will be the best way to improve performance and reduce costs, as a small increase in specific impulse can yield significant mass and cost reductions, especially for high delta-v routes like Ceres and the Asteroid Belt.
And what about cost?
Mention crewed missions to Mars, much less anywhere else, and people automatically assume you’re talking tens of billions of dollars as a starting point. We modeled approximate round-trip mission costs to destinations throughout the inner solar system, using a 40,000 kilogram (40 tonne) dry hull and SpaceX’s published launch costs to get materials, including water, into low earth orbit ($1,700/kg via Falcon 9 Heavy [8]), with electric propulsion (Isp between 1,500 to 3,000s) from there (electrode-less Lorentz force thrusters using water operate in this range). Among the missions we modeled were EML-2 (Earth Moon Lagrange point 2) to/from cislunar space, Martian moons, NEO interception, Venus orbit and Ceres. Even with engines operating at the low end of the electric propulsion performance envelope, our models predicted per mission costs in the hundreds of millions of dollars, a one or two order of magnitude reduction compared to conventional missions, some of which, such as a crewed mission to Ceres, simply are not possible via chemical propulsion.
Such large cost reductions are possible due to a combination of the fuel efficiency of electric engines, and the synergies created by using water as propellant. On one hand electric engines require far less propellant for a given delta-v. On the other, virtually everything the crew consumes or uses for life support can eventually be sent to the engines. As a result the only dead weight on the ship is the hull and whatever non-consumable materials and equipment are brought on board, which will also allow spacecoaches to carry larger crews. Reusability will also enable operators to amortize development and construction costs across many missions.
Spacecoaches are also well suited for in situ resource utilization. Should we reach low gravity destinations with accessible water (Ceres is an especially interesting location), it will eventually be possible to refuel spacecoaches at these destinations, or even ship water inbound to cislunar depots. We assume for now that spacecoaches are fully supplied from Earth, but exploring ISRU destinations and capabilities will be a high priority early on. Partially reusable launch vehicles offer another way to reduce costs. Water will be an ideal payload for a heavily re-used Falcon 9R booster. Unlike most payloads, it has essentially zero replacement cost, so the launch operator can fly the reusable boosters until they fail, and can learn about potential failure modes and fixes in the process (all while delivering more water to orbit).
If you are part of a team working on electric propulsion technology, here’s one way you can help make these a reality. Test your engine with water vapor, carbon dioxide and gasified waste (or a good analogue), and publish your results. The most important parameters ship designers will be interested in are specific impulse, efficiency (ideally the “wall plug” efficiency of the entire system so it can be modeled as a black box) and thrust/mass ratio. We already know several SEP technologies work reasonably well with water, but it will be great to examine all systems to see how well each works with water, compare performance across a variety of technologies, and identify opportunities for further improvement.
It is easy to be cynical about new spaceflight concepts, especially one that promises large cost reductions, but most of this can be validated on the ground and via uncrewed testbeds in a short time and at little expense. It is a paradigm shift, and that will take people some time to accept. The rocket + capsule design pattern served us well in the early years of spaceflight, so its hard to get away from that, but it’s time to move on to something that is more adaptable, something that’s more like a ship that can sail wherever her captain wants to go.
Spacecoaches will form the basis for a real world Starfleet, a fleet which will grow as ships are built, and which will reach new destinations as component technologies continue to improve in the coming decades. They will open the inner solar system out to the Asteroid Belt to human exploration and settlement, and with some spacecoaches operating in cislunar space, humanity will also have a rapid response capability should we be surprised by the discovery of an Earth threatening object.
Visit spacecoach.org to learn more, and to subscribe for notices about the upcoming book, which examines the spacecoach reference design and potential missions in detail. If you are interested in obtaining an advance copy of the book, acting as a technical reviewer or inviting us to speak, please get in touch.
References
[1] “Reference Design for a Simple, Durable and Refuelable Interplanetary Spacecraft”, B. S. McConnell; A. M. Tolley (2010), JBIS, 63, 108-119
[2] Image credit: Frank Tinsley/American Bosch Arma Corporation, 1954
[3] “Possibilities of Electrical Space Ship Propulsion,” E. Stuhlinger, Bericht über den V Internationalen Astronautischen Kongreß, Frederich Hecht, editor, 1955, pp. 100-119; paper presented at the Fifth International Astronautical Congress in Innsbruck, Austria, 5-7 August 1954
[4] “A Shape Grammar for Space Architecture – I. Pressurized Membranes”, Val Stavrev* Aeromedia, Sofia, Bulgaria, 40th International Conference on Environmental Systems, http://www.spacearchitect.org/pubs/AIAA-2010-6071.pdf
[5] Image credit: Rüdiger Klaehn
[6] “Super radiation tolerance of CIGS solar cells demonstrated in space by MDS-1 satellite”, Photovoltaic Energy Conversion, 2003. Proceedings of 3rd World Conference on, 18-18 May 2003, pp. 693 – 696 Vol.1
[7] Estimate based on BA330 mass per cubic meter of habitable space, per Bigelow Aerospace’s published specifications
[8] Per SpaceX published launch cost and delivery capacity for Falcon 9 Heavy, as of April 2015
Pluto/Charon: Surface Features Emerging
One of the more memorable moments from yesterday’s teleconference on the New Horizons mission was Alan Stern’s comment that the latest pixelated images of Pluto/Charon constituted his ‘meet Pluto moment.’ If anyone has an interest in meeting Pluto, it’s Stern (Southwest Research Institute), who serves as principal investigator and whose unflagging efforts made it possible. As for those pixelated views, well, they’re a glimpse of what is to come, but even now, they’re telling us helpful things about the target. The animation below speaks volumes, with the first showing Charon’s rotation with the center of Pluto fixed in the frame. The images were acquired with the Long Range Reconnaissance Imager (LORRI) camera.
Image: A series of LORRI images of Pluto and Charon taken at 13 different times spanning 6.5 days, from April 12 to April 18, 2015. During that time, the spacecraft’s distance from Pluto decreased from about 111 million kilometers to 104 million kilometers. Pluto and Charon rotate around a center-of-mass (also called the “barycenter”) once every 6.4 Earth days, and these LORRI images capture one complete rotation of the system. The direction of the rotation axis is shown in the figure. In one of these movies, the center of Pluto is kept fixed in the frame, while the other movie is fixed on the center of mass (accounting for the “wobble” in the system as Charon orbits Pluto). Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.
The second image shows the same view with the motion around the barycenter clearly revealed, a spectacular teaching tool for those trying to explain rotation around the center of mass.
Image: The 3x-magnified view of Pluto highlights the changing brightness across the disk of Pluto as it rotates. Because Pluto is tipped on its side (like Uranus), when observing Pluto from the New Horizons spacecraft, one primarily sees one pole of Pluto, which appears to be brighter than the rest of the disk in all the images. Scientists suggest this brightening in Pluto’s polar region might be caused by a “cap” of highly reflective snow on the surface. The “snow” in this case is likely to be frozen molecular nitrogen ice. New Horizons observations in July will determine definitively whether or not this hypothesis is correct. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.
The zoomed image is particularly notable because it clearly shows different brightness patterns as Pluto rotates (and notice that Charon, even though much smaller, still stays relatively the same, an indication that the surface is more uniform than Pluto, or as Stern said in the news conference, more ‘muted’). An apparent oddity here actually is not — If you look closely at the view of Pluto in the center, it seems to show an uneven surface. This is only an optical effect caused by the fact that dark areas on the dwarf planet are rotating in and out of view.
These albedo changes on the Plutonian surface are striking. Stern commented that it was unusual that we should be seeing such marked effects from such a distance. The possibility of a polar cap — probably made of highly reflective frozen molecular nitrogen ice — seems strongly enhanced by the bright area that persists in the zoomed image. JHU/APL has used a mathematical technique called deconvolution to improve the resolution of the raw LORRI images, giving us about the best resolution the camera and detector can provide at this distance.
And speaking of raw images, be aware of the raw image archive being assembled on the LORRI Images from the Pluto Encounter page. The imagery is, of course, going to keep getting better. We’ll get another sequence from an observing run scheduled for May 8 through May 14, producing another ‘movie’ in mid-May, followed by a lengthy pause to downlink the data that lasts until the 27th. The spacecraft will then begin observing Pluto/Charon daily at ever higher resolution. Fasten your seat belt.
HD 7924: Planets with a Robotic Assist
We’ve found a lot of planets far away from the Sun but know comparatively little about what may be circling nearby stars. The rationale is clear: The Kepler mission’s field of view was carefully chosen to provide a large sample (over 145,000 main sequence stars) that could be studied for transits by the spacecraft’s photometer. Looking out along the Cygnus arm of the Milky Way, far enough from the ecliptic plane to avoid the Sun, the Kepler stars have been providing statistical data to help us understand how common planets actually are in the galaxy.
But as we saw with the announcement of a candidate planet around Alpha Centauri B, the news of planets closer to home excites interest. These are places close enough to us that they could conceivably be the targets of future interstellar probes. As we continue to look at the Kepler inflow, we’re also anticipating missions like TESS (Transiting Exoplanet Survey Satellite), scheduled for a 2017 launch, and PLATO (PLAnetary Transits and Oscillations of stars), scheduled for 2024, both given the charter of finding habitable planets around nearby stars.
But we’re finding nearby worlds even now, as witness the discovery of two planets around the star HD 7924, a K-class main sequence star about 55 light years away in Cassiopeia. A single super-Earth was found here in 2009 using the 10-meter Keck I telescope working in conjunction with the HIRES spectrograph. Now further observations with Keck and a campaign using the Automated Planet Finder (APF) Telescope at Lick Observatory have uncovered two more planets in this system, making for a trio of ‘super-Earths.’
Image: Artist’s impression of a view from the HD 7924 planetary system looking back toward our Sun, which would be easily visible to the naked eye. Since HD 7924 is in our northern sky, an observer looking back at the sun would see objects like the Southern Cross and the Magellanic Clouds close to our sun in their sky. Credit: Karen Teramura & BJ Fulton, UH IfA.
The robotic contribution to this work is a story in itself. We’re looking at a new degree of automation in the exoplanet hunt with Earth-based observatories, one that co-author Andrew Howard (University of Hawaii) likens to “owning a driverless car that goes planet shopping.” The Automated Planet Finder instrument at the Lick Observatory operates robotically every clear night of the year, the installation consisting of a 2.4-meter automated telescope and the high-resolution Levy spectrometer, searching a preprogrammed list of nearby stars using radial velocity methods in the hunt for low-mass rocky worlds.
Lead author Benjamin Fulton (a University of Hawaii graduate student) notes that robotic searches like these can run all night without human oversight:
“We initially used APF like a regular telescope, staying up all night searching star to star. But the idea of letting a computer take the graveyard shift was more appealing after months of little sleep. So we wrote software to replace ourselves with a robot.”
The Automatic Photometric Telescope (APT) at Fairborn Observatory in Arizona also factored in. Now in operation for fourteen years, APT is one of a cluster of automated instruments at the observatory. Its observations of the brightness of HD 7924 complemented the APF and Keck data to confirm the new planets, both of which are seven to eight times the mass of Earth and, in a configuration we’re now finding common, orbiting close to their host star, with periods of 15.3 and 24.5 days. They join the previously discovered super-Earth with a period of a scant 5 days.
What to make of close-in super-Earths like these? From the paper (internal references omitted for brevity):
The large population of super-Earths orbiting close to their host stars was a surprise. Population synthesis models of planet formation had predicted that such systems would be rare. Planet cores were expected to mostly form beyond the ice line and rarely migrate to close orbits unless they first grew to become gas giants. Nevertheless, close-in, low-mass planets are common and often appear in compact multi-planet systems. Theoretical models are catching up, with refinements to the disk migration and multiplanet dynamics in the population synthesis family of models. A new class of “in situ” formation models have also been proposed in which systems of super-Earths and Neptunes emerge naturally from massive disks.
The first HD 7924 planet discovered (2009) grew out of the Eta-Earth Survey at Keck, in which Howard and colleagues searched for planets from a field of 166 nearby G and K dwarf stars. The team is now observing a subset of the Eta-Earth Survey stars with the Automated Planet Finder, with the two new HD 7924 worlds emerging from this work. With incident irradiation values 114, 28 and 15 times that of the Earth for the three planets, they’re not habitable by our standards of liquid water on the surface. But transits cannot be ruled out here and the paper adds that HD 7924 will be an excellent candidate for the James Webb Space Telescope. The upcoming launch of TESS offers the chance to observe the star closely for transit signatures.
The paper speaks of building a census of small planets in the local stellar neighborhood (within 100 light years) as the Automated Planet Finder continues its operations. There are doubtless many planets to find, considering Kepler’s discovery of so many compact, multi-planet systems.
The paper is Fulton et al., “Three super-Earths orbiting HD 7924,” accepted for publication in the Astrophysical Journal (preprint). But see also Howard and team’s 2010 paper on the Eta-Earth findings. It’s “The Occurrence and Mass Distribution of Close-in Super-Earths, Neptunes, and Jupiters,” Science Vol. 330, No. 6004 (2010), pp. 653-655 (abstract). This University of California Observatories news release is also helpful.
DSAC: Paradigm Changer for Deep Space Navigation?
We need to improve the way we handle data tracking and deep space navigation. While the near term is always uncertain because of budgetary issues, we can still take the long view and hope that we’re going to see a steadily increasing number of robotic and human spacecraft in the Solar System. That puts a strain on our existing facilities, and a premium on any methods we can find to make data return more precise and navigation more autonomous.
With these ideas in mind, keep your eye on the Deep Space Atomic Clock (DSAC). It’s a NASA technology demonstrator mission being built to validate a miniaturized, ultra-high precision mercury-ion atomic clock that researchers believe will be 100 times more stable than today’s best navigation clocks. Managed at the Jet Propulsion Laboratory, the DSAC has been tweaked and improved to the point where it allows drift of no more than a single nanosecond in ten days.
Image: Drawing of the DSAC mercury-ion trap showing the traps and the titanium vacuum tube that confine the ions. Credit: NASA/JPL.
We need improved atomic clocks in space to take spacecraft navigation to the next level and permit the next generation of studies of distant targets like Europa. Assuming Europa does have a subsurface ocean, this body of water will clearly be affected by tidal effects from Jupiter. Atomic clock measurements of DSAC’s caliber will be needed to provide the tracking data we’ll use to estimate Europa’s gravitational tide, helping to confirm the characteristics of its putative ocean. More on this thought and on the background of the DSAC in this JPL news release.
Europa, of course, is but one target whose investigation will be enhanced by projects like DSAC. Beyond this, improving the accuracy and stability of atomic clocks can change the way deep space navigation is done. Right now, we use a two-way paradigm for radiometric tracking, meaning that the same ground-based frequency standard is used as a reference for an uplink signal and a downlink detector. In other words, we track a spacecraft with our network on Earth and a ground-based team performs the necessary navigation. Improving the DSAC to allow it to operate in deep space will create a one-way tracking paradigm and autonomous navigation.
Think of the GPS unit you probably use to navigate with when you drive. GPS offers a one-way signal requiring no return signal from your car. The goal is to create the same one-way capability in deep space navigation. The smaller clock error (and DSAC is expected to be stable to less than 3 X 10-15 at one day, as measured by its Allan Deviation, a measure of frequency stability) enables one-way tracking with accuracies equal to or better than the two-way methods we currently use, a more flexible and efficient space navigation system.
The benefits of such a system will be striking, particularly in scenarios where we have several spacecraft either in orbit around or on the surface of a planet (Mars is the obvious reference for now). Let me pull a quote from Thornton and Border’s Radiometric Tracking Techniques for Deep-Space Navigation (Wiley, 2003):
…a single deep space antenna can acquire one-way Doppler and telemetry simultaneously from all spacecraft. Multiple uplink signals are not required. Consequently, this configuration results in more efficient use of ground-based resources and enhances orbit solutions and lander position estimates through the use of differential measurements.
Moreover, we get better signal-to-noise ratios for receiving spacecraft telemetry. Thornton and Border go on to explain the two reasons for this:
…one-way transmissions provide better short-term (< 1 s) stability, resulting in less signal loss in the detection process. This is because the short-term stability of two-way transmissions is degraded by solar plasma scintillations of the uplink signal and, for more distant spacecraft, by thermal noise in the spacecraft receiver. Second, the ground antennas are configured in a listen-only mode for one-way tracking, whereas the more complicated diplexer mode, required for simultaneous uplinking and downlinking, increases the effective system noise temperature of the ground receiver.
So DSAC technology can be a game-changer for deep space navigation, assuming the system checks out in flight. The plan is for the demonstration unit to be launched in 2016 aboard a SpaceX Falcon 9 Heavy booster, hosted on a spacecraft provided by Surrey Satellite Technologies. The equipment will be operated for at least one year, making use of GPS satellite signals to demonstrate precision orbital determination. Todd Ely (JPL), principal technologist for the DSAC Technology Demonstration Mission, describes the testing:
“Our in-orbit investigation has several phases beginning with commissioning, where we start up the clock and bring it to its normal operating state. After that we’ll spend the first few months confirming and updating our modeling assumptions, which we will use to validate the clock’s space-based performance. With these updates and our observation data, we’ll spend the next few months determining DSAC’s performance over many time scales…from seconds to days.”
Image: Overview of the mission architecture. Credit: NASA/JPL.
Following that period, the team will monitor clock telemetry to characterize its potential for long-term operations. The initial DSAC flight aims at producing the data that will help to make future units smaller and more efficient, readying them for the lengthy mission times that exploring deep space will demand. The kind of tracking data such a refined atomic clock will make available will improve spacecraft navigation and allow the precise tracking data we’ll need as we explore the moons of the gas giants and prepare for future targets even further out.
Exoplanet Spectrum in Visible Light
It’s the twentieth anniversary of the discovery of 51 Pegasi b, a ‘hot Jupiter’ that was the first planet to be discovered around a normal star. I always have to throw in that ‘normal’ qualifier because it was in 1992 that Aleksander Wolszczan and Dale Frail announced their discovery of planets around the pulsar PSR 1257+12, the first extrasolar planets ever found, and an extraordinary discovery in themselves. Michel Mayor and Didier Queloz announced the 51 Pegasi b discovery in 1995, and it was quickly confirmed by Geoff Marcy and Paul Butler.
51 Pegasi b, some 50 light years from Earth in the constellation Pegasus, is the prototypical ‘hot Jupiter,’ a gas giant orbiting in tight proximity to its star. A new paper from Jorge Martins (Universidade do Porto, Portugal) and team announces another first for this world, the first detection of the spectrum of visible light reflected off an exoplanet. The detection was made by painstakingly removing the host star’s spectrum to reveal the spectrum of the (by comparison) extremely faint planet.
Image: This artist’s view shows the hot Jupiter exoplanet 51 Pegasi b, sometimes referred to as Bellerophon, which orbits a star about 50 light-years from Earth in the northern constellation of Pegasus (the Winged Horse). This was the first exoplanet around a normal star to be found in 1995. Twenty years later this object was also the first exoplanet to be be directly detected spectroscopically in visible light. Credit: ESO/M. Kornmesser/Nick Risinger (skysurvey.org).
This is intriguing work because until now we’ve been largely confined to studying exoplanet atmospheres through a method called transmission spectroscopy, which allows scientists to study starlight filtered through the atmosphere during a transit. Also in common use is occultation photometry and spectroscopy, which measures the depth of the secondary eclipse as the planet passes behind the star (again, we’re talking about a transiting planet). Occultation spectroscopy is particularly useful in gauging the heat signature of the exoplanet.
The method deployed by Martins and team uses what is known as the cross-correlation function to extract meaning from the combined spectra of star and planet. Correlation is a mathematical operation that uses two signals to produce a third signal — this third signal is the cross-correlation of the two inputs. Using their spectroscopic data, Martins and team could amplify the signal of the planet and then remove the stellar signal to reveal the planet’s spectrum. The paper describes the basic principle at work:
…the cross-correlation function (hereafter CCF) can be used to mathematically enhance the S/N [signal-to-noise ratio] of our observations to a level where the extremely low S/N planetary signal can be recovered. The CCF of a spectrum with a binary mask (Baranne et al. 1996) has been extensively tested in detecting exoplanets with the radial velocities method. Briefly, this technique corresponds to mapping the degree of similarity between the stellar spectrum and a binary mask (representing the stellar type), which increases the S/N of the data by a factor proportional to the square root of the number of spectral lines identified in the mask.
The work was performed using the European Southern Observatory 3.6 meter telescope in conjunction with the HARPS spectrograph at La Silla (Chile). The results reveal that 51 Pegasi b is about half as massive as Jupiter although somewhat larger than Jupiter in diameter, with an orbital inclination of about nine degrees to the direction of the Earth. That means that the planet’s orbit is close to edge-on as seen from Earth, although not close enough to allow a transit. Remember, 51 Pagasi b was a radial velocity discovery, and the method used by Martins and his colleagues is not dependent on transits, unlike transmission and occultation spectroscopy.
Exposed to the intense starlight of its host star in such a tight orbit, the planet is highly reflective. And the fact that these properties can be deduced from the spectroscopic technique he has developed is promising for the future, as Martins explains:
“This type of detection technique is of great scientific importance, as it allows us to measure the planet’s real mass and orbital inclination, which is essential to more fully understand the system. It also allows us to estimate the planet’s reflectivity, or albedo, which can be used to infer the composition of both the planet’s surface and atmosphere.”
Moreover, the fact that existing equipment could produce these results points to the potential of future instruments on larger telescopes, such as the Very Large Telescope and the planned European Extremely Large Telescope. The ESPRESSO spectrograph (Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations) will greatly exceed the HARPS instrument to produce radial velocity measurements that can reveal Earth-like planets. Now being manufactured, the goal for first light of ESPRESSO on the VLT is 2016. The paper notes what the combination of advanced instruments and this new spectroscopic method means:
The sheer increase in precision and collecting power will allow for the detection of reflected light from smaller planets, planets on orbits with longer periods, or an increase in detail for larger planets like 51 Peg b.
The paper is Martins et al., “Evidence for a spectroscopic direct detection of reflected light from 51 Peg b,”
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