Centauri Dreams
Imagining and Planning Interstellar Exploration
Electric Sails: Fast Probe to Uranus
For years now Pekka Janhunen has been working on his concept of an electric sail with the same intensity that Claudio Maccone has brought to the gravitational focus mission called FOCAL. Both men are engaging advocates of their ideas, and having just had a good conversation with Dr. Maccone (by phone, unfortunately, as I’ve been down with the flu), I was pleased to see Dr. Janhunen’s electric sail pop up again in online discussions. It turns out that the physicist has been envisioning a sail mission to an unusual target.
Let’s talk a bit about the mission an electric sail enables. This is a solar wind-rider, taking advantage not of the momentum imparted by photons from the Sun but the stream of charged particles pushing from the Sun out to the heliopause (thereby blowing out the bubble’ in the interstellar medium we call the heliosphere). As Janhunen (Finnish Meteorological Institute) has designed it, the electric sail taps the Coulomb interaction in which particles are attracted or repulsed by an electric charge. The rotational motion of the spacecraft would allow the deployment of perhaps 100 tethers, thin wires that would be subsequently charged by an electron gun with the beam sent out along the spin axis.
Image: The electric sail is a space propulsion concept that uses the momentum of the solar wind to produce thrust. Credit: Alexandre Szames.
The electron gun keeps the spacecraft and tethers charged, with the electric field of the tethers extending tens of meters into the surrounding solar wind plasma — as the solar wind ‘blows,’ it pushes up against thin tethers that act, because of their charge, as wide surfaces against which the wind can push. The sail uses the attraction or repulsion of particles caused by the electric charge to ride the wind, the positively charged solar wind protons repelled by the positive voltage they meet in the charged tethers.
One disadvantage that electric sails bring to the mix, as opposed to solar sails like IKAROS, is that the solar wind is much weaker — Janhunen’s figures have it 5000 times weaker — than solar photon pressure at Earth’s distance from the Sun. This has come up before in comments here and it’s worth quoting Janhunen on the matter, from a site he maintains on electric sails:
The solar wind dynamic pressure varies but is on average about 2 nPa at Earth distance from the Sun… Due to the very large effective area and very low weight per unit length of a thin metal wire, the electric sail is still efficient, however. A 20-km long electric sail wire weighs only a few hundred grams and fits in a small reel, but when opened in space and connected to the spacecraft’s electron gun, it can produce several square kilometre effective solar wind sail area which is capable of extracting about 10 millinewton force from the solar wind.
Computer simulations using tethers up to 20 kilometers in length have yielded speeds of 100 kilometers per second, a nice step up from the 17 kps of Voyager 1, and enough to get a payload into the nearby interstellar medium in fifteen years. Or, as Janhunen describes in the recent paper on a Uranus atmospheric probe, an electric sail could reach the 7th planet in six years. Janhunen sees such a probe as equally applicable for a Titan mission and, indeed, missions to Neptune and Saturn itself, but notice that none of these are conceived as orbiter missions. A significant amount of chemical propellant is needed for orbital insertion unless we were to try aerocapture, but the problem with the latter is that it is at a much lower technical readiness level.
A demonstrator electric sail mission, then, is designed to keep costs down and reach its destination as fast as possible, with the interesting spin that, because we’re in need of no gravitational assists, the Uranus probe will have no launch window constraints. As defined in the paper on this work, the probe would consist of three modules stacked together: The electric sail module, a carrier module and an entry module. The entry module would be composed of the atmospheric probe and a heat-shield.
At approximately Saturn’s distance from the Sun, the electric sail module would be jettisoned and the carrier module used to adjust the trajectory as needed with small chemical thrusters (50 kg of propellant budgeted for here). And then the fun begins:
About 13 million km (8 days) before Uranus, the carrier module detaches itself from the entry module and makes a ~ 0.15 km/s transverse burn so that it passes by the planet at ~ 105 km distance, safely outside the ring system. Also a slowing down burn of the carrier module may be needed to optimise the link geometry during flyby.
Now events happen quickly. The entry module, protected by its heat shield, enters the atmosphere. A parachute is deployed and the heat shield drops away, with the probe now drifting down through the atmosphere of Uranus (think Huygens descending through Titan’s clouds), making measurements and transmitting data to the high gain antenna on the carrier module.
Thus we get atmospheric measurements of Uranus similar to what the Galileo probe was able to deliver at Jupiter, measuring the chemical and isotopic composition of the atmosphere. A successful mission builds the case for a series of such probes to Neptune, Saturn and Titan. Thus far Jupiter is the only giant planet whose atmosphere has been probed directly, and a second Jupiter probe using a similar instrument package would allow further useful comparisons. Our planet formation models, which predict chemical and isotope composition of the giant planet atmospheres, can thus be supplemented by in situ data.
Not to mention that we would learn much about flying and navigating an electric sail during the testing and implementation of the Uranus mission. The paper is Janhunen et al., “Fast E-sail Uranus entry probe mission,” submitted to the Meudon Uranus workshop (Sept 16-18, 2013) special issue of Planetary and Space Science (preprint).
Cloudy Encounter at the Core
The supermassive black hole at the center of our galaxy comes to Centauri Dreams‘ attention every now and then, most recently on Friday, when we talked about its role in creating hypervelocity stars. At least some of these stars that are moving at speeds above galactic escape velocity may have been flung outward when a binary pair approached the black hole too closely, with one star being captured by it while the other was given its boost toward the intergalactic deeps.
At a mass of some four million Suns, Sagittarius A* (pronounced ‘Sagittarius A-star’) is relatively quiet, but we can study it through its interactions. And if scientists at the University of Michigan are right, those interactions are about to get a lot more interesting. A gas cloud some three times the mass of the Earth, dubbed G2 when it was found by German astronomers in 2011, is moving toward the black hole, which is 25,000 light years away near the constellations of Sagittarius and Scorpius.
What’s so unusual about this is the time-frame. We’re used to thinking in million-year increments at least when discussing astronomical events, but G2 was expected to encounter Sagittarius A* late last year. The event hasn’t occurred yet but astronomers think it will be a matter of only a few months before it happens. Exactly what happens next isn’t clear, says Jon Miller (University of Michigan), who along with colleague Nathalie Degenaar has been making daily images of the gas cloud’s approach using NASA’s orbiting Swift telescope.
“I would be delighted if Sagittarius A* suddenly became 10,000 times brighter,” Miller adds. “However it is possible that it will not react much—like a horse that won’t drink when led to water. If Sagittarius A* consumes some of G2, we can learn about black holes accreting at low levels—sneaking midnight snacks. It is potentially a unique window into how most black holes in the present-day universe accrete.”
Image: The galactic center as imaged by the Swift X-ray Telescope. This image is a montage of all data obtained in the monitoring program from 2006-2013. Credit: Nathalie Degenaar.
We have much to learn about the feeding habits of black holes. The Milky Way’s black hole isn’t nearly as bright as those in some galaxies. While we can’t see black holes directly because no light can escape from within, we can see the evidence of material falling into them, and it would be useful to know why some black holes consume matter at a slower pace than others. The X-ray wavelengths that Swift studies should give us our best data on the upcoming black hole encounter. A sudden spike in X-ray brightness would presumably mark the event, and the researchers will post the images online.
In studying black hole behavior, we’re also looking at key information about how galaxies live out their lives. After all, these objects are consuming matter and radically affecting the region around the very heart of the galaxy. “The way they do that influences the evolution of the entire galaxy—how stars are formed, how the galaxy grows, how it interacts with other galaxies,” says Nathalie Degenaar. Those of us of a certain age can delight in the recollection of Fred Hoyle’s 1957 novel The Black Cloud, in which a gas cloud approaching the Solar System turns out to be a bit more than astronomers had bargained for. Don’t miss this classic if you haven’t read it yet — you should have plenty of time to finish it before the G2 event.
Stars at Galactic Escape Velocity
How do you boost the velocity of a star up to 540 kilometers per second? Somewhere in that region, with a generous error range on either side, is the speed it would take to escape the galaxy if you left from our Solar System’s current position. Here on Centauri Dreams we often discuss exotic technologies that could propel future vehicles, but it’s hard to imagine mechanisms that would drive natural objects out of the galaxy at such speeds. Even so, there are ways, as explained by Vanderbilt University’s Kelly Holley-Bockelmann:
“It’s very hard to kick a star out of the galaxy. The most commonly accepted mechanism for doing so involves interacting with the supermassive black hole at the galactic core. That means when you trace the star back to its birthplace, it comes from the center of our galaxy.”
The mechanism works like this: A binary pair of stars moving a bit too close to the massive black hole at the center of the Milky Way loses one star to the black hole while flinging the other outward at high velocity. When you calculate that the black hole has a mass equal to some four million Suns, this works: Stars can indeed be accelerated to galactic escape velocity, and so far a number of blue hypervelocity stars have been found that could be explained this way.
But Holley-Bockelmann and grad student Lauren Palladino have run into something that casts doubt on this explanation, or at least makes us wonder about other methods for making stars travel this fast. Calculating stellar orbits with data from the Sloan Digital Sky Survey, the duo have found about twenty stars the size of the Sun that appear to be hypervelocity stars. Moreover, these are stars whose composition mirrors normal disk stars, leading the researchers to believe they were not formed in the galaxy’s central bulge or its halo.
Image: Top and side views of the Milky Way galaxy show the location of four of the new class of hypervelocity stars. These are sun-like stars that are moving at speeds of more than a million miles per hour relative to the galaxy: fast enough to escape its gravitational grasp. The general directions from which the stars have come are shown by the colored bands. (Graphic design by Julie Turner, Vanderbilt University. Top view courtesy of the National Aeronautics and Space Administration. Side view courtesy of the European Southern Observatory).
Holley-Bockelmann and Palladino are working on possible causes for the movement of these stars, including interaction with globular clusters, dwarf galaxies or even supernovae in the galactic disk. The list of possibilities is surprisingly long, as noted in the paper on this work (internal references omitted for brevity):
While the SMBH [supermassive black hole] at the Galactic center remains the most promising culprit in generating HSVs [hypervelocity stars], other hypervelocity ejection scenarios are possible, such as a close encounter of a single star with a binary black hole… In this case, the star gains energy from the binary black hole and is flung out of the Galaxy while the orbit of the black hole binary shrinks…Another alternative hypervelocity ejection model involves the disruption of a stellar binary in the Galactic disk; here a supernova explosion in the more massive component can accelerate the companion to hypervelocities…
We may know more soon, for the paper points out that a nearby supernova should have contaminated the spectrum of a hypervelocity star. As they delve into these and other possibilities, the researchers are also expanding their search for hypervelocity stars to a larger sample within the Sloan data to include all spectral types.
The findings were announced at the meeting of the American Astronomical Society in Washington this week. The paper is Palladino et al., “Hypervelocity Star Candidates in the SEGUE G and K Dwarf Sample,” The Astrophysical Journal Vol. 780, No. 1 (2014), with abstract and preprint available.
Stormy Outlook for Brown Dwarfs
“Weather on Other Worlds” is an observation program that uses the Spitzer Space Telescope to study brown dwarfs. So far 44 brown dwarfs have fallen under its purview as scientists try to get a read on the conditions found on these ‘failed stars,’ which are too cool to sustain hydrogen fusion at their core. The variation in brightness between cloud-free and cloudy regions on the brown dwarf gives us information about what researchers interpret as torrential storms, and it turns out that half of the brown dwarfs investigated show these variations.
Given the chance nature of their orientation, this implies that most, if not all, brown dwarfs are wracked by high winds and violent lightning. The image below could have come off the cover of a 1950’s copy of Astounding, though there it would have illustrated one of Poul Anderson’s tales with Jupiter as a violent backdrop (“Call Me Joe” comes to mind). Brown dwarfs are, of course, a much more recent find, and in many ways a far more fascinating one.
Image: This artist’s concept shows what the weather might look like on cool star-like bodies known as brown dwarfs. These giant balls of gas start out life like stars, but lack the mass to sustain nuclear fusion at their cores, and instead, fade and cool with time. Credit: NASA/JPL-Caltech/University of Western Ontario/Stony Brook University.
Storms like these inevitably suggest Jupiter’s Great Red Spot, too, but we want to be careful with analogies considering how much we still have to learn about brown dwarfs themselves. What we can say is this: Brown dwarfs are too hot for water rain, leading most researchers to conclude that any storms associated with them are made up of hot sand, molten iron or salts.
The idea that brown dwarfs have turbulent weather is not surprising, but it is interesting to learn that such storms are evidently commonplace on them. Even more interesting is what the Spitzer work has revealed about brown dwarf rotation. Some of the Spitzer measurements found rotation periods much slower than any previously measured. Up to this point the assumption had been that brown dwarfs began rotating quickly shortly after they formed, a rotation that did not slow down as the objects aged. Aren Heinze (Stony Brook University) had this to say:
“We don’t yet know why these particular brown dwarfs spin so slowly, but several interesting possibilities exist. A brown dwarf that rotates slowly may have formed in an unusual way — or it may even have been slowed down by the gravity of a yet-undiscovered planet in a close orbit around it.”
Whatever the case, brown dwarfs do seem to be opening a window into weather systems in exotic places, systems that can be studied and characterized by their variations in brightness. Heinze presented this work at the 223rd annual meeting of the American Astronomical Society in Washington for principal investigator Stanimir Metchev (University of Western Ontario).
Will We Find Habitable ‘Super-Earths?’
As the 223rd meeting of the American Astronomical Society continues in Washington, we’re continuing to see activity on the subject of mini-Neptunes and ‘super-Earths,’ the latter often thought to be waterworlds. Given how fast our picture of planets in this domain is changing, I was intrigued to see that Nicolas Cowan (Northwestern University) and Dorian Abbot (University of Chicago) have come up with a model that allows a super-Earth with active plate tectonics to have abundant water in its mantle and oceans as well as exposed continents.
If Cowan and Abbot are right, such worlds could feature a relatively stable climate even if the amount of water there is far higher than Earth. Focusing on the planetary mantle, the authors point to a deep water cycle that moves water between oceans and mantle, a movement made possible by plate tectonics. The Earth itself has a good deal of water in its mantle. The paper argues that the division of water between ocean and mantle is controlled by seafloor pressure, which is proportional to gravity.
As planetary size increases, in other words, a super-Earth’s gravity and seafloor pressure go up as well. Rather than a waterworld with surface completely covered by water, the planet could have many characteristics of a terrestrial-class world, leading Cowan to say: “We can put 80 times more water on a super-Earth and still have its surface look like Earth. These massive planets have enormous sea floor pressure, and this force pushes water into the mantle.”
Image: Artist’s impression of Kepler-62f, a potential super-Earth in its star’s habitable zone. Could a super-Earth like this maintain oceans and exposed continents, or would it most likely be a water world? Credit: NASA/Ames/JPL-Caltech.
The implications for habitability emerge when we regard super-Earth oceans as relatively shallow. Exposed continents allow a planet to undergo the deep carbon cycle, producing the kind of stabilizing feedback that cannot exist on a waterworld. A super-Earth with exposed continents is much more likely to have an Earth-like climate, with all of this being dependent on whether or not the super-Earth has plate tectonics, and on the amount of water it stores in its mantle. Cowan calls the argument ‘a shot from the hip,’ but it’s an interesting addition to our thinking about this category of planet as we probe our ever growing database of new worlds.
On the matter of the amount of water, not all super-Earths would fit the bill, and it shouldn’t be difficult to turn a planet into a waterworld — Cowan argues that if the Earth were 1 percent water by mass, not even the deep water cycle could save the day. Instead, the researchers are considering planets that are one one-thousandth or one ten-thousandth water. From the paper:
Exoplanets with sufficiently high water content will be water-covered regardless of the mechanism discussed here, but such ‘ocean planets’ may betray themselves by their lower density: a planet with 10% water mass fraction will exhibit a transit depth 10% greater than an equally-massive planet with Earth-like composition… Planets with 1% water mass fraction, however, are almost certainly waterworlds, but may have a bulk density indistinguishable from truly Earth-like planets. Given that simulations of water delivery to habitable zone terrestrial planets predict water mass fractions of 10-5 – 10-2… we conclude that most tectonically active planets — regardless of mass — will have both oceans and exposed continents, enabling a silicate weathering thermostat.
The paper is Cowan and Abbot, “Water Cycling Between Ocean and Mantle: Super-Earths Need Not be Waterworlds,” The Astrophysical Journal Vol. 781, No. 1 (2014). Abstract and preprint available.
Thinking About ‘Mini-Neptunes’
Yesterday’s look at the exoplanet KOI-314c showed us a world with a mass equal to the Earth, but sixty percent larger than the Earth in diameter. This interesting planet may be an important one when it comes to studying exoplanet atmospheres, for KOI-314c is a transiting world and we can use transmission spectroscopy to analyze the light that passes through the atmosphere as the planet moves in front of and then behind its star. A space-based observatory like the James Webb Space Telescope should be able to tease useful information out of KOI-314c.
But the American Astronomical Society meeting in Washington DC continues, and it’s clear that the technique of studying transit timing variations (TTV) is coming into its own as a tool for exoplanet investigation. David Kipping and colleagues use TTV to look for exomoons, and it was during such a search that they discovered KOI-314c. But consider the other AAS news. At Northwestern University, Yoram Lithwick has been measuring the masses of approximately sixty exoplanets larger than the Earth and smaller than Neptune.
Learn the mass and the size of a planet, and you can make a call on its density, and thus learn something about its probable composition. And guess what?
“We were surprised to learn that planets only a few times bigger than Earth are covered by a lot of gas,” said Lithwick. “This indicates these planets formed very quickly after the birth of their star, while there was still a gaseous disk around the star. By contrast, Earth is thought to have formed much later, after the gas disk disappeared.”
That resonates nicely with Kipping and company’s work on KOI-314c, and Lithwick, working with graduate student Sam Hadden, used transit timing variation to achieve his results. Among the duo’s sample, planets two to three times larger than the Earth have very low density (compare with KOI-314c, which turned out to be only thirty percent denser than water). These are worlds something like Neptune except smaller and covered in massive amounts of gas.
Image: Chart of Kepler planet candidates as of January 2014. Credit: NASA Ames.
Transit timing variations occur when two planets orbiting the same star pull on each other gravitationally, so that the exact time of transit for each planet is affected. These are complicated interactions, to be sure, but we’re beginning to see radial velocity measurements confirming trends that have been originally uncovered with TTV. I ran this by David Kipping, asking whether TTV wasn’t coming into its own, and he agreed. “My bet is that when we measure the mass of Earth 2.0,” Kipping wrote, “it will be via TTVs.”
We can also look at the work of Ji-Wei Xie (University of Toronto), who used TTV to measure the masses of fifteen pairs of Kepler planets. These ranged in size from close to Earth to a little larger than Neptune, The results appeared in The Astrophysical Journal in December and were presented at the AAS meeting. The work complements reports from the Kepler team at AAS presenting mass measurements of worlds between Earth and Neptune in size. Here the follow-up used for the Kepler findings was based on Doppler measurements. In fact, six of the planets under investigation are non-transiting and seen only in Doppler data.
So we’re seeing both radial velocity and TTV used to study this interesting category of planets. 41 planets discovered by Kepler were validated by the program of ground-based observation, and the masses of sixteen of these were determined, allowing scientists to make the call on planetary density. In the Kepler study, ‘mini-Neptune’ planets with a rocky core show up with varying proportions of hydrogen, helium and hydrogen-rich molecules surrounding the core. The variation is dramatic, and some of these worlds show no gaseous envelope at all.
Kepler mission scientist Natalie Batalha sums up the questions all this raises:
“Kepler’s primary objective is to determine the prevalence of planets of varying sizes and orbits. Of particular interest to the search for life is the prevalence of Earth-sized planets in the habitable zone. But the question in the back of our minds is: are all planets the size of Earth rocky? Might some be scaled-down versions of icy Neptunes or steamy water worlds? What fraction are recognizable as kin of our rocky, terrestrial globe?”
Plenty of questions emerge from these findings, but the Kepler team’s report tells us that more than three-quarters of the planet candidates the mission has discovered have sizes between Earth and Neptune. Clearly this kind of planet, which is not found in our own Solar System, is a major player in the galactic population, and learning how such planets form and what they are made of will launch numerous further investigations. The usefulness of transit timing variations at determining mass will likely place the technique at the forefront of this ongoing work.
The paper by Ji-Wei Xie is “Transit Timing Variation of Near-Resonance Planetary Pairs: Confirmation of 12 Multiple-Planet Systems,” Astrophysical Journal Supplement Series Vol. 208, No. 2 (2013), 22 (abstract). I don’t have the citation for the Kepler report, about to be published in The Astrophysical Journal, but will run it as soon as I can. Yoram Lithwick’s presentation at AAS was based on Hadden & Lithwick, “Densities and Eccentricities of 163 Kepler Planets from Transit Time Variations,” to be published in The Astrophysical Journal and available as a preprint.
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