Centauri Dreams
Imagining and Planning Interstellar Exploration
Brown Dwarfs at the Boundary
We spend a lot of time probing the borderlines of astronomy, wondering what the boundaries are between a large gas giant and a brown dwarf, for example. The other end of that question is also intriguing: When does a true star get small enough to be a brown dwarf? For main sequence stars don’t operate the same way brown dwarfs do. Add hydrogen to a main sequence star and its radius increases. But brown dwarfs work the opposite way, with additional mass causing them to shrink. We see this beginning to happen at the high end of the brown dwarf mass range, somewhere between 60 and 90 Jupiter masses.
Electron degeneracy pressure, which occurs when electrons are compressed into a very small volume, is at play here. No two electrons with the same spin can occupy the same energy state in the same volume — this is the Pauli exclusion principle. When the lowest energy level is filled, added electrons are forced into higher energy states and travel at faster speeds, creating pressure. We see this in other kinds of objects as well. A star of less than four solar masses, for example, having gone through its red giant phase, will collapse and move off the main sequence until its collapse is halted by the pressure of electron degeneracy — a white dwarf is the result.
New work out of the RECONS (Research Consortium on Nearby Stars) group at Georgia State has now found observational evidence that helps us pinpoint the distinction between very low mass stars and brown dwarfs. Let me quote from the preprint to their upcoming paper, slated to appear in The Astronomical Journal, to clarify the electron degeneracy issue:
One of the most remarkable facts about VLM [Very Low Mass] stars is the fact that a small change in mass or metallicity can bring about profound changes to the basic physics of the object’s interior, if the change in mass or metallicity places the object in the realm of the brown dwarfs, on the other side of the hydrogen burning minimum mass limit. If the object is unable to reach thermodynamic equilibrium through sustained nuclear fusion, the object’s collapse will be halted by non-thermal electron degeneracy pressure. The macroscopic properties of (sub)-stellar matter are then ruled by different physics and obey a different equation of state… Once electron degeneracy sets in at the core, the greater gravitational force of a more massive object will cause a larger fraction of the brown dwarf to become degenerate, causing it to have a smaller radius. The mass-radius relation therefore has a pronounced local minimum near the critical mass attained by the most massive brown dwarfs…
With these facts in mind, the RECONS team used data from two southern hemisphere observatories, the SOAR (Southern Observatory for Astrophysical Research) 4.1-m telescope and SMARTS (Small and Moderate Aperture Research Telescope System) in Chile, to take measurements of objects thought to lie at the star/brown dwarf boundary. Says Sergio Dieterich, lead author of the paper:
“In order to distinguish stars from brown dwarfs we measured the light from each object thought to lie close to the stellar/brown dwarf boundary. We also carefully measured the distances to each object. We could then calculate their temperatures and radii using basic physical laws, and found the location of the smallest objects we observed. We see that radius decreases with decreasing temperature, as expected for stars, until we reach a temperature of about 2100K. There we see a gap with no objects, and then the radius starts to increase with decreasing temperature, as we expect for brown dwarfs.”
The diagram below makes the distinction clearer:
Image: The relation between size and temperature at the point where stars end and brown dwarfs begin (based on a figure from the publication). Credit: P. Marenfeld & NOAO/AURA/NSF.
Notice the temperature drop as the size of main sequence stars declines, then the break between true stars and brown dwarfs. The RECONS research indicates that the boundaries can be precisely drawn: Below temperatures of 2100 K, a radius 8.7 percent that of the Sun, and a luminosity of 1/8000th of the Sun, we leave the main sequence. The team identifies the star 2MASS J0513-1403 as an example of the smallest of main sequence stars. Is the gap between true stars and brown dwarfs right after 2MASS J0513-1403 real or is it the effect of an insufficient sample? To find out, the team is planning a larger search to test these initial results.
Note, too, the interesting distinction in the ages of these objects. Small M-dwarfs can live for trillions of years. Brown dwarfs, on the other hand, have much shorter lifetimes, continually fading over time. Whether they could produce a habitable zone over timeframes sufficient to support life is an open question, one that we’ve looked at in Brown Dwarf Planets and Habitability. Our study of small, dim objects and astrobiology is only beginning.
The paper is Dieterich et al., “The Solar Neighborhood XXXII. The Hydrogen Burning Limit,” accepted at the Astronomical Journal and available as a preprint. See this NOAA/SOAR news release for more.
Distant Companions: The Case of HD 106906 b
When the pace of discovery is as fast as it has been in the realm of exoplanet research, we can expect to have our ideas challenged frequently. The latest instance comes in the form of a gas giant known as HD 106906 b, about eleven times as large as Jupiter in a young system whose central star is only about 13 million years old. It’s a world still glowing brightly in the infrared, enough so to be spotted through direct imaging, about which more in a moment. For the real news about HD 106906 b is that it’s in a place our planet formation models can’t easily explain.
Image: This is a discovery image of planet HD 106906 b in thermal infrared light from MagAO/Clio2, processed to remove the bright light from its host star, HD 106906 A. The planet is more than 20 times farther away from its star than Neptune is from our Sun. AU stands for Astronomical Unit, the average distance of the Earth and the Sun. (Image: Vanessa Bailey).
Start with the core accretion model and you immediately run into trouble. Core accretion assumes that the core of a planet like this forms from the accretion of small bodies of ice and rock called planetesimals. The collisions of these objects bulk up the core, which then attracts an outer layer of gas. The problem with applying this model to HD 106906 b is that it’s about 650 AU from its star, far enough out that the core doesn’t have enough time to form before radiation from the hot young star dissipates the already thin gas of the outer protoplanetary disk. Get beyond 30 AU or so and it gets harder and harder to explain how a gas giant forms here.
Gravitational instability is the key to the other major theory of planet formation, but it’s challenged by HD 106906 b as well. Here the instabilities in a dense debris disk cause it to collapse into knots of matter, a process that can theoretically form planets in mere thousands rather than the millions of years demanded by core accretion. But at distances like 650 AU, there shouldn’t be enough material in the outer regions of the protoplanetary disk to allow a gas giant to form.
Grad student Vanessa Bailey (University of Arizona), lead author on the paper presenting this work, describes one of two alternative hypotheses that could explain the planet’s placement:
“A binary star system can be formed when two adjacent clumps of gas collapse more or less independently to form stars, and these stars are close enough to each other to exert a mutual gravitational attraction and bind them together in an orbit. It is possible that in the case of the HD 106906 system the star and planet collapsed independently from clumps of gas, but for some reason the planet’s progenitor clump was starved for material and never grew large enough to ignite and become a star.”
Making this explanation problematic is the fact that the mass ratio of the two stars in a binary system is usually more like 10 to 1 rather than this system’s 100 to 1, but the fact that the remnants of HD 106906’s debris disk are still observable may prove useful in untangling the mystery. A second formation mechanism suggested for planets at this distance from their star is that the planet may have formed elsewhere in the disk and been forced into its current position by gravitational interactions. The preprint notes the problem with this scenario:
Scattering from a formation location within the current disk is unlikely to have occurred without disrupting the disk in the process… We also note that the perturber must be > 11 MJup; we do not detect any such object beyond 35 AU…, disfavoring formation just outside the disk’s current outer edge. While it is possible that the companion is in the process of being ejected on an inclined trajectory from a tight initial orbit, this would require us to observe the system at a special time, which is unlikely. Thus we believe the companion is more likely to have formed in situ in a binary-star-like manner, possibly on an eccentric orbit.
HD 106906 b was found through deliberate targeting of stars with unusual debris disks in the hope of learning more about planet/disk interactions. The team used the Magellan Adaptive Optics system and Clio2 thermal infrared camera mounted on the Magellan telescope in Chile. The Folded-Port InfraRed Echellette (FIRE) spectrograph at Magellan was then used to study the planetary companion. The Magellan data were compared to Hubble Space Telescope data taken eight years earlier to confirm that the planet is indeed moving with the host star.
This unusual gas giant thus joins the small but growing group of widely separated planetary-mass companions and brown dwarf companions that challenge our planet formation models. The massive, ring-like debris disk around HD 106906 helps us constrain the formation possibilities, but we’ll need more data from systems whose disk/planet interactions can be observed before we can speak with any confidence about the origin of these objects.
The paper is Bailey et al., “HD 106906 b: A planetary-mass companion outside a massive debris disk,” accepted at The Astrophysical Journal Letters and available as a preprint. This University of Arizona news release is also useful.
Cosmic Loneliness and Interstellar Travel
Nick Nielsen’s latest invokes the thinking of Carl Sagan, who explored the possibilities of interstellar ramjets traveling at close to the speed of light in the 1960’s. What would the consequences be for the civilization that developed such technologies, and how would such starships affect their thinking about communicating with other intelligent species? Sagan’s speculations took humans not just to the galactic core but to M31, journeys made possible within a human lifetime by time dilation. Nielsen, an author and contributing analyst with strategic consulting firm Wikistrat, ponders how capabilities like that would change our views of culture and identity. Fast forward to the stars, after all, means you can’t go home again.
by Nick Nielsen
In my previous Centauri Dreams post, I discussed some of the possible explanations of what Paul Davies has called the “eerie silence” – the fact that we hear no signs of alien civilizations when we listen for them – in connection with existential risk. Could the eerie silence be a sign that older civilizations than ours have been risk averse to the point of plunging the galaxy into silence, perhaps even silencing others (making use of the Rezabek maneuver)? It is a question worth considering.
For one answer is that we are alone, or very nearly alone, in our galaxy, and probably also in our local cluster of galaxies, and perhaps also alone even in our local supercluster of galaxies. I think this may be the case partly due to the eerie silence when we listen, but also due to what may be called our cosmic loneliness. Not only are our efforts to listen for other intelligences greeted with silence, but also the attempts to demonstrate any alien visitation of our planet or our solar system have turned up nothing. When we listen, we hear only silence, and when we look, we find nothing.
The question, “Are we alone?” has come to take on a scientific poignancy that few other questions hold for us, and we ask this question because of our cosmic loneliness. We are beginning to understand the Copernican revolution not only on an intellectual level, but also on a visceral level, and for many who experience this visceral understanding the result is what psychoanalyst Viktor Frankl called the existential vacuum; the whole cosmos now appears as an existential vacuum devoid of meaning, and that is why we ask, “Are we alone?” We ask the question out of need.
Image credit: TM-1970, Russia (via Dark Roasted Blend).
While talk of alien visitation is usually dominated by discussions of UFOs (and merely by mentioning the theme I risk being dismissed as a crackpot), due to the delay involved in EM spectrum communications, it is at least arguable that communication is less likely than travel and visitation. That being said, I do not find any of the claimed accounts of extraterrestrial visitation to be credible, and I will not discuss them, but I will try to show why visitation is more likely than communication via electromagnetic means.
An organic life form having established an industrial-technological civilization on its homeworld – rational beings that we might think of as peer species – would, like us, have risen from biological deep time, possessing frail and fragile bodies as we do, subject to aging and deterioration. An advanced technological civilization could greatly extend the lives of organic beings, but how long such lives could be extended (without being fully transformed into non-organic beings, i.e., without becoming post-biological) is unknown at present.
For EM spectrum communications across galactic distances, even the most long-lived organic being would be limited in communications to only a small portion of its home galaxy. If civilizations are a rarity within the galaxy, the likelihood of living long enough to engage in even a single exchange is quite low. In fact, we can precisely map the possible sphere of communication of a being with a finite life span within our galaxy (or any given galaxy) based on the longevity of that life form. Even an extraordinarily long-lived and patient ETI would not wish to wait thousands of years between messages, especially in view of the quickening pace of civilization that comes about with the advent of telecommunications.
It could be argued that non-organic life forms take up where organic life forms leave off, and for machines to take over our civilization would mean that length of life becomes much less relevant, but the relative merits and desirability of mechanistic vs. organic bearers of industrial-technological civilization (not to speak of being bearers of consciousness) is a point that needs to be argued separately, so I will not enter into this at present. But whether ETI is biological or post-biological, no advanced intellect is going to send out a signal and wait a thousand years for a response, since in that same thousand year period it would be possible to invent the technologies that would allow for travel to the same object of your communication in a few years’ time (i.e, a few years in terms of elapsed shipboard time).
Our perfect ETI match as a peer civilization in the Milky Way will have already realized that electromagnetic communications mean waiting too long to talk to planetary systems that can be visited directly. If they are a hundred years ahead of us, they may already have started out and may find us soon. If they are a hundred years behind us, they will not yet even have the science to conceive of these possibilities as realizable technological aims. But what is the likelihood, in the universe in which intelligent life is rare (and we know by now that there are no “super-civilizations” nearby us in cosmic terms – cf. my Searching the Sky), that in all the vast space and time of the universe, a peer civilization should arise within a hundred years’ development of our own civilization? Not very likely.
The further we push out the temporal parameters of this observation, the more likely there is another civilization within these temporal parameters, but the further such a civilization is from being a peer civilization. Take a species a thousand years behind us or a thousand years ahead of us: the former cannot form a conception of the universe now known to observational cosmology; the latter will have technological abilities so far beyond ours (having had an industrial-technological civilization that has been in existence five times longer than ours) that we would not be in any sense their peer. And they would have already visited us. If we set the parameters of temporal radius from the present at ten thousand years, or a hundred thousand years, we are much more likely to find life on other worlds, but the further from our present level of development, the less likely any life found would be recognizable as a peer civilization.
How would we visit other worlds directly? With the breakthrough technology of a 1G starship (i.e., a starship than can accelerate or decelerate at a constant of one gravity) [2], all of the waiting to discover the universe and what lies beyond virtually disappears for those willing to make the journey. And while I have called this 1G starship a “breakthrough technology,” it is not likely to happen all at once in a breakthrough, but will probably take decades (if not centuries) of development. Our first interstellar probes, Voyager 1 and Voyager 2, are already headed to the stars [3]. It would take tens of thousands of years for the Voyager probes to arrive at another solar system, should they survive so long. Incremental improvements even in known propulsion technologies will yield gradually more efficient and effective interstellar travel (and will not require any violations of the laws of physics). While we don’t yet have full breakeven in inertial confinement fusion [4], we can in fact achieve inertial confinement fusion at an energy loss, meaning that an inertial confinement fusion starship drive is nearly within the capability of present technology. All of this leaves aside the possibility of breakthrough technologies that would be game-changers (such as the Alcubierre drive).
If we assume that a peer species would emerge from an Earth twin, we can assume that such a peer species would be subject to roughly similar gravitational limitations, so that an ETI 1G starship would be something similar in terms of velocity. Human beings or a peer ETI species, while unable to engage in any but the most limited EM spectrum communications over galactic distances, would find the galaxy opened up to them by a 1G starship, able to explore the farthest reaches of the universe within the ordinary biological lifetime of intelligent life forms even as we know such life forms today (i.e., ourselves), limited to a mere three score and ten, or maybe a bit more.
I have mentioned inertial confinement fusion above as a possible starship propulsion system, but this example is not necessary to my argument. If there existed only a single propulsion proposal for interstellar travel, and all our hopes for such travel rested on an unknown science and an unknown technology, we would have good reason to be skeptical that interstellar travel would ever be possible under any circumstances. This, however, is not the case. There are a wide variety of potential interstellar propulsion technologies, including inertial confinement fusion, matter-antimatter, quantum vacuum thrusters, and other even more exotic ideas. As long as industrial-technological civilization continues its development, some advanced propulsion idea is likely to prove successful, if only marginally so, but marginally will be enough for the first pioneers who are willing to sacrifice all for the chance at a new world.
It is humbling that we know so little about these technologies and the science that underlies them that we are not today in a position to say which among these might prove to be robust and durable drives for a starship, but the very fact that we know so little implies that we have much to learn and we cannot yet exclude any of these exotic starship drive possibilities, much less dismiss them as impossible. While no one has yet produced a proof of concept of any of these proposed forms of propulsion, it is also the case that no one has yet falsified the science upon which they are based.
Even the most successful of the drives mentioned above (with the exception of the Alcubierre drive) will involve time dilation as a condition of interstellar travel. There has been a tendency to view time dilation as a cosmic “fun spoiler” that prevents us seeing the universe on our own terms, since the elapsed time on one’s home world means that no one can return to the world that they left. We need to get beyond this limiting idea and come to see time dilation as a resource that will allow us to travel throughout the galaxy. It is true that time dilation is a limitation, but it is also an opportunity. As Carl Sagan noted:
“Relativity does set limits on what humans can ultimately do. But the universe is not required to be in perfect harmony with human ambition. Special relativity removes from our grasp one way of reaching the stars, the ship that can go faster than light. Tantalizingly, it suggests another and quite unexpected method.” [5]
Human ambition, as Sagan suggests, wants interstellar travel without the price exacted by time dilation, but the universe is not going to accommodate this particular ambition. We have had to reconcile ourselves with the fact that historical transmission is a unidirectional process. We can read Shakespeare, but we cannot talk to Shakespeare. Shakespeare’s contemporary Queen Elizabeth could make it known that she wanted to see Falstaff in love, and “The Merry Wives of Windsor” resulted, but we cannot approach the Bard to write the perfect comedy of manners in which smart phones and text messages figure in the plot.
Just so, we are all unidirectional time travelers, and the eventual development of travel at relativistic velocities will not give us the ability to travel backward in time, nor will it allow us to travel to the stars without giving thought to the inertial frame of reference of our homeworld, but it will give us more alternatives for going forward in time. We will have the opportunity to choose between an inertial framework at rest (presumably relative to our homeworld), and some accelerated inertial framework in which time passes more slowly, allowing us to travel farther and, incidentally, to see more of the universe. Even if we can never go back, we can always go forward. Relativistic interstellar travel will mean that we have a choice as to how rapidly we move forward in time. The possibility of always going farther forward in time has consequences for existential risk mitigation that I will discuss in my next Centauri Dreams post.
Notes
[1] Cf. Viktor Frankl, Man’s Search for Meaning
[2] Carl Sagan discussed the 1G starship in his Cosmos, Chapter VIII, “Travels in Space and Time”; I quote from this same chapter below.
[3] As of this writing, Voyager 1 has passed into interstellar space, while Voyager 2 has not yet emerged from the heliosheath and into interstellar space.
[4] A near breakeven in inertial confinement fusion was recently achieved, in which produced more energy that was absorbed by the fuel for the reaction, but this is not the same as producing as much energy from the fusion reaction as was pumped into the lasers making the reaction happen. (Cf. Nuclear fusion milestone passed at US lab.)
[5] Carl Sagan, Cosmos, Chapter VIII, “Travels in Space and Time” (cf. note [2] above)
The Winds of Deep Space
If we can use solar photons to drive a sail, and perhaps use their momentum to stabilize a threatened observatory like Kepler, what about that other great push from the Sun, the solar wind? Unlike the stream of massless photons that exert a minute but cumulative push on a surface like a sail, the solar wind is a stream of charged particles moving at speeds of 500 kilometers per second and more, a flow that has captured the interest of those hoping to create a magnetic sail to ride it. A ‘magsail’ interacts with the solar wind’s plasma. The sailing metaphor remains, but solar sails and magsails get their push from fundamentally different processes.
Create a magnetic field around your spacecraft and interesting things begin to happen. Those electrons and positively charged ions flowing from the Sun experience a force as they move through the field, one that varies depending on the direction the particles are moving with respect to the field. The magsail is then subjected to an opposing force, producing acceleration. The magsail concept envisions large superconducting wire loops that produce a strong magnetic field when current flows through them, taking advantage of the solar wind’s ‘push.’
A magsail sounds like a natural way to get to the outer Solar System or beyond, but the solar wind introduces problems that compromise it. One is that it’s a variable wind indeed, weakening and regaining strength, and although I cited 500 kilometers per second in the introductory paragraph, the solar wind can vary anywhere from 350 to 800 kilometers per second. An inconstant wind raises questions of spacecraft control, an issue Gregory Matloff, Les Johnson and Giovanni Vulpetti are careful to note in their 2008 title Solar Sails: A Novel Approach to Interplanetary Travel
While technically interesting and somewhat elegant, magsails have significant disadvantages when compared to solar sails. First of all, we don’t (yet) have the materials required to build them. Second, the solar wind is neither constant nor uniform. Combining the spurious nature of the solar wind flux with the fact that controlled reflection of solar wind ions is a technique we have not yet mastered, the notion of sailing in this manner becomes akin to tossing a bottle into the surf at high tide, hoping the currents will carry the bottle to where you want it to go.
Interstellar Tradewinds and the Local Cloud
We have much to learn about the solar wind, but missions like Ulysses and the Advanced Composition Explorer have helped us understand its weakenings and strengthenings and their effect upon the boundaries of the heliosphere, that vast bubble whose size depends upon the strength of the solar wind and the pressures exerted by interstellar space. For we’re not just talking about a wind from the Sun. Particles are also streaming into the Solar System from outside, and data from four decades and eleven different spacecraft have given us a better idea of how these interactions work.
A paper from Priscilla Frisch (University of Chicago) and colleagues notes that the heliosphere itself is located near the inside edge of an interstellar cloud, with the two in motion past each other at some 22 kilometers per second. The result is an interstellar ‘wind,’ says Frisch:
“Because the sun is moving through this cloud, interstellar atoms penetrate into the solar system. The charged particles in the interstellar wind don’t do a good job of reaching the inner solar system, but many of the atoms in the wind are neutral. These can penetrate close to Earth and can be measured.”
Image: The solar system moves through a local galactic cloud at a speed of 50,000 miles per hour, creating an interstellar wind of particles, some of which can travel all the way toward Earth to provide information about our neighborhood. Credit: NASA/Adler/U. Chicago/Wesleyan.
We’re learning that the interstellar wind has been changing direction over the years. Data on the matter go back to the 1970s, and this NASA news release mentions the U.S. Department of Defense’s Space Test Program 72-1 and SOLRAD 11B, NASA’s Mariner, and the Soviet Prognoz 6 as sources of information. We also have datasets from Ulysses, IBEX (Interstellar Boundary Explorer), STEREO (Solar Terrestrial Relations Observatory), Japan’s Nuzomi observatory and others including the MESSENGER mission now in orbit around Mercury.
Usefully, we’re looking at data gathered using different methods, but the flow of neutral helium atoms is apparent with each, and the cumulative picture is clear: The direction of the interstellar wind has changed by some 4 to 9 degrees over the past forty years. The idea of the interstellar medium as a constant gives way to a dynamic, interactive area that varies as the heliosphere moves through it. What we don’t know yet is why these changes occur when they do, but our local interstellar cloud may experience a turbulence of its own that affects our neighborhood.
The interstellar winds show us a kind of galactic turbulence that can inform us not only about the local interstellar medium but the lesser known features of our own heliosphere. Ultimately we may learn how to harness stellar winds, perhaps using advanced forms of magnetic sails to act as brakes when future probes enter a destination planetary system. As with solar sails, magsails give us the possibility of accelerating or decelerating without carrying huge stores of propellant, an enticing prospect indeed as we sort through how these winds blow.
The paper is Frisch et al., “Decades-Long Changes of the Interstellar Wind Through Our Solar System,” Science Vol. 341, No. 6150 (2013), pp. 1080-1082 (abstract)
Can Kepler be Revived?
Never give up on a spacecraft. That seems to be the lesson Kepler is teaching us, though it’s one we should have learned by now anyway. One outstanding example of working with what you’ve got is the Galileo mission, which had to adjust to the failure of its high-gain antenna. The spacecraft’s low-gain antenna came to the rescue, aided by data compression techniques that raised its effective data rate, and sensitivity upgrades to the listening receivers on Earth. Galileo achieved 70 percent of its science goals despite a failure that had appeared catastrophic, and much of what we’ve learned about Europa and the other Galilean satellites comes from it.
Image: Galileo at Jupiter, still functioning despite the incomplete deployment of its high gain antenna (visible on the left side of the spacecraft). The blue dots represent transmissions from Galileo’s atmospheric probe. Credit: NASA/JPL.
Can we tease more data out of Kepler? The problem has been that two of its four reaction wheels, which function like gyroscopes to keep the spacecraft precisely pointed, have failed. Kepler needs three functioning wheels to maintain its pointing accuracy because it is constantly being bathed in solar photons that can alter its orientation. But mission scientists and Ball Aerospace engineers have been trying to use that same issue — solar photons and the momentum they impart — to come up with a mission plan that can still operate.
You can see the result in the image below — be sure to click to enlarge it for readability. By changing Kepler’s orientation so that the spacecraft is nearly parallel to its orbital path around the Sun, mission controllers hope to keep the sunlight striking its solar panels symmetric across its long axis. We may not have that third reaction wheel, but we do have the possibility of this constant force acting as a surrogate. Re-orientation of the probe four times during its orbit will be necessary to keep the Sun out of its field of view. And the original field of view in the constellations of Cygnus, Lyra and Draco gives way to new regions of the sky.
Testing these methods, mission scientists have been able to collect data from a distant star field of a quality within five percent of the primary mission image standards. That’s a promising result and an ingenious use of the same photon-imparted momentum that’s used in the design of solar sails like JAXA’s IKAROS and the upcoming Sunjammer sail from NASA. We now continue with testing to find out whether this method will work not just for hours but for days and weeks.
Image (click to enlarge): This concept illustration depicts how solar pressure can be used to balance NASA’s Kepler spacecraft, keeping the telescope stable enough to continue searching for transiting planets around distant stars. Credit: NASA Ames/W Stenzel.
If a decision is made to proceed, the revised mission concept, called K2, will need to make it through the 2014 Senior Review, in which operating missions are assessed. We should expect further news by the end of 2013 even as data from the original mission continue to be analyzed.
All of this may take you back to the Mariner 10 mission to Mercury in 1973, itself plagued by problems. Like Galileo, its high-gain antenna malfunctioned early in the flight, although it would later come back to life, and like Kepler, Mariner 10 proved difficult to stabilize. Problems in its star tracker caused the spacecraft to roll, costing it critical attitude control gas as it tried to stabilize itself. The guidance and control team at the Jet Propulsion Laboratory was able to adjust the orientation of Mariner 10’s solar panels, testing various tilt angles to counter the spacecraft’s roll. It was an early demonstration of the forces at work in solar sails.
We can hope that ingenuity and judicious use of solar photons can also bring Kepler back to life in an extended mission no one would have conceived when the spacecraft was designed. What we’ll wind up with is about 4.5 viewing periods (‘campaigns’) per orbit of the Sun, each with its own field of view and the capability of studying it for approximately 83 days. As the diagram shows, the proper positioning of the spacecraft to keep sunlight balanced on its solar panels is crucial. It’s a tricky challenge but one that could provide new discoveries ahead.
Addendum, from a news release just issued by NASA:
Based on an independent science and technical review of the Kepler project’s concept for a Kepler two-wheel mission extension, Paul Hertz, NASA’s Astrophysics Division director, has decided to invite Kepler to the Senior Review for astrophysics operating missions in early 2014.
The Kepler team’s proposal, dubbed K2, demonstrated a clever and feasible methodology for accurately controlling the Kepler spacecraft at the level of precision required for scientifically valuable data collection. The team must now further validate the concept and submit a Senior Review proposal that requests the funding necessary to continue the Kepler mission, with sufficient scientific justification to make it a viable option for the use of NASA’s limited resources.
To be clear, this is not a decision to continue operating the Kepler spacecraft or to conduct a two-wheel extended mission; it is merely an opportunity to write another proposal and compete against the Astrophysics Division’s other projects for the limited funding available for astrophysics operating missions.
Putting the Solar System in Context
Yesterday I mentioned that we don’t know yet where New Horizons will ultimately end up on a map of the night sky like the ones used in a recent IEEE Spectrum article to illustrate the journeys of the Voyagers and Pioneers. We’ll know more once future encounters with Kuiper Belt objects are taken into account. But the thought of New Horizons reminds me that Jon Lomberg will be talking about the New Horizons Message Initiative, as well as the Galaxy Garden he has created in Hawaii, today at the Arthur C. Clarke Center at UC San Diego. The talk will be streamed live at: http://calit2.net/webcasting/jwplayer/index.php, with the webcast slated to begin at approximately 2045 EST, or 0145 UTC.
While both the Voyagers and the Pioneers carried artifacts representing humanity, New Horizons may have its message uploaded to the spacecraft’s memory, its collected images and perhaps sounds ‘crowdsourced’ from people around the world after the spacecraft’s encounter with Pluto/Charon. That, at least, is the plan, but we need your signature on the New Horizons petition to make it happen. The first 10,000 to sign will have their names uploaded to the spacecraft, assuming all goes well and NASA approval is forthcoming. Please help by signing. In backing the New Horizons Message Initiative, principal investigator Alan Stern has said that it will “inspire and engage people to think about SETI and New Horizons in new ways.”
Artifacts, whether in computer memory or physical form like Voyager’s Golden Record, are really about how we see ourselves and our place in the universe. On that score, it’s heartening to see the kind of article I talked about yesterday in IEEE Spectrum, discussing where our probes are heading. When the Voyagers finished their planetary flybys, many people thought their missions were over. But even beyond their continued delivery of data as they cross the heliopause, the Voyagers are now awakening a larger interest in what lies beyond the Solar System. Even if they take tens of thousands of years to come remotely close to another star, the fact is that they are still traveling, and we’re seeing our system in this broader context.
The primary Alpha Centauri stars — Centauri A and B — are about 4.35 light years away. Proxima Centauri is actually a bit closer, at 4.22 light years. It’s easy enough to work out, using Voyager’s 17.3 kilometers per second velocity, that it would take over 73,000 years to travel the 4.22 light years that separate us from Proxima, but as we saw yesterday, we have to do more than take distance into account. Motion is significant, and the Alpha Centauri stars (I am assuming Proxima Centauri is gravitationally bound to A and B, which seems likely) are moving with a mean radial velocity of 25.1 ± 0.3 km/s towards the Solar System.
We’re talking about long time frames, to be sure. In about 28,000 years, having moved into the constellation Hydra as seen from Earth, Alpha Centauri will close to 3.26 light years of the Solar System before beginning to move away. So while we can say that Voyager 1 would take 73,000 years to cross the 4.22 light years that currently separate us from Proxima Centauri, the question of how long it would take Voyager 1 to get to Alpha Centauri given the relative motion of each remains to be solved. I leave this exercise to those more mathematically inclined than myself, but hope one or more readers will share their results in the comments.
Image: A Hubble image of Proxima Centauri taken with the observatory’s Wide Field and Planetary Camera 2. Centauri A and B are out of the frame. Credit: ESA/Hubble & NASA.
We saw yesterday that both Voyagers are moving toward stars that are moving in our direction, Voyager 1 toward Gliese 445 and Voyager 2 toward Ross 248. When travel times are in the tens of thousands of years, it helps to be moving toward something that is coming even faster towards you, which is why Voyager 1 closes to 1.6 light years of Gl 445 in 40,000 years. But these are hardly the only stars moving in our direction. Barnard’s Star, which shows the largest known proper motion of any star relative to the Solar System, is approaching at around 140 kilometers per second. Its closest approach should be around 9800 AD, when it will close to 3.75 light years. By then, of course, Alpha Centauri will have moved even closer to the Sun.
When we talk about interstellar probes, we’re obviously hoping to move a good deal faster, but it’s interesting to realize that our motion through the galaxy sets up a wide variety of stellar encounters. Epsilon Indi, currently some 11.8 light years away, is moving at about 90 kilometers per second relative to the Sun, and will close to 10.6 light years in about 17,000 years, a distance roughly similar to Tau Ceti’s as it will be in the sky of 43,000 years from now.
And as I learned from Erik Anderson’s splendid Vistas of Many Worlds, the star Gliese 710 is one of the most interesting in terms of close encounters. It’s currently 64 light years away in the constellation Serpens, but give it 1.4 million years and Gl 710 will move within 50,000 AU. That’s clearly in our wheelhouse, for 50,000 AU is the realm of the Oort Cloud comets, and we can only imagine what effects the passage of a star this close to the Sun will have on disturbing the cometary cloud. If humans are around this far in the future, GL 710 will give us an interstellar destination right on our doorstep as it swings by on its galactic journey.
Follow with RSS or E-Mail
