by Paul Gilster | Nov 13, 2009 | Exoplanetary Science |
Friday is a travel day for me, so be aware that comment moderation will be slow and sporadic. I just have time to get in word about the upcoming launch of the WISE mission, slated for December 7. NASA is planning a media briefing next Tuesday (November 17) to discuss the mission, which is designed to scan the entire sky at infrared wavelengths, spotting perhaps hundreds of thousands of asteroids and studying a wide range of stars and galaxies.
The technology is fascinating in and of itself. WISE will image the entire sky in the infrared, using detectors kept below 15 Kelvins (which is only 15 degrees C above absolute zero) by a solid hydrogen cryostat. The telescope will be oriented to look out at right angles to the Sun, always pointing away from the Earth, so that its observations sweep out a circle in the sky. After six months, the instrument will have observed the entire sky, producing nearly 1.5 million images and creating, ultimately, an atlas of the entire celestial sphere.
This is exciting stuff. For one thing, WISE should be able to measure the diameters of more than 100,000 asteroids. For another (and this may be of the most interest to Centauri Dreams readers), WISE will be able to detect stars much dimmer than the Sun. These brown dwarfs, many of which have yet to be discovered, should be readily apparent to the WISE instrument, and of course we hope for one that ranks as the closest star to the Earth. And beyond all this, WISE will be able to produce a global map of the galaxy and its associated dust.
But back to the brown dwarf issue. WISE principle investigator Ned Wright refers to the chart below. WISE is sensitive to radiation with wavelengths of five microns, useful for our purpose because from brown dwarfs down to Jupiter-class gas giants, a large fraction of the emitted radiation appears at five microns, as the figure shows:
About which Wright has this to say on a page of his Web site devoted to the brown dwarf hunt:
These low mass stars are expected to be more numerous than the more massive stars like red dwarfs, and thus there should be brown dwarf stars closer to the Solar system than Proxima Centauri [italics mine]. The green curve shows a 200 K model atmosphere calculation from Burrows et al. (1997) for an object with the radius of Jupiter at the distance of Proxima Centauri. WISE will easily be able to detect these nearby brown dwarfs.
Image: Plot showing nearby objects compared to the sensitivity of WISE. A free floating Jupiter at 1 light year (FFP), and a 200 K brown dwarf at the distance of Proxima Centauri (BD). Credit: Edward Wright/UCLA.
So if we are dealing with a brown dwarf closer than any other star, WISE ought to be the mission to find it. Surveys like the Two-Micron All Sky Survey (2MASS) and the Sloan Digital Sky Survey (SDSS) have discovered numerous brown dwarfs, but have been unable to locate any cooler than 750 K. We can expect WISE to see 450-K brown dwarfs out to a distance of 75 light years, and brown dwarfs as cool as 150-K out as far as ten light years. All eyes may be on Kepler and CoRoT for terrestrial exoplanets, but a nearby brown dwarf would be huge, putting WISE on the front pages.
by Paul Gilster | Nov 12, 2009 | Exoplanetary Science |
The exoplanet watch among our readers is clearly in full operation, to judge from the number of backchannel messages I received about the latest work from HARPS (High Accuracy Radial Velocity Planet Searcher). The remarkable ESO spectrograph attached to the La Silla 3.6-meter telescope now offers evidence that Sun-like stars that host planets will show a sparser lithium signature than stars without planets. Says Garik Israelian, lead author of the paper now appearing in Nature:
“For almost ten years we have tried to find out what distinguishes stars with planetary systems from their barren cousins. We have now found that the amount of lithium in Sun-like stars depends on whether or not they have planets.”
All of this helps us understand our own star, whose low levels of lithium (140 times less than it should have had when formed) have long been apparent. The HARPS work draws on an analysis of some 500 stars, including seventy known to host planets. Ace planet hunter Michel Mayor calls the sample the best available to date “to understand what makes planet-bearing stars unique.” The majority of the stars hosting planets showed less than one percent of the lithium possessed by other stars. Most stars should possess roughly the same amount of lithium unless the element, produced just after the Big Bang, had been destroyed inside the star.
Image: Artist’s impression of a baby star still surrounded by a protoplanetary disc in which planets are forming. Using ESO’s very successful HARPS spectrograph, a team of astronomers has found that Sun-like stars which host planets have destroyed their lithium much more efficiently than planet-free stars. This finding does not only shed light on the low levels of this chemical element in the Sun, solving a long-standing mystery, but also provides astronomers with a very efficient way to pick out the stars most likely to host planets. Credit: ESO.
So what exactly is going on here? How does a planet disturb the matter in a host star to rearrange the chemical elements found within its outer atmosphere, where lithium can exist? The Israelian’s team points to loss of angular momentum caused by the presence of planets, causing host stars to spin less rapidly and allowing their atmospheres to mix more freely. Lithium in the stellar atmospheres would thus be drawn to the stellar surface and destroyed. But stellar ages could also account for the difference, as this article in Nature News points out, noting comments to that effect by Jorge Melendez (University of Porto, Portugal).
The Israelian team disputes that idea, noting that all the stars in its sample are more than a billion years old, and arguing that its findings cannot be coincidence. If they’re right, we may have an observational tool that could speed up the exoplanet search. The paper is Israelian et al., “Enhanced lithium depletion in Sun-like stars with orbiting planets,” Nature 462 (12 November 2009), pp. 189-191 (abstract).
by Paul Gilster | Nov 11, 2009 | Sail Concepts |
From the fusion-powered Project Icarus, designed to handle the interstellar long-haul, to the first tentative solar sail experiments in near-Earth space seems like quite a jump. But we needed to be reminded of the need for research on both ends of the spectrum, the things that are doable today and the concepts we want to shape for tomorrow. How heartening, then, to see the Planetary Society’s new commitment to the solar sail in the form of a project called LightSail, which will include several missions designed to demonstrate the potential of photons for propulsion.
Make no mistake about it, solar sail technology is a practical solution for leaving the fuel at home. We know the principle works. Photons carry no mass but they do carry momentum. Missions designed to operate close to the Sun already have to factor in the pressure exerted by the photon stream, and in a famous case, controllers used the solar panels aboard the Mariner 10 spacecraft (launched in 1973 to explore Mercury) to re-orient the vehicle after it had lost its fix on Canopus, the star it was using for celestial guidance. We know the principle works, and we’ve gotten the solar sail up to near flight-readiness at NASA.
Image: An artist’s conception of Lightsail-1. Credit: Rick Sternbach/Planetary Society.
What we now know is that we cannot, in economic times like these, count on government agencies to proceed with the next step. The Planetary Society has raised the needed money (boosted considerably by a $1 million donation) to build Lightsail-1, creating the vehicle out of three Cubesat spacecraft. Based on early reports, the spacecraft sounds much like the NanoSail-D sailcraft created at Marshall Space Flight Center. Planetary Society vice-president Bill Nye notes, “To get sunlight to push us through space, we need a large sail attached to a small spacecraft. Lightsail-1 fits into a volume of just three liters before the sails unfurl to fly on light. It’s elegant.” NanoSail-D, likewise, is small enough to fit into a suitcase.
But Lightsail-1 will clearly be the first to reach space, assuming the Planetary Society fulfills its promise not to try the launch on another faulty Volna booster. Lightsail-1 represents early solar sail technology, 32 square meters of mylar arranged into four triangular sails placed in an orbit some 800 kilometers above the Earth, where the pressure of sunlight ought to increase its orbital energy. A second sail is intended to demonstrate a longer duration flight, with Lightsail-3 sent to L1, that interesting place where a sail can be ‘parked’ to serve as a solar weather station. The Planetary Society is hoping the craft will fly by the end of 2010.
Mylar is a workable material for these experiments, though as we continue our investigations, we’ll need to move to much lighter materials, and more durable ones. You can certainly work with aluminized 0.5-micron mylar for the short term, but it is degraded by ultraviolet radiation. Areal density (mass divided by the area of the sail) is key, and advances in materials technology will help us achieve ever thinner sails. Much promise exists in so-called carbon-carbon, a carbon fiber material whose density is less than 1/10 ounce per square yard, the equivalent of flattening one raisin to the point that it covers a square yard. This stuff is light as smoke. But we have to get our early experiments flying before we push into these areas.
Image: NASA’s Les Johnson (MSFC) holding the rigid, lightweight carbon fiber called carbon-carbon. An artist’s concept of a solar sail is on the right. Credit: NASA MSFC.
In speaking of solar sails, I always like to go back and quote J.D. Bernal, the British physicist and author of The World, the Flesh & the Devil (1929):
However it is effected, the first leaving of the earth will have provided us with the means of traveling through space with considerable acceleration and, therefore, the possibility of obtaining great velocities—even if the acceleration can only be maintained for a short time. If the problem of the utilization of solar energy has by that time been solved, the movement of these space vessels can be maintained indefinitely. Failing this, a form of space sailing might be developed which used the repulsive effect of the sun’s rays instead of wind. A space vessel spreading its large, metallic wings, acres in extent, to the full, might be blown to the limit of Neptune’s orbit. Then, to increase its speed, it would tack, close-hauled, down the gravitational field, spreading full sail again as it rushed past the sun.
Remember, Bernal was writing in 1929! Someday we’ll see missions like the one he describes, which is for all practical purposes a Sun-diver mission with a close solar pass to maximize acceleration. But we have much work to do before we can make this happen. Sail technologies are in my view the most likely to power our first true interstellar missions, beamed via laser or microwave to high velocities. Getting to that point in this early going will involve private and commercial funding. The Planetary Society’s Lightsail project reminds us that if government won’t do it, the private sector can respond.
by Paul Gilster | Nov 10, 2009 | Missions |
by Pat Galea
Project Icarus will update the Project Daedalus starship designed by the British Interplanetary Society in the 1970s. As we saw yesterday, developments in technology allow new options in a number of areas, but also raise questions about the mission’s scope and choice of targets. Pat Galea now concludes his discussion of the recent Project Icarus symposium in London, after which the terms of reference for the project, now frozen, are listed.
Propulsion Options
Richard Obousy tackled the part of the system that is the most obvious, the most important and probably the most difficult: the engine. As I’ve noted above, the Daedalus propulsion system uses pulsed nuclear fusion to accelerate the craft to about 12% of the speed of light. Even at such an immense speed, the mission time to Barnard’s Star (5.9 light years from Sol) is about fifty years.
Obousy took us through different methods of propulsion that have been proposed, such as chemical rockets, electric ion engines and nuclear thermal rockets. He gave an overview of how nuclear fusion works, and various enhancements that could be made to the fusion process.
One such trick is to use an antimatter seed to initiate the nuclear fusion reaction. This is a compromise design, a hybrid of the extremely powerful pure matter-antimatter engine and the simpler nuclear fusion engine. Small amounts of antimatter are used in this system to initiate the nuclear fusion reaction. The advantage of this idea is that we get some of the power boost of the antimatter reaction, without either the expense of producing large amounts of the stuff, or the problem of storing huge quantities during the mission (as the antimatter cannot be kept in a container made of matter).
Image: The original Daedalus concept, shown here as the second stage separates. Credit: Adrian Mann.
Current Earth-based production of antimatter is terribly meager at the moment. However, Obousy pointed out that much of the criticism we hear of the economics of current antimatter production is unfair, because the facilities were never built to produce antimatter efficiently. We can reasonably expect that truly dedicated industrial techniques to come will give us a much better return on our energy investment because they will be optimized for the purpose.
Onboard Power and Computing
Andreas Tziolas examined the power and computer systems that would be needed for Daedalus-like missions. Despite having a massive fusion pulse engine sitting at the back of the craft, there is still a need for separate systems that can provide the electrical power required for running the ship’s equipment. The engine won’t be running for the whole mission, and there needs to be some redundancy to cope with the inevitable failures which will occur over the lifetime of the voyage.
Tziolas examined several different power systems, such as batteries, fuel cells, flywheels and radio thermal generators (RTGs). Each system has its own advantages and disadvantages, so choosing the right systems depends upon a clear understanding of the mission parameters. For example, some systems work better over certain temperature ranges; other systems might have greater reliability over longer periods. By building a clear idea of the craft and the situations it will be operating in, it becomes possible to choose the most appropriate power systems.
Tziolas also explored the computer systems required on the craft. He emphasized that for such a long duration unmanned mission, there was a clear need for computer systems that have an advanced decision tree to cope with the huge variety of scenarios that the craft will encounter. These systems need to be fault tolerant, having no single point of failure, and be able to recover from any faults that do arise.
Parameters of the Project
Kelvin Long introduced Project Icarus, the aim of which is to produce an updated mission along similar lines to Daedalus, taking advantage of the progress that has been made in science and technology over the last thirty years. The terms of reference for the study have not been finalized yet, though Long presented a rough skeleton as a starting point. In open-floor discussions, the symposium attendees explored the range of mission parameters. It is clear that there is still a lot of work to do here before these parameters can be nailed down.
A couple of the most important parameters are closely related to each other:
- Mission time
- Fly-by or rendezvous
The Daedalus team decided that it was important to set an upper limit for the mission time such that a young person at the start of a professional career could join the project at launch time, and still be working when the probe returns data from the destination system. This requires a mission time of no more than 40-50 years. The motive for this restriction stems from the desire to maintain interest in the mission throughout the entire duration, and hence to make the project appear worth doing in the first place.
There was much discussion on this point, as several attendees pointed out that there have been many projects in human history that have taken significantly longer than a single lifetime to complete, such as cathedrals and pyramids. Whether this kind of commitment could be made to support a super-lifetime mission is an interesting question.
Deceleration at the Target System?
The second mission parameter concerns whether we decelerate the probe at the target system, perhaps to place it in orbit around the star. The benefits of doing so are discussed briefly above, in the summary of Crawford’s talk. It would clearly be desirable to do so, but the penalty is that the mission time would be extended significantly, as the ship would have to hold on to a lot of its fuel in order to burn it in deceleration.
So we have a trade-off between the two parameters. It is possible that there may be hybrid solutions available, where the main probe carries on at full speed, while sub-probes are decelerated. There are many options here, but nature is harsh, so the choices are tough. If it’s decided that a super-lifetime mission is acceptable, then a rendezvous becomes more feasible.
Image: Daedalus was designed for a mission to Barnard’s Star. The Icarus team hopes to allow for a variety of target stars. Credit: Adrian Mann.
However, a long mission entails consideration of another potential problem: the “overtake” scenario. If we launch a probe that will take, say, 150 years to reach the target star, we may worry that in fifty years time we’ll be able to launch a probe with greater speed that can reach the target in only fifty years. That would mean we’d get results back from the later probe fifty years before the data from the earlier probe. So we might decide that there’s no point in launching the slow probe at all; we just wait until we can launch the fast one. Some argue that it’s still worth launching the slower probe, because data are data. We may get results from the newer fast probe first, but we’ll still be glad to get data from the older slow probe when it arrives.
Icarus and the Years Ahead
After hammering out and fixing the basic mission parameters, the Icarus project will take several years of hard work to produce a new design for an interstellar craft. How much this design will resemble Daedalus is anyone’s guess at this point, and it’s quite possible that scientific data obtained during the design project (such as exoplanets) will throw a few curve balls along the way. Ultimately, the designs will be completed and submitted for publication in the Journal of the British Interplanetary Society (JBIS), just as Daedalus was all those years ago.
The Daedalus team mentioned that working on the project changed the way they view the universe. I hope Icarus will do the same, not only for those working on it, but for everyone else who takes an interest in humanity’s future.
Terms of Reference
On November 4, the Icarus team froze the terms of reference for the Icarus project. The text below constitutes the Terms of Reference (ToR) for Project Icarus and sets out what is to be achieved by the design study. This is frozen for the duration of the project and essentially represents the initial requirements.
The Terms of Reference are as follows:
- To design an unmanned probe that is capable of delivering useful scientific data about the target star, associated planetary bodies, solar environment and the interstellar medium.
- The spacecraft must use current or near future technology and be designed to be launched as soon as is credibly determined.
- The spacecraft must reach its stellar destination within as fast a time as possible, not exceeding a century and ideally much sooner.
- The spacecraft must be designed to allow for a variety of target stars.
- The spacecraft propulsion must be mainly fusion based (i.e. Daedalus).
- The spacecraft mission must be designed so as to allow some deceleration for increased encounter time at the destination.
There is also the addition of a project scope as follows:
‘The required milestones should be defined in order to get to a potential launch of such a mission. This should include a credible design, mission profile, key technological development steps and other aspects as considered appropriate.’
by Paul Gilster | Nov 9, 2009 | Missions |
by Pat Galea
The Tau Zero Foundation has been working with the British Interplanetary Society on Project Icarus, a starship study that updates the famous Project Daedalus work from the 1970s. Pat Galea, a software engineer with a lively interest in the physics of interstellar flight, attended the recent symposium that launched the project, and here provides us with a report that I will publish in two parts, concluding tomorrow.
Just over thirty years ago, British Interplanetary Society (BIS) members carried out one of the most complete studies of an interstellar vehicle ever made. Even today, Project Daedalus retains its status as an outstandingly comprehensive reference design. Its final report sits on the shelf of many a starship enthusiast.
In the intervening years, technology and science have advanced in many of the areas that are crucial to the Daedalus mission plan and design. The time has come to re-examine Daedalus in light of the progress that has been made, so Kelvin Long and Ian Crawford organized a one-day symposium held at BIS headquarters in London, UK, on 30 September 2009. This meeting gathered experts on the subject areas to be examined, as well as some of the original Daedalus team, not only to look at the original design, but also to launch Project Icarus, a new project that will produce an updated interstellar craft design.
The Need for Infrastructure
The Daedalus craft at launch has a mass of 54,000 tons; most of that is fuel. Building and fueling this thing is, to say the least, an immense undertaking. Daedalus would require an enormous amount of infrastructure in place before it could be built and launched. Rather than build this all up solely for producing interstellar craft, we could (and probably would) have most of this capacity in place for supporting interplanetary transport. Bob Parkinson gave a presentation on the use of Daedalus-type vehicles for vehicles that can shuttle around our solar system on missions such as Mars colonization, asteroid diversion, and fast transit missions to the outer planets.
Image: Project Icarus logo by Alexandre Szames.
Daedalus calls for a fusion pulse engine, in which high-energy electron beams are fired at pellets containing Deuterium (D) and Helium-3 (He3), causing them to undergo nuclear fusion, and releasing a huge amount of energy. Some of this energy is captured by the craft, and the subsequent release of energy from this capture propels the ship forward. The Daedalus plan requires a system for obtaining the He3 from the atmosphere of Jupiter. This is, indeed, a big engineering challenge, but as Parkinson pointed out, with a fleet of vehicles being used for local transport anyway, we are only worrying about the marginal cost of producing the extra fuel required for the interstellar probe.
Parkinson presented some background on the Jovian extraction process, and examined some other options that have been suggested. One possibility is to extract He3 from the lunar regolith. Unfortunately, it appears that while this is technically possible, the huge energy required to extract meager amounts of He3 from immense masses of regolith renders the process economically unfeasible. In brief, the Jupiter mining system is still the preferred option for fueling Daedalus.
Propulsion Choices and Supporting Technologies
Other nuclear reactions, not involving He3, are possible candidates. The reason the original team chose the D/He3 reaction is that it is relatively clean, producing very few neutrons. We prefer to have charged particles coming from the reaction as we can ‘bend’ the path of these using electromagnetic fields. Uncharged neutrons are a nuisance, hitting the craft and causing structural damage over time. Other reactions can have different advantages, however, such as being easier to initiate, so the case is far from closed.
Image: Daedalus prior to departure, in orbit around Europa. Credit: Adrian Mann.
Richard Osborne considered other aspects of the infrastructure question. While we cannot know the details of the systems that will be in place at the time a Daedalus-like craft is launched, we can sketch out the types of systems that will be required for launch and assembly of these probes. Osborne drew a high-level picture of the process as a whole, and broke it down into its constituent elements. For example, a Single-Stage-To-Orbit (SSTO) vehicle will be essential for launching components from Earth, and a space dockyard is needed for assembling the craft (and presumably building other local transport vehicles). Overall, we were seeing from Osborne effectively a first draft plan for industrializing our solar system.
Astrobiology and the Need to Linger
Ian Crawford explored the planetary science and astrobiology case for pursuing interstellar flight. He explained the concept of habitable zones (those areas around stars that are suitable for life), and discussed the kinds of planets that can support life. In our own solar system, we have expanded our view of the candidates for habitability over the last thirty years. It is quite common these days to read discussions of the potential for life being found in moons of the gas giants, and in even more exotic locations. Crawford discussed the kinds of chemical signatures that we might look for when examining planets and moons that could indicate the presence of biological activity. These tests could play a role in the extensive search for exoplanets that is being undertaken right now. Similar tests could also be carried out by a probe actually in a candidate solar system, where its proximity to the planets would give it a huge advantage in terms of sensitivity to the signatures it is looking for.
One aspect of the Daedalus mission profile has a very significant impact on this type of exploration. To keep the total mission time down to an acceptable duration, the Daedalus probe does not brake. Once it is accelerated up to its cruising speed of approximately 12% of the speed of light, it maintains this even through the encounter phase at the destination. This means that after a flight of about fifty years, the craft actually spends less than a day in the target system. Fitting useful science into this short window of opportunity would be a challenge, particularly if close-up operations are desired on a variety of planetary targets. Daedalus partially mitigated this problem by launching a series of sub-probes once it arrived in the destination system, each of which could be assigned to a different target. However, these probes would inherit the fast speed of the main craft, so although they would be able to get good close looks at the planets, they too would only have short periods in which to do so.
Crawford expressed a strong preference for a mission that is decelerated in the target solar system so that the craft (and any sub-probes) can hang around for longer, taking a good look around, and returning much more science. Daedalus itself could be configured to fly this way, but at the cost of significantly extending the mission time to 100 years or more. It comes down to a decision about whether the people building the Daedalus probe would be prepared to accept a mission of such a long duration in return for better science.
Image: The Daedalus design. Credit: Adrian Mann.
The Choice of Targets
Martyn Fogg considered the star systems ‘near’ Sol using various criteria for selecting a suitable destination for a Daedalus craft. In the original plan, the chosen target was Barnard’s Star. At the time, there appeared to be evidence that there was a planet orbiting the star. This made it a good candidate for the mission, as no other exoplanets had been discovered at that time, and it was not known whether planet formation around stars was common or rare. It now appears that the evidence for this planet was not as good as had been thought. In the meantime, over 300 exoplanets have been discovered in recent years around a great many stars.
Although we have detected many exoplanets, there is still a great deal to learn about how the planets form, and the types of planets that form around different types of stars. Fogg and others have been running computer simulations of stars and their surrounding environments in an attempt to establish this kind of information. More problematic than single stars (like the Sun) are the multiple star systems, which pose a challenge for planets because the presence of extra stars restricts the potential for planetary orbits that are stable over a prolonged period of time. The simulations that Fogg runs track the progress of the system as a whole over billions of years, from the formation of the planets through to the era of stable orbits. By running the simulation again and again, it is possible to observe (as Fogg demonstrated) the zones around the stars in which we might find planets.
We are hopefully going to be deluged with data about extra-solar planets over the next few years from sources such as Kepler. As more information comes in, we will be building up a better picture of the stars that will make the best candidates for a mission. Fogg has his work cut out!
To be concluded tomorrow.
