by Paul Gilster | Jul 23, 2015 | Exoplanetary Science |
I’ve come to dislike the term ‘Earth 2.0.’ It’s not so much the idea of a second Earth as the use of 2.0, which in our technological era invariably recalls software updates. Windows 2.0 was better than Windows 1.0, but Windows 3.0 was the one that really took off — the idea here is that progressive iterations improve the product. I’d rather see us use ‘Earth 2’ than ‘Earth 2.0,’ for the latter implies a new and improved Earth, and I’m not sure just what that would be. Speculating about that is, I suppose, a key activity of philosophers.
But Earth 2.0 has stuck as a way of designating a planet much like our own. Here too we have to be careful. A planet with liquid water on at least parts of its surface might exist around a red dwarf, packed into a tidally-locked orbit and divided between a frigid night side and a day side with, perhaps, only a few zones where life might flourish. It’s not Earth 2.0 because it has a star that never moves in its sky and its susceptibility to solar flares offers evolutionary challenges much different from those life has experienced around our G-class Sun.
So we can reserve Earth 2.0 for planets that orbit around their star in roughly the same way we do, meaning a star much like the Sun and a planet of Earth size in a more or less circular orbit at about 1 AU. To really hammer home the comparison, we should ask for a star of a certain age. We might find a planet meeting all these characteristics circling a star so young that life is unlikely to have taken hold, assuming that life takes the same kind of path it did on Earth (obviously, nothing more than an assumption). But the Earth 2.0 that seizes the popular imagination will so closely mirror our own in age, orbit, and size as to suggest a living world.
We’re getting close.
Image: The sweep of NASA Kepler mission’s search for small, habitable planets in the last six years. The first planet smaller than Earth, Kepler-20e, was discovered in December 2011 orbiting a Sun-like star slightly cooler and smaller than our sun every six days. But it is scorching hot and unable to maintain an atmosphere or a liquid water ocean. Kepler-22b was announced in the same month, as the first planet in the habitable zone of a sun-like star, but is more than twice the size of Earth and therefore unlikely to have a solid surface. Kepler-186f was discovered in April 2014 and is the first Earth-size planet found in the habitable zone of a small, cool M dwarf about half the size and mass of our sun. Kepler-452b is the first near-Earth-Size planet in the habitable zone of a star very similar to the sun. Credit: NASA Ames/W. Stenzel.
Enter Kepler (again)
The beauty of the Kepler mission is that it just keeps on giving. The Kepler team has already identified over 4,000 planet candidates, and now we have a new catalog of more than 500. All planets are subjects of interest in their own right, but life-bearing ones still have considerable cachet, as witness Jeffrey Coughlin (SETI Institute), who comments thus:
“This catalog contains our first analysis of all Kepler data, as well as an automated assessment of these results. Improved analysis will allow astronomers to better determine the number of small, cool planets that are the best candidates for hosting life.”
Twelve planet candidates in the new catalog are less than twice Earth’s diameter and orbit in the habitable zone of their star, meaning that region where liquid water can exist on the surface. Among these, Kepler-452b, about 1400 light years from us, has now been confirmed as a planet, and it’s an interesting world, one that orbits a star much like the Sun, being about 5 percent more massive and 10 percent brighter. The planet itself is about 5 times the mass of the Earth, with a radius 50 to 60 percent larger. Moreover, Kepler-452b orbits only 5 percent farther from its parent star than Earth orbits the Sun, with a 385-day year. Jon Jenkins (NASA Ames) is lead author on the paper on this work. He pointed out at the NASA news briefing today that gravity on this world would be about 50 percent larger than that of Earth, on a world with a thicker atmosphere and a larger degree of cloud cover. The star is also older than our Sun, which has predictable consequences:
“Kepler-452b receives 10 percent more energy than the Earth. Bear in mind that stars in their youth are smaller and dimmer, but they get brighter with age. Kepler-452’s star is more than 6 billion years old, and should leave its habitable zone at about the 9 or 10 billion year mark. Earth will receive the same energy as Kepler-452 does today in about one and a half billion years.”
Image: This artist’s concept depicts one possible appearance of the planet Kepler-452b, the first near-Earth-size world to be found in the habitable zone of star that is similar to our sun. The habitable zone is a region around a star where temperatures are right for water — an essential ingredient for life as we know it — to pool on the surface. Scientists do not know if Kepler-452b can support life or not. What is known about the planet is that it is about 60 percent larger than Earth, placing it in a class of planets dubbed “super-Earths.” While its mass and composition are not yet determined, previous research suggests that planets the size of Kepler-452b have a better than even chance of being rocky. Kepler-452b orbits its star every 385 days. The planet’s star is about 1,400 light-years away in the constellation Cygnus. It is a G2-type star like our sun, with nearly the same temperature and mass. This star is 6 billion years old, 1.5 billion years older than our sun. As stars age, they grow in size and give out more energy, warming up their planets over time. Credits: NASA Ames/JPL-Caltech/T. Pyle
This is a planet that has been in its star’s habitable zone for longer than the age of the Earth, ample time, as Jenkins noted, for life to begin. Although the size of the world — intermediate between Earth and Neptune — makes it too large to be a true Earth analogue, Jenkins believes that it has a “better than even chance of being rocky.” Thus we could be looking at a world that models changes our planet will be making in the remote future.
We’ll get more habitable zone planets out of the Kepler data, according to Jeff Coughlin (SETI Institute), because we’re getting much better in our planet extraction techniques, but Coughlin noted at the news conference that for every planet we’ve detected, there are at least fifty we cannot see because they are not oriented so as to make transits possible. “Earth-like planets,” Coughlin said, “are common throughout the galaxy.”
Image: Twelve Exoplanet discoveries from Kepler that are less than twice the size of Earth and reside in the habitable zone of their host star. The sizes of the exoplanets are represented by the size of each sphere. These are arranged by size from left to right, and by the type of star they orbit, from the M stars that are significantly cooler and smaller than the sun, to the K stars that are somewhat cooler and smaller than the sun, to the G stars that include the sun. The sizes of the planets are enlarged by 25X compared to the stars. The Earth is shown for reference. Credits: NASA/JPL-CalTech/R. Hurt
Earth 2.0? Not if we’re dealing with a super-Earth. But what an interesting world Kepler-452b seems to be. We have the example of planets like Kepler-438b and Kepler-442b to remind us of worlds that might be rocky like the Earth, but orbiting different kinds of stars, in this case red dwarfs. No Earth 2.0 among that lot either, but it’s clear we’re moving in the right direction.
Image: Since Kepler launched in 2009, twelve planets less than twice the size of Earth have been discovered in the habitable zones of their stars. These planets are plotted relative to the temperature of their star and with respect to the amount of energy received from their star in their orbit in Earth units. The light and dark shaded regions indicate the conservative and optimistic habitable zone. The sizes of the blue disks indicate the sizes of these exoplanets relative to one another and to the image of Earth, Venus and Mars, placed on this diagram for reference. Note that all the exoplanets discovered up until now are orbiting stars which are somewhat to significantly cooler and smaller than the sun. Kepler-452b is the first planet less than twice the size of Earth discovered in the habitable zone of a G-type star. Credit: NASA Ames/N. Batalha and W. Stenzel.
by Paul Gilster | Jul 22, 2015 | Outer Solar System |
We’ve already had the pleasure of naming features on Pluto, at least informally, highlighted by the moment when the heart-shaped area revealed by New Horizons was named Tombaugh Regio, after the world’s discoverer. The fact that two of Clyde Tombaugh’s children were in the audience during the news briefing where this occurred made it all the more powerful. Now we are turning to smaller features, as witness the mountain range near the southwest margin of Tombaugh Regio, viewed by New Horizons from a distance of 77,000 kilometers on July 20.
Image: What a glorious view on what had previously been nothing more than a barely resolved dot. This is the region of Tombaugh Regio containing a range of mountains evidently less elevated than those previously seen near Pluto’s equator (see First Post-Flyby Imager). Features as small as one kilometers across are visible in this image. Credit: NASA/JHUAPL/SWRI.
The mountains in the equatorial region — now known as Norgay Montes, after sherpa Tenzing Norgay — rise as high as 3500 meters, and evidently formed no more than 100 million years ago, an indication that they may still be in a geologically active area (the debate on the matter should be lively). The new range is west of the region now called Sputnik Planum, some 110 kilometers northwest of Norgay Montes. These peaks reach between 1 and 1.5 kilometers in height. This NASA news release likens them to the Appalachian Mountains of the US, compared to the loftier Norgay Montes region with peaks similar to those in the Rockies.
We’re also seeing interesting topography along the western edge of Tombaugh Regio, as Jeff Moore (NASA Ames), who leads the New Horizons Geology, Geophysics and Imaging Team (GGI), points out:
“There is a pronounced difference in texture between the younger, frozen plains to the east and the dark, heavily-cratered terrain to the west. There’s a complex interaction going on between the bright and the dark materials that we’re still trying to understand.”
If Sputnik Planum is thought to be less than 100 million years old, the darker region probably goes back billions of years. Moore notes the bright, sediment-like material that seems to be filling in the older craters — see the circular feature to the lower left of center in the image.
Glimpses of Pluto’s Moons
While Charon obviously took pride of place in the New Horizons flyby, the lesser satellites were not neglected, as we see in the just released imagery of Nix and Hydra. These were the second and third moons to be discovered respectively, and they are of roughly the same size. But just as both Pluto and Charon have already delivered their share of surprises, so has Nix, which sports a region with a reddish tint and patterning that suggests the area is a crater. Nix is, overall, gray, but the red tint is obvious in the first color image of Nix, with colors enhanced.
Image: Pluto’s moon Nix (left), shown here in enhanced color as imaged by the New Horizons Ralph instrument, has a reddish spot that has attracted the interest of mission scientists. The data were obtained on the morning of July 14, 2015, and received on the ground on July 18. At the time the observations were taken New Horizons was about 165,000 km from Nix. The image shows features as small as approximately 3 kilometers across on Nix, which is estimated to be 42 kilometers long and 36 kilometers wide. Pluto’s small, irregularly shaped moon Hydra (right) is revealed in this black and white image taken from New Horizons’ LORRI instrument on July 14, 2015 from a distance of about 231,000 kilometers. Features as small as 1.2 kilometers are visible on Hydra, which measures 55 kilometers in length. Credit: NASA/JHUAPL/SWRI.
On Hydra (which NASA’s news release likens to the US state of Michigan in terms of its shape), we can see two evident craters, one largely in shadow. Here again we seem to have differences in surface composition, to judge from the darker aspect of the upper portion of the moon.
“Before last week, Hydra was just a faint point of light, so it’s a surreal experience to see it become an actual place, as we see its shape and spot recognizable features on its surface for the first time,” said mission science collaborator Ted Stryk (Roane State Community College, Tennessee).
Indeed. But that’s been the experience of this entire flyby, with puzzling terrain, striking mountains, and satellite features that will long keep us occupied. When I think about how much New Horizons has already brought us, I’m reminded to send you to Kenneth Chang’s fine piece in the New York Times on the history of New Horizons. This is a mission that was threatened from the beginning by everything from budget problems to plutonium sourcing, and but for the tenacity of those who believed in it, we wouldn’t be having this discussion. “If you wrote a novel about it, I don’t think people would buy it,” said mission PI Alan Stern, and rightly so. Don’t miss Chang’s The Long Strange Trip to Pluto, and How NASA Nearly Missed It.
by Paul Gilster | Jul 21, 2015 | Advanced Rocketry |
If you’re a Centauri Dreams regular, you’re familiar with Adam Crowl, an Australian polymath who is deeply involved in the ongoing Project Icarus starship design study. Adam maintains a blog called Crowlspace where interesting and innovative ideas emerge, some of them related to earlier work that has been largely forgotten in our era. A recent post that caught my eye was on Ernst Stuhlinger’s ‘umbrella ship,’ a kind of spacecraft that, when introduced to the world on Walt Disney’s 1957 TV show Mars and Beyond, surely surprised most viewers.
The umbrella ship, as Adam notes, looks nothing like what readers of the famous space series in Collier’s (1952-1954) had come to associate with manned travel to other worlds. Wernher von Braun was then championing massive rockets to be engaged in the exploration of Mars, an exploratory operation that would send a fleet of vessels to the Red Planet. Unlike tiny capsules of the kind we used to reach Earth orbit and explore the Moon, these would be large vessels to be sent in great numbers. The expedition would be described by its designer in a book von Braun wrote in 1948 called Das Marsprojekt (translated into English in 1953).
What von Braun depicted and what both Collier’s and Disney immortalized was a fleet of ten spacecraft that would send 70 crew members to Mars, the spacecraft to be built in Earth orbit using reusable space shuttles. While von Braun radically revised the plan in 1956 and scaled it back substantially, Ernst Stuhlinger was working with an entirely different concept.
Image: Ernst Stuhlinger’s Umbrella Ship, built around ion propulsion. Notice the size of the radiator, which disperses heat from the reactor at the end of the boom. As Adam notes in his blog piece, the source for this concept was a Stuhlinger paper called “Electrical Propulsion System for Space Ships with Nuclear Power Source,” which ran in the Journal of the Astronautical Sciences 2, no. Pt. 1 in 1955, pp. 149-152 (online version here). Credit: Winchell Chung.
No chemical rockets for Stuhlinger. While von Braun envisioned his fleet using a nitric acid/hydrazine propellant, Stuhlinger was interested in electrical propulsion, producing thrust by expelling ions and electrons instead of combustion gases. He noted in the paper that using chemical reactions to produce thrust created a high initial mass as compared to the payload. To reduce this mass problem, he saw, it would be necessary to increase the exhaust velocity of the propellant. Accelerating propellant particles by electrical fields made the numbers more attractive, as the paper notes in its summary:
A propulsion system for space ships is described which produces thrust by expelling ions and electrons instead of combustion gases. Equations are derived from the optimum mass ratio, power, and driving voltage of a ship with given payload, travel time, and initial acceleration. A nuclear reactor provides the primary power for a turbo-electric generator; the electric power then accelerates the ions. Cesium is the best propellant available because of its high atomic mass and its low ionization energy. A space ship with 150 tons payload and an initial acceleration of 0.67 x 10-4 G, traveling to Mars and back in a total travel time of about 2 years, would have a takeoff mass of 730 tons.
Image: Wernher von Braun and Ernst Stuhlinger discuss the Umbrella Ship concept at Walt Disney Studios. Credit: NASA MSFC.
Adam works out the details, drawing from the Stuhlinger paper itself and deriving some quantities through his own work. We get a payload, including landing vehicle and crew habitat, that is about 20.5 percent of launch mass, an impressive figure indeed. We’re also saddled with low acceleration, as you would expect. The Umbrella Ship would take about a year to reach Mars, while a chemically propelled ship as analyzed by Stuhlinger would make the journey in about 260 days. The longer the travel time, the greater the hazard, which was in many ways unknown to Stuhlinger, as Adam comments:
These days we wouldn’t want a crewed vehicle spending weeks crawling through the Van Allen Belts, but back when Stuhlinger computed his trajectory and even when the design aired, the Belts were utterly unknown. Now we’d have to throw in a solar radiation “storm shelter” and I’d feel rather uncomfortable making astronauts spend two years soaking up cosmic-rays in interplanetary space. Even so, the elegance of the design, as compared with the gargantuan Von Braun “Der Mars Projekt” for example, is a testament to Stuhlinger’s advocacy of electric propulsion.
But what an interesting design to emerge in the 1950s, and it’s ironic given the above remark that when Explorer 1 was launched in 1958, Stuhlinger was at the controls of the timer that, in those relatively primitive days of space technology, handled rocket staging. Explorer 1 was the satellite that discovered the Van Allen belts in the first place. A German infantryman (he was wounded outside Moscow and later served at Stalingrad), Stuhlinger joined the German V-2 effort and worked closely with von Braun, later coming to the United States as part of Operation Paperclip. In the 1950s, he actively collaborated with von Braun on the Disney films Man in Space, Man and the Moon and Mars and Beyond.
Stuhlinger would spend a great deal of time on ion thrusters using either cesium or rubidium vapor, accelerating positively charged ions through a grid of electrodes. Today, he is considered a pioneer of ion propulsion, well known for his book Ion Propulsion for Space Flight (McGraw-Hill, 1964). He would serve as director of Marshall Space Flight Center’s Space Science Laboratory until 1968 and later as MSFC’s associate director for science, going on to become a professor at the University of Alabama in Huntsville and a senior research assistant with Teledyne Brown Engineering. Ernst Stuhlinger died in Huntsville in May of 2008.
by Paul Gilster | Jul 20, 2015 | Astrobiology and SETI |
SETI received a much needed boost this morning as Russian entrepreneur Yuri Milner, along with physicist Stephen Hawking and a panel including Frank Drake, Ann Druyan, Martin Rees and Geoff Marcy announced a $100 million pair of initiatives to reinvigorate the search. The first of these, Breakthrough Listen, dramatically upgrades existing search methods, while Breakthrough Message will fund an international competition to create the kind of messages we might one day send to other stars, although the intention is also to provoke the necessary discussion and debate to decide the question of whether such messages should be sent in the first place.
With $100 million to work with, SETI suddenly finds itself newly affluent, with significant access to two of the world’s largest telescopes — the 100-meter Green Bank instrument in West Virginia and the 64-meter Parkes Telescope in New South Wales. The funding will also allow the Automated Planet Finder at Lick Observatory to search at optical wavelengths. Milner’s Breakthrough Prize Foundation is behind the effort through its Breakthrough Initiatives division, a further indication of the high-tech investor’s passion for science.
Image: Internet investor Yuri Milner announcing the Breakthrough Listen and Breakthrough Message initiatives in London at The Royal Society. Credit: Breakthrough Prize Foundation.
Organizers explained that the search will be fifty times more sensitive than previous programs dedicated to SETI, and will cover ten times more of the sky than earlier efforts, scanning five times more of the radio spectrum 100 times faster than ever before. Covering a span of ten years, the plan is to survey the one million stars closest to the Earth, as well as to scan the center of the Milky Way and the entire galactic plane. Beyond the Milky Way, Breakthrough Listen will look for messages from the nearest 100 galaxies.
According to the news release from Breakthrough Initiatives, if a civilization based around one of the thousand nearest stars transmits to us with the power of the aircraft radar we use today, we should be able to detect it. A civilization transmitting from the center of the Milky Way with anything more than twelve times the output of today’s interplanetary radars should also be detectable. At optical wavelengths, a laser signal from a nearby star even at the 100-watt level is likewise detectable.
Frank Drake noted the changes in technology that have made such searches possible:
“Today we have major developments in digital technology and also the necessary telescopes to monitor billions of channels at the same time. But we needed the funding to allow all this to proceed. Fortunately there are private benefactors who realize the significance of the search. We will finally have stable funding so we can plan from one year to the next. This will be the most enduring search ever launched, a great milestone and our best chance for success.”
Image: Martin Rees, Frank Drake, Ann Druyan and Geoff Marcy at the announcement. Credit: Breakthrough Prize Foundation.
Geoff Marcy (UC-Berkeley) pointed out that we simply have no idea whether the nearest civilization is ten light years or 10 million light years away, but the Breakthrough Listen project will attempt to find out by scanning 10 billion frequencies simultaneously.
“We will listen to the cosmic piano every time we point a radio telescope, but instead of 88 keys, we’ll be using ten billion keys, with software designed to pick out any note with a frequency that is ringing consistently true against the background noise of all the other frequencies.”
Milner spoke of bringing a ‘Silicon Valley approach’ to SETI, one that will develop its own software tools using open source methods and maintaining open databases. Organizers estimate that what Breakthrough Listen generates will amount to the largest amount of scientific data ever made available to the public. Thanks to its open source nature, the software effort will be flexible enough to allow scientists and members of the public to use it and to develop their own applications for data analysis. As part of the crowdsourced aspect of Breakthrough Listen, Milner announced that the effort will join the SETI@home project at UC-Berkeley, in which nine million volunteers donate spare computing power to assist in the SETI search.
The project leadership team listed on the Breakthrough Initiatives site:
- Martin Rees, Astronomer Royal, Fellow of Trinity College; Emeritus Professor of Cosmology and Astrophysics, University of Cambridge.
- Pete Worden, Chairman, Breakthrough Prize Foundation.
- Frank Drake, Chairman Emeritus, SETI Institute; Professor Emeritus of Astronomy and Astrophysics, University of California, Santa Cruz; Founding Director, National Astronomy and Ionosphere Center; Former Goldwin Smith Professor of Astronomy, Cornell University.
- Geoff Marcy, Professor of Astronomy, University of California, Berkeley; Alberts SETI Chair.
- Ann Druyan, Creative Director of the Interstellar Message, NASA Voyager; Co-Founder and CEO, Cosmos Studios; Emmy and Peabody award winning Writer and Producer.
- Dan Werthimer, Co-founder and chief scientist of the SETI@home project; director of SERENDIP; principal investigator for CASPER.
- Andrew Siemion, Director, Berkeley SETI Research Center.
Image: Stephen Hawking addressing the audience at the Breakthrough Initiatives announcement. Credit: Breakthrough Prize Foundation.
As to the Breakthrough Message initiative, it should be stressed that it is not an effort to actually send signals to other stars. This last is an important point, so let me quote directly from the news release: “This initiative is not a commitment to send messages. It’s a way to learn about the potential languages of interstellar communication and to spur global discussion on the ethical and philosophical issues surrounding communication with intelligent life beyond Earth.”
The news of these two Breakthrough Initiatives comes on July 20, the day humans first landed on the Moon in 1969. Hawking noted the scope of the challenge. We already know that potentially habitable planets are plentiful, and that organic molecules are common in the universe. Intelligence remains the great unknown. While it took 500 million years for life to evolve on Earth, it took two and a half billion years to get to multicelled animals, and technological civilization has appeared only once on our planet. Is intelligent life, then, rare? And if it exists, is it as fragile and as prone to self-destruction as we ourselves?
“We can explain the light of the stars through physics, but not the light that shines from planet Earth,” Hawking said. “For that, we must know about life, and acknowledge that there must be other occurrences of life in an infinite universe. There is no bigger question. We must know.”
by Paul Gilster | Jul 17, 2015 | Sail Concepts |
Today we look beyond Pluto/Charon toward possible ways of getting a payload to another star. Centauri Dreams readers are familiar with the pioneering work of Robert Forward in developing concepts for large-scale laser-beamed missions to Alpha Centauri and other destinations. But what if we go smaller, much smaller? Project Dragonfly, in progress at the Initiative for Interstellar Studies, proposes to explore this space, and as Andreas Hein explains below, it was recently examined in a workshop giving student teams a chance to present their ideas. A familiar figure in these pages, Andreas received his master’s degree in aerospace engineering from the Technical University of Munich and is now working on a PhD there in the area of space systems engineering, having conducted part of his research at MIT.
by Andreas M. Hein
The Project Dragonfly Design Competition, organized by the Initiative for Interstellar Studies (i4is) was concluded on the 3rd of July in the rooms of the British Interplanetary Society (BIS). To choose the rooms of the society is no coincidence. The BIS conducted the Lunar Lander study in the 1930s, which foreshadowed in an almost uncannily precise way the Apollo mission. Forty years later, in 1978, the BIS presented the first design of an interstellar probe: Daedalus. And in 2015, it was a natural choice to choose the BIS’ rooms for what might again be the stage for imagining things to come: Four international teams, almost exclusively consisting of students, are going to present their design for an interstellar probe. And this time, things get small.
Background
The field of interstellar studies can be divided into two categories. First, the field of interstellar studies is teaming with huge spacecraft, often as large and heavy as today’s largest skyscrapers. However, there is a second stream of concepts for interstellar spacecraft, beginning with Robert Forwards’ Starwisp probe and Freeman Dysons’ Astrochicken [1, 2]. These concepts stimulated thinking about the opposite: How small can we get? A small interstellar probe has a fundamental advantage. It needs less energy to accelerate to the same velocity. This is particularly relevant for interstellar missions, as we usually talk about required energies that often surpass current global energy consumption. Hence, any reduction in size might considerably increase the feasibility of an interstellar mission. Figure 2 shows an overview of some of the most relevant interstellar concepts and designs. The columns indicate the mass of a spacecraft from a particular concept or design in orders of magnitude. The rows indicate the level of detail. High-level concepts often consist of a basic feasibility analysis. System-level designs go deeper and describe key spacecraft systems in considerable detail. Subsystem-level designs add additional detail to all relevant spacecraft systems. The objective of Project Dragonfly is to address the gap in the lower left: To design an interstellar spacecraft down to subsystem level, ideally with a mass below 10 tons.
Figure 2: Some of the most relevant interstellar concepts and designs
The idea behind Project Dragonfly emerged in early 2013 when I visited Professor Gregory Matloff in New York. Greg is one of the key figures in interstellar research. That night we talked about different propulsion methods for going to the stars. We realized that nobody had yet done a design for a small interstellar laser-propelled mission. Soon after this conversation Project Dragonfly was officially announced by i4is. The name “Dragonfly” was chosen in order to pay credit to Robert Forward, who wrote the novel The Flight of the Dragonfly in the early 1980s, featuring a laser sail spacecraft. Later in 2013, i4is organized the “Philosophy of the Starship” Symposium at the BIS, where first presentations on laser-propelled interstellar probes were given by Kelvin F. Long and Martin Ciupa. Further vital preparatory work was done by Kelvin that year that fed into defining the competition requirements. With the first set of requirements defined, we finally got to the point where we were able to organize an international design competition in 2014. The purpose of the competition would be to speed up our search for a feasible mission to another star, based on technologies of the near future.
The Project Dragonfly Design Competition focused on small, laser-sail-propelled interstellar probes. Why small and laser-sail-propelled? In getting small, we are following a trend which started in the last decade. With the emergence of the CubeSat Standard first universities and then companies started to develop satellites, often not larger than a shoebox. Today, NASA and ESA are even thinking about sending small satellites to the Asteroids and Mars [3, 4]. However, the spacecraft still needs to be big enough to get enough science data back, setting a lower limit to spacecraft size, sufficient to host and supply power to the communication subsystem.
Why laser-sail-propelled? Laser sails are similar to solar sails. They both use light pressure for accelerating the spacecraft. Solar-sail-based spacecraft are today developed by various space agencies and organizations, from JAXA’s Ikaros mission to The Planetary Society’s LightSail 1. The elegance of solar sails is that they are scalable and use an abundant energy source: the Sun. Most types of solar sails could be used as a laser sail and vice versa. Hence, using a potential laser sail on a solar sail precursor would be possible in most cases. This would lower the barrier for testing a new type of sail, as operating a solar sail does not depend on a laser infrastructure.
Project Dragonfly leverages these two technology trends, as they seem to be promising to realize an interstellar mission in a scalable way.
The Project Dragonfly Design Competition
In August 2014, international university teams were invited to participate in the competition. All candidate teams had to submit solutions to a problem set first. This problem set included a range of small problems that were intended to train the teams in the key areas relevant for the competition, such as the basics of laser sail propulsion, laser systems, and in-space communication systems. The objective of this initial problem set was two-fold. First, it was intended as an entry barrier for all teams that do not have a serious intention or the capabilities to participate in the competition. Eliminating teams that would not make the cut later on also had the purpose of avoiding overburdening the reviewers. The reviewers we invited are very busy individuals. We wanted to use their time as effective as possible, giving them the opportunity to focus on the best teams. Second, the successful teams would be able to develop or strengthen their capabilities to solve the main competition task of designing a laser-sail-propelled interstellar probe. They would also get familiar with the existing literature on the topic and get a “feeling” for the subject.
The teams that were able to pass this hurdle were then confronted with the mission requirements. These requirements used the requirements developed during Project Icarus as a starting point. Project Icarus is an ongoing collaborative interstellar study between the BIS and Icarus Interstellar, in which I am participating [5]. However, the requirements were adapted and extended to the laser sail case. The requirements are depicted in Figure 3 in a graphical fashion.
Figure 3: Graphical representation of the Project Dragonfly requirements
Written out, the mission requirements are:
1. To design an unmanned interstellar mission that is capable of delivering useful scientific data about the Alpha Centauri System, associated planetary bodies, solar environment and the interstellar medium.
2. The spacecraft will use current or near-future technology.
3. The Alpha Centauri system shall be reached within a century of its launch.
4. The spacecraft propulsion for acceleration must be mainly light sail-based.
5. The mission shall maximize encounter time at the destination.
6. The laser beam power shall not exceed 100 gigawatts
7. The laser infrastructure shall be based on existing concepts for solar power satellites
These requirements were deliberately fine-tuned in order to be challenging. The 100 GW beam power requirement constrains the design space considerably. The particular value was selected as it constrains the mass of the spacecraft to tens of tons. Furthermore, it is a beam power that is very challenging to generate with an in-space infrastructure within the 21st century but not completely out of reach. The 100 year time constraint sets a theoretical minimum average trip velocity of 4.3% of speed of light in order to reach the Alpha Centauri star system. With the power constraint only the spacecraft mass, its sail system parameters, and the duration of acceleration / deceleration are left as key variables. A long acceleration duration allows for reaching a high velocity. However, a long acceleration duration means that the laser beam has to be steered over long distances. This in turn makes pointing and focusing the beam challenging.
The science data requirement is also challenging to fulfill. If the teams decide to reduce the spacecraft mass, they need to shrink their communication system as well. However, communication over interstellar distances requires large amounts of power, if useful science data is to be collected and sent back.
The teams needed to navigate in this design space, making careful trade-offs between different parameters. The competition included two intermediate stage gates and a final review of the reports. Each stage gate required a different set of deliverables that are commonly required for concept studies in the space domain, such as an initial feasibility analysis, a technology readiness assessment, and detailed calculations for key aspects of the mission. The stage gate process allowed us to check and adjust our expectations for the next stage of the competition and provide targeted support if teams were struggling in a particular area. Furthermore, each of the deliverables covered a vital aspect that is commonly needed for a concept study. The staged approach enabled the teams to work on a limited set of deliverables at each stage, reducing the difficulty of the overall task.
The competition wouldn’t have been possible without our reviewers and advisors. Fortunately, we were able to recruit experts with considerable experience in solar and laser sailing studies, such as Les Johnson, who is working as the Deputy Director of the Advanced Concepts Team at NASA and Bernd Dachwald, a German professor.
Four international teams, out of initially six contestants were able to take all hurdles and submit a final design report:
Technical University of Munich
University of Cairo
University of California Santa Barbara
CranSEDS, consisting of students of Cranfield University, UK, the Skolkovo Institute of Science and Technology in Russia, and the Université Paul Sabatier in France.
These reports were this time graded by the reviewers. Furthermore, the teams were invited to present their designs at the final workshop in London, on the 3rd of July 2015.
The workshop
The main purpose of the workshop was to mimic a typical design review in the space sector. The teams would give a presentation, covering all vital aspects of their design and would then answer questions from the audience and review panel. The review panel consisted mostly of aerospace engineers. Notable members were the Executive Director of i4is, Kelvin F. Long, and Chris Welch, who is working as a professor at the International Space University.
The first team to present was the team from the Technical University of Munich. Their spacecraft would be accelerated up to a distance of 2.2 light years and a velocity of 9% of the speed of light. The laser infrastructure would be placed on the Moon. Their laser sail would consist of a graphene sandwich material. Deceleration is enabled by a staged magnetic / electric sail. The team chose this staged approach, as the magnetic sail is very efficient at high velocities but gets increasingly less efficient at lower velocities. The electric sail is capable of decelerating at lower velocities. The spacecraft reaches the Alpha Centauri system after 100 years. Communication is enabled by a laser communication system. Power is supplied, either by solar cells that generate energy from the laser beam or the electric sail, which generates electric power when flying through the interstellar medium.
Figure 4: The spacecraft of the Technical University of Munich. Sail is not to scale.
The overall spacecraft is relatively heavy, compared to the other designs. It’s mass is 14t. Part of the reason is that the team aimed at maximizing the payload mass. A higher payload mass leads to a higher scientific yield but also leads to a heavier communication system, due to the higher data rate. Another effect is that a heavier spacecraft needs a longer duration to accelerate, imposing pointing requirements on the laser optics that are difficult to meet. Another difficulty with the overall architecture of the mission is that the laser system is located on the lunar surface. Although in principle feasible, installing such a system is very costly, unless a large amount of in-situ materials are used.
Figure 5: UCSB team wafer spacecraft design
The second presentation was given by the UCSB team. This team’s design combined the highest number of innovative technologies. It distinguished itself in numerous ways from the other teams. First, the concept of the spacecraft was a “wafer-based” design. This means that the spacecraft is basically imprinted onto a chip with all spacecraft subsystem integrated into it. The sail would consist of a dielectric material with an extremely high reflectivity, in order to withstand the enormous power density of several gigawatts per square meter. Note that sunlight in Earth orbit has a power density of about 1.4kW per square meter. Hence, the power density of the laser is roughly a million times higher than what today’s spacecraft are usually facing.
A highly reflecting surface avoids that part of the energy that is absorbed by the spacecraft, immediately melting it. The spacecraft is also accelerated rapidly, within a distance of three astronomical units, up to a velocity of 25%c. The team was able to consider such high velocities, as the spacecraft does not contain any deceleration system. Using deceleration systems such as a magnetic sail or an electric sail would lead to significant deceleration durations that may nullify any decrease in trip duration as a result of the high cruise velocity. However, the lack of a deceleration system does not comply with the mission requirements. The laser is a phased-array fiber-fed laser.
Figure 6: Samar Eldiary presenting the Cairo Team’s spacecraft
The third presentation was given by the Cairo University Team. The team’s mission design is based on an initial acceleration via laser beam, based on the DE-STAR system developed by the UCSB team. The spacecraft is decelerated via magnetic sail, and then separates into two sub-probes. One probe will collect data from Proxima Centauri, the other data from the Alpha Centauri A and B system. The laser sail is made out of aluminum. A laser communication system is used for sending back data to the Solar System. Power is provided by three Radioisotope Thermoelectric Generators (RTGs). The team presented an innovative approach for attitude control during the acceleration phase by changing the shape of the sail.
Figure 7: CranSEDS spacecraft design. The left image depicts the spacecraft bus with payload. The three cylindrical objects are the RTGs.
The last presentation was given by the CranSEDS Team. The interesting thing about their mission architecture is the use of a staged approach. A total of three spacecraft are launched within 33 year intervals. The rationale behind these intervals is to use each subsequent spacecraft as a communication relay station as well as exploiting technological advances that have occurred in the meantime.
First, the spacecraft is accelerated up to a velocity of 5%c at a distance of about 5,800 astronomical units. This phase takes about 3.7 years. The laser sail, used during acceleration, is then jettisoned. The subsequent cruise phase takes up to 77.5 years. Deceleration then starts via magnetic sail and the spacecraft enters the Alpha Centauri system after about 98 years of flight. 33 years after launch, a second spacecraft is launched with a similar configuration, but mission phases shifted by 33 years. The third spacecraft is launched after a similar interval of 33 years.
The team’s spacecraft is propelled by a Silicon Carbide sail. Power is provided by three RTGs. Data is sent back via laser communication. The overall mass of one of the spacecraft is about 4.5 tons. Each spacecraft hosts a scientific payload of 93kg, consisting of various instruments such as spectrometers, a magnetometer, and a cosmic dust analyzer.
The team presented detailed trade-off analyses for each of the critical aspects of the mission such as how many spacecraft to send and each of the spacecraft subsystems. The reviewers, however, remarked that the mission architecture, consisting of three separate spacecraft might induce programmatic risks, as the mission would need to be sustained over a period longer than a century. Furthermore, the so-called waiting paradox might come into play: A spacecraft launched later could overtake an earlier one due to a more sophisticated technology or laser infrastructure.
After the teams’ presentations, the review panel retreated for ranking the teams. As mentioned earlier, the team reports already contributed to the overall grading with two-third of the points. One-third would consist of the presentation and the teams’ performance during the Q&A session. After a few discussions, the review panel reached a conclusion and got back to the teams, waiting eagerly to hear the results.
Figure 8: The teams and members of the i4is leadership
We then announced the winners:
4. Cairo University
3. UCSB
2. CranSEDS
1. Technical University of Munich
The first prize, which went to the team of the Technical University of Munich, went along with one of the Alpha Centauri Prizes, which i4is awards to contributions advancing the field of interstellar travel.
Figure 9: Alpha Centauri Prize logo
Figure 10: The team from the Technical University of Munich (TUM) is awarded the Project Dragonfly Alpha Centauri Prize (left to right: Kelvin Long (i4is), Johannes Gutsmiedl (TUM), Nikolas Perakis (TUM), Andreas Hein (i4is))
After the ranking was announced, the next steps for Project Dragonfly were presented. First, the teams are requested to submit a summary of their report to a peer-reviewed international journal. The purpose is to receive another independent validation of the designs. Furthermore, the teams would gain experience in writing scientific publications. Another step is a technology roadmap, based on the technologies that were selected by the teams. Some of the technologies were common, such as laser communication and a magnetic sail for deceleration. However, the teams diverged in other technologies such as the laser sail material and power supply. With the teams, we will select key technologies and think about what steps are needed for developing them, along with prototypes and demonstration missions.
Later in the afternoon, the workshop participants gathered at the local bar and restaurant, the Riverside: A traditional gathering place after BIS events. Here, new friendships were forged between the participants and the future of Project Dragonfly was hotly debated.
Conclusions
The main conclusion is that a small, laser-sail-propelled interstellar mission is in principle feasible by using a laser infrastructure providing a 100GW laser beam. The Alpha Centauri system could be reached within 100 years. The spacecraft mass would be somewhere between 15 and a few tons. With the use of innovative technologies, even masses below one ton could be achieved.
The following conclusions can be drawn from the presented spacecraft designs:
- Laser communication seems to be a promising approach for achieving communication over interstellar distances.
- Magnetic sails seem to be the currently most promising way to achieve deceleration from velocities of a few percent of the speed of light.
- The trade-offs for the best laser sail material are non-trivial and there seem to be several promising materials.
- Most teams have used RTGs as power supply.
- More research needs to be done on the laser infrastructure. In particular, where to place it and how to leverage on potential future solar power satellite infrastructures.
However, there are several other feasibility issues that need to be addressed, such as beam pointing requirements over distances of several to thousands of astronomical units. Manufacturing and deployment of kilometer-sized solar sails is also an issue. Furthermore, spacecraft autonomy during the mission is a huge challenge as well. Deployment of magnetic sails with several kilometers in radius remains another feasibility issue.
Despite these challenges, let us not forget where we came from: Missions using the whole energy consumption of humankind. We were able to decrease that by two orders of magnitude or more. Yes, building such an infrastructure is an immense challenge but it is less a challenge than for example mining Jupiter for Helium 3 for two decades, as proposed for Project Daedalus, or harvesting large quantities of antimatter.
Maybe, and just maybe, some of the ideas presented during the workshop might one day open up the pathway to the stars. Until then, a lot of work remains to be done.
Let’s get started!
References
[1] Forward, R. L. (1985). Starwisp-An ultra-light interstellar probe. Journal of Spacecraft and Rockets, 22(3), 345-350.
[2] Matloff, G. L. (2005). The incredible shrinking spaceprobe. Deep-Space Probes: To the Outer Solar System and Beyond, pp.61-69.
[3] https://www.nasa.gov/press-release/nasa-prepares-for-first-interplanetary-cubesats-on-agency-s-next-mission-to-mars
[4] Asteroid Impact Mission (ESA).
[5] Long, K. F., Fogg, M., Obousy, R., Tziolas, A., Mann, A., Osborne, R., & Presby, A. (2009). Project Icarus-Son of Daedalus-Flying Closer to Another Star. Journal of the British Interplanetary Society, 62, 403-414.
