Future exoplanet missions: NASA and the world (part 1)by Philip Horzempa
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| Few, though, note that the biggest source of error in estimating the size of planets discovered by Kepler is the lack of knowledge of stellar diameters. |
One of the key challenges of this effort is not just the detection of extrasolar planets, or exoplanets, but of one type in particular, known as Earth analogs. These are Earth-sized worlds circling a Sun-like star in an Earth-like orbit. They represent the Holy Grail of exoplanet research. These exoplanets represent a new challenge for humanity. They are difficult to detect and to observe, but offer the excitement of the discovery of new worlds, akin to the great of age of discovery 500 years ago. We now have the means to find these new lands, but there is the question of this generation’s will to explore.
NASA has recently granted a five-year mission extension to the Kepler transit-detecting space telescope. This will allow it more time to detect the presence of Earth analogs that circle Sun-like stars. Kepler stares at a sector of our Milky Way galaxy located about 2,000 light-years away and is, essentially, gathering statistics on the presence and architectures of other solar systems. It has found an amazing variety. Its continuing mission will allow scientists to gauge the likelihood of finding worlds resembling the Earth. Carl Sagan, in the 1980’s, estimated that perhaps half of stars would harbor an “Earth.” Now, with data from Kepler, we can start to get a handle on the real picture.
Limited data from early in Kepler’s mission has led different teams to quite different conclusions. One team estimated eta-Earth (the fraction of Sun-like stars with an Earth analog) to be about 2%, while another came up with 35%. An analysis by John Rehling, (Space Daily, “Kepler Statistical Analysis Suggests Earthlike Planets Extremely Rare”, March 15, 2012) supported the lower figure. However, since that article, using more complete data from Kepler, he has shown that the real picture is more complex. His analysis has shown that the abundance curves for certain planet types, such as super-Earths, do not follow a smooth distribution curve. Rather, there is a peak, followed by a falloff, with increasing distance from the parent star. The distribution curve for Earth-sized worlds shows a positive slope, so far. Therefore, at present the best that can be said about eta-Earth is that it probably has a value of 2–12%. Only an extended Kepler mission will be able to determine the actual fraction of stars that have an Earth orbiting it. A low value for eta-Earth will mean that the search for Earth analogs in nearby solar systems will need to be pursued with vigor, and with multiple approaches.
Few, though, note that the biggest source of error in estimating the size of planets discovered by Kepler is the lack of knowledge of stellar diameters. These diameters are known only to a precision of 30%! This means that, for an exoplanet with an estimated diameter of 1.5 times that of the Earth, the real size could range from a bit larger than Earth’s to a value twice that of Earth. In other words, we could be seeing a virtual twin of the Earth or a super-Earth, a type of planet that is still unconstrained.
Next in the queue is a mission that is not designed to search for exoplanets per se, but which should, nonetheless, make a major contribution to the field. This is the Gaia space telescope, an ESA star-mapping mission that is in the final testing phase and should be operating at the Earth-Sun L2 point by early 2014. Gaia will be nothing less than a revolution for astronomers. Its main goal is to precisely measure the distance to one billion stars in our sector of the Milky Way galaxy. In addition, it will also record the positions of those stars with exquisite accuracy.
| Gaia’s keen vision will allow it to detect Jupiter-sized planets orbiting stars out to a distance of 600 light-years. |
As a byproduct of its principal work, Gaia will be able to detect thousands of exoplanets. Like Kepler, Gaia will be a “planet factory.” However, unlike Kepler, Gaia will provide planetary masses directly, as well as orbital inclinations. Gaia will do this by detecting the subtle “wobbles” in a star’s position caused by the gravitational pull of orbiting planets. Gaia’s accuracy will be at the level of 10–20 microarcseconds, or millionths of an arcsecond in angular measurement. For comparison, the Moon is half of a degree, or 1,800 arcseconds in apparent diameter. An accuracy of 10 microarcseconds is equivalent to detecting, from the Earth, a separation of 20–40 centimeters on the Moon! This is a daunting task, but the ability to accomplish it is necessary if we are to find neighboring worlds.
Gaia’s keen vision will allow it to detect Jupiter-sized planets orbiting stars out to a distance of 600 light-years. Within 100 light-years, it will be able to detect Saturn-mass planets. Depending on the abundance of planets and the amount of effort directed to analysis of the data, hundreds or thousands of exoplanets should be detected.
Gaia will contribute to exoplanet research in other ways. It is designed to monitor the brightness of its target stars and so will be able to detect planetary transits, much like Kepler. It will not monitor the stars almost continuously, as does Kepler, but may be able to discover several hundred exoplanets by this method. Also, as mentioned earlier, the uncertainty in our knowledge of star sizes affects the analysis of Kepler’s data. Gaia should help immensely with this problem by determining parallaxes of the Kepler stars. When this data is combined with spectra taken of those same stars, the stellar radii can be pinned down. This will allow a “recalculation” of planet sizes from the Kepler data, and enable a more accurate census of exoplanet sizes. In addition, these stellar radii will enable all transit missions to get better statistics on the abundance of Earth-like planets.
 The NEAT spacecraft uses formation flying to create a baseline long enough to create resolutions that allow the detection of Earth analogs up to 50 light-years away. (credit: NEAT) |
With its keen vision, Gaia will be a “proof-of-concept” mission for using astrometry to discover exoplanets. However, to detect planets as small as the Earth using this method, we will need to wait until the next generation of spacecraft. These advanced missions will have a precision that is about 10 to 100 times that of Gaia. SIM was planned to be that mission, but with its cancellation by NASA, it may be some time before such measurements are attempted by an American spacecraft (“SIM and the ‘ready, aim, aim’ syndrome”, Space Review, October 18, 2010). However, there is a European mission, the Nearby Earth Astrometric Telescope (NEAT), which could serve as a less expensive stand-in for SIM. It has been proposed to ESA as a candidate for one of their M-class missions. If funded, NEAT will be able to capture those nearby Earths that are the Holy Grail for the exoplanet community.
To achieve its accuracy, NEAT uses a different approach to conducting astrometry than SIM’s interferometer. NEAT utilizes a pair of spacecraft that would fly in formation at a separation of 40 meters. This provides the long focal length necessary to generate high angular resolution. One spacecraft carries a 1-meter mirror, while the other “detector” probe collects the focused light onto an array of moveable CCDs. It is an ingenuous design that uses only a few CCDs as opposed to the 100 CCDs in the Gaia spacecraft. Whereas Gaia will provide an accuracy of 10 microarcseconds, NEAT will provide an accuracy much less than 1 microarcsecond. In fact, for selected targets, it should be able to achieve an accuracy of 0.05 microarcseconds.
| NEAT will benefit immensely by the demonstration, in space, of two technologies: astrometry and formation flying. |
The main goal of the NEAT mission will be the detection of Earth analogs within 50 light-years. The NEAT duo will observe a list of 200 nearby Sun-like stars over several years. It will be able ascertain whether planets down to a mass of 1 Earth, or larger, orbit those stars. In addition, NEAT will be able to survey the solar systems discovered by Kepler, detecting giant planets that were missed by Kepler. These would be planets that did not transit their parent star during the Kepler mission, either because of the “wrong” orbital inclination or because their orbital period is too long. This will help to “fill in” the architecture of those solar systems, allowing a better understanding of how those systems are formed.
NEAT will benefit immensely by the demonstration, in space, of two technologies: astrometry and formation flying. As mentioned earlier, Gaia will be the “proof-of-concept” for astrometry. However, there are plans by the NEAT team to conduct an exciting in-space demo of the formation flying method. There is still a stigma attached to formation flying. At first, the use of a deployable mast may seem the safer route for a long-focal length mission. However, for exoplanet missions, target stars will need to be observed numerous times during the mission requiring frequent re-orientations. This presents several significant pitfalls for long masts, including thermal distortion, jitter, and micrometeoroid damage. Formation flying avoids those drawbacks.
Significant progress in formation flying has been achieved with Sweden’s PRISMA satellite program. This project consists of a pair of smallsats that, since their launch two years ago, have demonstrated several methods of formation flying. However, the importance of precursor missions for spacecraft designed for exoplanet hunting cannot be overstated. It is with this in mind that the French space agency CNES provided funding for a NEAT Pathfinder experiment in September. This demo utilized the PRISMA satellite duo as NEAT “stand-ins” and demonstrated the formation flying algorithm that the NEAT pair will use to find exoplanets. To make the experiment even more valuable, several of NEAT’s target stars were “observed.” This marks the first exoplanet formation flying demonstration to be flown in space and should allow the NEAT team to gain valuable real-life experience using actual space hardware.
However, the competition for limited funding is just as intense in Europe as it is in the United States. This may be why the NEAT team is proposing a smallsat version based on PRISMA technology. This concept, micro-NEAT, would take the next step after NEAT-Pathfinder towards a full NEAT mission. Instead of 40 meters, the micro-NEAT duo would fly at a separation of 12 meters. This would limit the number of targets that would be observed, but it would still produce results on the prevalence of exoplanets around the nearest stars. The lower mass limit for exoplanets discovered by micro-NEAT would be 10 Earth masses for 25 of the nearest stars. However, it will be able to detect planets down to one Earth mass for the closest stellar twins, Alpha Centauri A and B. This demonstration of the capabilities of formation flying will set the stage for more ambitious astrometry missions.
 The formation flying technology needed for NEAT is being tested with the PRISMA spacecraft. (credit: SSC) |
On the other side of the Atlantic, NASA’s plans for exoplanet exploration beyond Kepler are not well defined. The Astro 2010 Decadal team gave their vote of approval to a specific type of exoplanet mission: direct imaging and spectroscopy. One of two technical approaches they highlighted was the New Worlds starshade design. This mission design originated with a NASA Institute of Advanced Concepts (NIAC) grant to Webster Cash in 2004. Beginning with that seed grant, Cash has refined the design through several iterations so that it has reached a high level of maturity. This mission would use a free-flying occulting spacecraft in combination with a large imaging space telescope. The occulting craft would block the light of selected stars as viewed by the space telescope. With the bright light of the parent star prevented from reaching the telescope, that optical unit would be able to image any planets orbiting that star. It is a brilliantly simple concept. The trick is in deploying the occulter so that it assumes the shape necessary to block starlight. This occulter would be a flat disk 50 meters in diameter. In practice, it would have an outline resembling a giant flower. The petals of this disk would be designed to high tolerance to reduce the amount of light that is diffracted around its edge and into its shadow.
| The main challenge that the New Worlds team now faces, however, is the lack of fiscal support. |
The Astro 2010 report directed NASA to conduct technology development on several options for this mission, with the aim of conducting a mid-decade review in 2015. That review would choose a final design and press forward with a goal of having a proposal to present to the 2020 Astrophysics Decadal review. Ideally, a New Worlds Flagship mission would utilize a dedicated telescope tailored to this mission. However, considering the legacy of the massive overruns associated with the James Webb Space Telescope (JWST) program, there may not be another large space telescope launched for another decade or two. In that case, the quickest, and cheapest, way forward to achieving Astro 2010’s direct imaging mandate would be to utilize an already existing asset like JWST. Just such a mission has been proposed, the New Worlds Probe. The New Worlds Probe would cost about as much as a New Frontiers mission, as it would only need to launch the starshade. Since this mission requires that there be no modification to an already over-budget JWST, there would be some sacrifice to the data return. However, for the exoplanet community, this is probably the best way forward.
The New Worlds’ approach should allow it to exceed the goals of the original Terrestrial Planet Finder proposal, achieving those goals more quickly and at a lower cost. Those goals include obtaining images to show the location, orbit, and size of an exoplanet, even one as small as the Earth. In addition, for the first time, good quality spectra would be obtained telling us something about the conditions on extrasolar terrestrial worlds—are there clouds, oceans, continents? These will not be maps per se, but they will be the first steps on the road to constructing those real maps.
The main challenge that the team now faces, however, is the lack of fiscal support. Astro 2010 directed that NASA, as its number one priority in the medium mission cost class, devote $100–200 million to the development of technology over the next decade for direct imaging. However, NASA has yet to initiate a roadmap by which it would implement a starshade mission, and has only budgeted a trickle of funding to the effort. In spite of this fiscal challenge, the New Worlds team is pressing forward, endeavoring to test the concept through ingenious methods. As with the NEAT Pathfinder, the New Worlds team is pursuing demos of the concept. This summer, Webster Cash and the New Worlds team began flight tests on a zeppelin that would be used as a platform for a small-scale starshade to be observed by a telescope on the ground. They are still working out the bugs, but, within a few months, the New Worlds team plans to begin gathering serious astronomical data using this starshade pathfinder. Their next step will be to keep the starshade attached to the zeppelin, but observe it with a telescope carried aloft by a balloon that will be several hundred kilometers away. This will allow the observers to peer at closer (and therefore warmer) regions around the target stars. Gaining real world experience should boost the case for funding, and flying, a space-based starshade.
As mentioned, the star shade has a shape that resembles a giant flower. A ground test that unfolded one full-scale petal was successfully conducted in 2011. However, it would be extremely useful if a reduced-scale version of the starshade could be tested in a weightless environment. Any deployable structure on a spacecraft carries risk. Good design and lots of testing are the way to buy down that risk. This is one area in which NASA’s human spaceflight program can assist. John Grunsfeld, NASA associate administrator for science, has advocated a synergy between human and robotic programs since he assumed office earlier this year. In May of 2011, Grunsfeld appeared at an exoplanet workshop discussing the future of exoplanet research. During his talk, he showed a slide of an astronaut on a spacewalk holding onto the side of a large starshade. (Figure 4.) An inspiring vision! It may have been Grunsfeld’s way of highlighting the positive aspects of once again involving human spaceflight in the world of space telescopes.
| Beyond the appearances, the inclusion of men and women in exoplanet missions may be the best engineering approach in the long run |
It can be further argued that tying the Hubble Space Telescope (HST) to the human spaceflight program ensured its survival. Would Congress have funded a replacement HST after the discovery of the almost mission-ending mirror deformation? Would Hubble still be a world-class space observatory without the support of NASA’s human spaceflight program? While some argue that Congress should support pure science projects, the reality is that the inclusion of astronauts generates public interest and government funds. For whatever reasons, the Congress and the public, and even NASA headquarters itself, pays much more attention to missions with people.
Therefore, there is much logic in Grunsfeld’s allusion to a new human-robotic partnership in space science. In fact, the outgoing chairman of NASA’s exoplanet advisory group, James Kasting, advocated such cooperation. It would combine the magic of other worlds and the magic of people being involved in this new frontier. Beyond the appearances, the inclusion of men and women in exoplanet missions may be the best engineering approach in the long run. The new telescopes needed to find and characterize other worlds will be complex. There is much that can go wrong. Perhaps it is best to include the option of human involvement in deployment and servicing.
In a real starshade demo mission to the ISS, an astronaut might not actually touch the structure. However, the presence of an astronaut crew to observe and, if necessary, assist deployment would be invaluable—reminiscent of the shuttle servicing and repair missions to the HST. An ISS test of a subscale starshade could help to evaluate several engineering issues: deployment assurance, shape control, jitter, and thermal deformation. The exoplanet program would obviously benefit from this technology demonstration. However, NASA’s Office of Chief Technologist and its human spaceflight program would both gain priceless experience in the handling of large structures in space. Joint funding by those three divisions could allow this demo to launch.
