Alternative Starshade Missions

Transcription

1 Alternative Starshade Missions W. Cash a, T. Glassman b, A. Lo b, R. Soummer c a University of Colorado, b Northrop-Grumman Aerospace Systems, c Space Telescope Science Institute Starshades have been shown to hold the potential to reveal -like planets around nearby stars and to allow detailed follow-up study including spectroscopy. Ideally this would be performed with a starshade in excess of 5m diameter and a telescope over 4m in diameter. However, such a flagship-class mission is unlikely to be realized in under fifteen years. But much can be accomplished in substantially less expensive missions. I will review the alternatives and provide an assessment of various architectures and what they can accomplish. These alternatives will include using JWST as the telescope, using small dedicated telescopes, and using smaller starshades. Keywords: Astrobiology, exoplanets, coronagraphy 1. The Goals of Exoplanet Research The last decade has seen an explosion in the knowledge we have about the planets that circle other stars. We have measured the approximate orbit and mass of hundreds of large planets in nearby systems, and our sensitivity is approaching the ability to detect s in the Habitable Zone. We are even performing spectra of exoplanets in absorption to their parent stars. This has to be the most exciting and dynamic astronomy discipline right now and for the coming decade 1. While there is much more to be learned from astrometric and photometric techniques, it is direct observation of exoplanets with spectroscopy and the full suite of astronomical analysis techniques that will allow astronomers to move to the next level of understanding. And, of course, we all share the goal of finding and studying true -like planets planets with oceans, continents, and temperate surfaces that might harbor life. Three classes of mission architectures to address these goals have been studied 2. The first is characterized as mid-infrared interferometry and has been largely abandoned at this point as too technically challenging and expensive. The next approach is internal coronagraphy, which is approaching the sensitivity level necessary to detect -like planets. But it requires very large, very high quality optics to detect s. And it is unclear whether spectroscopy can be effected through this route. In the last few years there has been growing interest in external occulters, often Venus Saturn Mercury Mars 1 Zodi Jupiter Figure 1: This is the test image used for the architecture comparisons. All the planets out to Saturn are shown at random phases, except for which was deliberately placed at maximum elongation. Zodiacal light, which can limit sensitivity is also included. Space Telescopes and Instrumentation 21: Optical, Infrared, and Millimeter Wave, edited by Jacobus M. Oschmann Jr., Mark C. Clampin, Howard A. MacEwen, Proc. of SPIE Vol. 7731, 77312J 21 SPIE CCC code: X/1/$18 doi: / Proc. of SPIE Vol J-1

2 called starshades 3. At the moment, this appears to the fastest and most cost-effective route to the goal of spectroscopy. In this paper we compare a variety of mission architectures to determine which are likely to be the most practical in the coming years. We selected five architectures that have been studied and subjected each to the same analysis. We first simulated its ability to image our Solar System as viewed from 1pc at an (average) inclination angle of 6 degrees. We do this with zodiacal light at the Solar System level, and then again with no exo-zodi. We then perform simulations of the spectra of -twins likely to be captured with the architecture in question. Together these simulations give a good comparative handle on the expected capability. 2. ATLAST a 1m telescope for the future Recently, Mark Postman and his colleagues performed a detailed study of the feasibility and cost of building a true next-generation Hubble Space Telescope 4. They showed that telescopes in the 8 to 16m diameter range, diffraction limited to the near ultraviolet are both achievable and affordable on a ten to fifteen year timescale. They included a starshade option as part of the study. In Figure 2 we show the simulations of the data acquired with such an instrument. We have assumed that if a 1mclass telescope can be built, then a 1m-class starshade would also be affordable, reducing the Inner Working Angle to about 2milli-arcsec. It can detect all the planets from Mercury outward at 1pc. The spectra are rich in detail. It is clear that such a mission would not only improve the ability to study details at 1pc, but could carry the study of systems out to 25 or even 5pc. 3. New Worlds Observer The New Worlds Observer architecture features a 4m diameter telescope and a 5m diameter starshade 5. This allows for an inner working angle of 5mas and imaging resolution of 25mas. This system was studied as part of the Astrophysics Strategic Mission Concept Study. A nearly identical concept was simultaneously studied by a separate group (THEIA). The simulations are presented in Figure 3. Mercury is lost because of the Inner Working Angle, but all the other planets (including Mars) can be seen. The spectra are of high quality, showing biomarkers clearly. Such a mission would allow detailed study of systems at 1pc. 4. James Webb Space Telescope One salient feature of starshades is that they can be designed independently from the telescope they are to be used with. They can even be used with existing telescopes if that telescope is in the right orbital environment. There is a high spatial resolution telescope now being prepared for launch to L2 mid-decade. The James Webb Space Telescope is primarily an infrared telescope designed to chase to galaxies to high redshift, but its resolution will be somewhat better than that of HST. Luckily, its bandpass does extend into the red end of the visible and so it can be used with starshades. More details of how JWST can be used in concert with a starshade are presented by Soummer et al in this conference 6. Proc. of SPIE Vol J-2

3 ATLAST 4 2 C) Figure 2: The test case of the Solar System as viewed from 1pc with a 1m diameter diffraction limited telescope as exemplified by ATLAST. The upper images are the case with no exozodiacal light and the middle images contain an exozodi comparable to the Solar System s. The left panels show the images at 4Å while to the right are images at 8Å where diffraction coarsens the image. The bottom panels contain simulated spectra of the. To the left without exozodiacal light and to the right with one zodi of background. These are raw spectrum including both signal and zodi background. The background level is shown by the solid line. It is clear that ATLAST can study such a system in detail and could perform well on much more distant systems. Proc. of SPIE Vol J-3

4 New Worlds Observer 1 DUD BUD 5DD 2DU U 4DDD GDDD BDDD a Figure 3: The test case of the Solar System as viewed from 1pc with a 4m diameter diffraction limited telescope as exemplified by NWO. The upper images are the case with no exozodiacal light and the middle images contain an exozodi comparable to the Solar System s. The left panels simulate the images at 4Å while to the right they simulate 8Å. The bottom images are simulated spectra of the. To the left without exozodiacal light and to the right with one zodi of background. These are raw spectra including both signal and zodi background. The background level is shown by the solid line. It is clear that NWO can study such a system in detail. Proc. of SPIE Vol J-4

5 C) James Webb Space Telescope 2DDD '1l 6DDD 5DDD 1 5DD I 1 5D I I C) 4 3 2DDD 1 6x1 ü3 Bx io Thox io 6x1 o3 Bx io Thox io Th2x io 1.4x1 Figure 4: The test case of the Solar System as viewed from 1pc with JWST. The upper images are the case with no exozodiacal light and the middle images contain an exozodi comparable to the Solar System s. The images to the right assume the baseline resolution of JWST, which is to be diffraction limited at 2microns (about 7mas). While there is no guarantee, it is likely that the effective resolution at one micron will be closer to 35mas, and this resolution is simulated to the left. The bottom images are simulated spectra of the. They are raw spectra including both signal and zodi background. The background level is shown by the solid line. Such an architecture would clearly be able to achieve the goal of finding and identifying like planets. Proc. of SPIE Vol J-5

6 ACCESS Figure 5: The test case of the Solar System as viewed from 1pc with a 1.5m diameter diffraction limited telescope as exemplified by ACCESS. The upper images are the case with no exozodiacal light and the middle images contain an exozodi comparable to the Solar System s. To the left are images at 4Å while to the right are images at 8Å. The bottom images are simulated spectra of the. They are raw spectra including both signal and zodi background. The background level is shown by the solid line. It is very clear that ATLAST can study such a system in detail and could perform well on much more distant systems. Proc. of SPIE Vol J-6

7 DEMO 1 DUD Jupiter BUD 2DU U 5DDD 6DDD 7DDD BDDD UDDD 1DDDD Anqstrome Figure 6: The test case of the Solar System as viewed from 1pc with a.5m diameter diffraction limited telescope at 4Å. The left image is the case of no zodiacal light and themiddle contains an exozodi comparable to the Solar System s. The right image is a simulated spectrum of a Jupiter, as there is no capability of acquiring spectra beyond two or three parsecs. Simulations of JWST performance are shown in Figure 4. The images to the right are created assuming that JWST is diffraction-limited at 2microns and that the resolution does not improve shortward of that point. The images to the left assume that another factor of two in resolution is achieved as one moves into the visible. That assumption is based on some simple extrapolation of tolerances, so it is likely, but not guaranteed. In both cases, clear images are achieved, although Mars is largely lost into the glare of the zodiacal light in the lower resolution case. Spectra of s can be achieved with longer observations as explained by Soummer et al. 5. ACCESS Trauger et al have designed a 1.5m class telescope for direct observation of exoplanets 7. They include an internal coronagraph that will allow direct observation of Jupiters without a starshade. We have here simulated what such a telescope would be able to accomplish it used in series with a starshade. The internal coronagraph would be used when the starshade is travelling between targets, but would not be used when the starshade is in place. Simple imaging and spectroscopy would be performed when the starshade is on target. In Figure 5 we show the simulations of ACCESS plus a starshade. To the left are images taken at 4Å and to the right at 8Å. The image at 4Å is gratifyingly clear. Venus,, Jupiter and Saturn are all easily seen. At 8Å the image is badly blurred and the planets are being hidden by even one zodi. The spectra take a long time (~1week) to acquire, but do show the major features of the s atmosphere. Thus it appears that a 1.5m architecture is practical but probably represents the smallest system that can reach twins. 6. DEMO Figure 6 shows what happens when small, inexpensive telescopes are used. The idea is to use a.5m telescope and a 12m starshade. The inner working angle provided by the starshade more than doubles, covering the. The low telescope resolution blurs planets into the zodiacal light. But Jupiter remains. So such a system would not achieve the prime goal of finding s but would be very useful for the study of major planets in the outer parts of planetary systems. It s main Proc. of SPIE Vol J-7

8 attraction is that a few systems could be studied for under $1M, establishing likely levels of zodiacal light and establishing the behavior of starshades at a low cost. 7. Conclusion In conclusion, we have shown that Starshade diameter is all about Inner Working Angle. Which planets are revealed follows from wavelength and starshade diameter considerations. The telescope diameter determines spatial resolution (through the diffraction limit) and thereby controls how well the planetary features can be resolved and studied. If it turns out that many systems are free of zodiacal light, then pulling the planets out of the noise is much easier and telescope size may be relaxed. It is clear that a starshade, as a component of a large UVOIR mission for the future would be very powerful, opening up planetary systems out to 25pc to study. Detailed spectroscopy of like planets would be possible. The smallest UVOIR telescope that would be effective in pursuit of s would be about 1.5m in diameter, similar to ACCESS, assuming it operates well and remains diffraction limited down to 4Å. The best candidates would be identified through color studies and then spectroscopy requiring a week or more would reveal their true nature. Such a mission would require two spacecraft and, as such, would end up costing over a billion dollars, putting it just outside the probe class. But not by much. Finally, using a starshade with JWST appears highly appealing. It would require a high quality starshade costing between $6M and $8M. But the images would clearly show all major planets in nearby systems. The spectrographs of JWST can be used to capture spectra adequate to classify the planets detected. Overall, use of JWST appears the fastest and most cost-effective way to detect and characterize s. But, should that not prove feasible for some reason, then a 1.5m-class telescope like ACCESS could be built to provide the needed capability in a dedicated way. While this would break the (somewhat arbitrary) barrier into the flagship class, it would not exceed that barrier by much and could be afforded by NASA should they choose to give it high priority. REFERENCES [1] [2] [3] Cash, W., Detection of -like planets around nearby stars using a petal-shaped occulter Nature, 442, (26) [4] [5] Cash, Webster, and 48 co-authors of the New Worlds Study Team, The New Worlds Observer: the astrophysics strategic mission concept study, Proc. SPIE, 7436, pp (29) [6] Soummer, Remi, et al, Direct imaging and spectroscopy of habitable planets using JWST and a starshade, SPIE, 7731, this volume (21) [7] Proc. of SPIE Vol J-8