Starshade system for direct observation of exoplanets (June 10, 2018)
[Direct Observation of Exoplanets by using Sharshade]

The following member is studying on this topic; S. Shitara, T. Nakamura (design of occulter shape), M. Fukunaga, S. Kataoka (Structural analysis and system design), D. Kawarabayashi, D. Kousaka (Structural design), Y. Miyazaki(PM).

There are found lots of planets beyond our solar system where extraterrestrial life may be able to exist, which are called "habitable exoplanets". There are the database of habitable explanets opened for anybody in overseas and in Japan. NASA/JPL is studying on a "Habitable Exoplanet Imaging Mission (HabEx)".

The habitable exoplanet ( or its candidate) is generally orbiting around a star that brights. It is quite difficult to image directly the exoplanet by space telescope because the star light is too strong. Actually, there is no case in which the exoplant beyond the solar system is imaged directly.

Therefore, the exoplanets has been observed indirectly so far. In the indirect observation, we find the evidence that the exoplanet exists "there" by observing someting other than the exoplanet itself. There are several indirect observational method, e.g. radial velocity method (doppler spectroscopy), transit photometory, and many exoplanets were found by using those methods. The information about its mass, dimension, composition has been obtained, too.

[Starshade] On the other hand, NASA is studying on the method to image the exoplanets directly using space telescope by blocking the starlight. There are two methods, i.e. coronagraph and starshade [1,2]. The coronagraph system puts something just before the detector of the telescope to block only the starlight and to detect the light from the exoplanet. The starshade system puts large shield called occulter with several tem meters in diameter between the exoplanet and the star to block the starlight. It is possible to combine these two methods. The right figure illustrates the concept of starshade. NASA/JPL proposes the occulter with 30-80 m in diameter, and several ten km in the distance between the occulter satellite and the space telescope.

In order to realize the starshade system, we need the following three technologies; 1) formation flight of the space telescope and the occulter satellite, 2) deployment of the occulter with appropriate shape, and 3) spectroscopic observation of the faint light from the exoplanet. The technology 3) can be obtained by appling the indirect observation technology, but the technologies 1) and 2) have not been realized yet. Our laboratory is now studing on the science of observation of exoplanets and researching on the design method of the occulter shape.

[Appotized mask] Considering the diffraction of light from the star, the optimized circular occulter has continuously changing transmittance A(r) in the radial direction r as illustrated in the right figure. However, it is not easy to produce the membrane with continuously changing transmittance.

[Petalised occulter]Therefore, there has been proposed an occulter with the transmittance equivallent to the continuously changing transmittance as in the left figure [3-5]. The left counter illustrate the distribution of the contrast at the pupil plane of the telescope in case of the occulter is located between the star and the telescope. In this figure, the starlight is blocked more with smaller contrast.

Our laboratory has been proposing a self-deployable membrane truss that consists of self-deployable truss and a membrane to realize such an occulter shape. The self-deployable truss consists of slender booms with convex cross-section, which can be stored in small volue by wrapping the booms with free-rotationg small cylindrical hubs that is latched in the launch phase. On the orbit in space, the latch is released and the boom extened automatically, and the truss deploys without any external power. We have been demonstrating the planer self-deployable truss with 20m diameter and 3D self-deployable truss with 2m in its side on the ground under 1G environment, and investigated the characteristics of those structures (see right movies).

[Starshade] We expect it is possible to deploy the petalized membrane by employing the self-deployable truss like the right figure. However, it is not easy to form a membrane with such a curved edge and to keep the shape and the stiffness of the structure on otbit. Actually, the tip of the petal may be a thin flexible plate with curved frame same as that proposed by NASA/JPL, and the problem of the shape accuracy and the stiffness is remained.
[New Occulter] So, we are researching on the occulter with outer circumference consisting of self-deployable truss with straight booms that has high contrast as shown in the left figure. We will publish this concept and its design theory in the conferences.

[1] S. Shitara, Y. Miyazaki, "Effect of Shape of Starshade on High-Contrast Imaging,", 26th Space Engineering Conference, 1A4, pp.1-8, December 22-23, 2017 (written in Japanese).
[2] W. Cash, "Detection of Earth-like planets around nearby stars using a petal-shaped occulter, Nature," Vol. 442, pp.51-52, 2006, DOI: 10.1038/nature04930.
[3] R. Soummer., W. Cash, R. A. Brown, I. Jordan, A. Roberge, T. Glassman, , A. Lo, S. Seager, and L. Puero, "A Starshade for JWST: Science Goals and Optimization," Proc. SPIE, Vol. 7440, pp. 1-15, 2009, DOI: 10.1117/12.826702.
[4] The Probe Scale Science and Technology Definition Teams of NASA's Astrophysics Division, "Exo-S: Starshade Probe-Class Exoplanet Direct Imaging Mission Concept, Final Report," 2015.
[5] R. J. Vanderbei, E. Cady, and N. J. Kasdin, "Optimal Occulter Design for Finding Extrasolar Planets," The Astrophysical Journal, Vol. 665, No.1, pp.794-798, 2007, DOI: 10.1086/519452.

College of Science and Technology, Nihon University
Department of Aerospace Engineering
Space Structure Systems Laboratory
College of Science and Technology, Nihon University
7-24-1 Narashinodai, Funabashi, Chiba 274-8501, Japan
e-mail: asel (at) forth.aero.cst.nihon-u.ac.jp