P. Douglas Lisman

A Starshade Petal Error Budget for Exo-Earth Detection and Characterization

We present a starshade error budget with engineering requirements that are well within the current manufacturing and metrology capabilities. The error budget is based on an observational scenario in which the starshade spins about its axis on timescales short relative to the zodi-limited integration time, typically several hours. The scatter from localized petal errors is smoothed into annuli around the center of the image plane, resulting in a large reduction in the background flux variation while reducing thermal gradients caused by structural shadowing. Having identified the performance sensitivity to petal shape errors with spatial periods of 3-4 cycles/petal as the most challenging aspect of the design, we have adopted and modeled a manufacturing approach that mitigates these perturbations with 1-m long precision edge segments positioned using commercial metrology that readily meets assembly requirements. We have performed detailed thermal modeling and show that the expected thermal deformations are well within the requirements as well. We compare the requirements for four cases: a 32 m diameter starshade with a 1.5 m telescope, analyzed at 75 and 90 mas, and a 40 m diameter starshade with a 4 m telescope, analyzed at 60 and 75 mas.

Starshade optical edge modeling, requirements, and laboratory tests

In conjunction with a space telescope of modest size, a starshade can be used as an external occulter to block light from a target star, enabling the detection of exoplanets in close orbits. Typically, the starshade will be placed some 50,000 km from the telescope and the system oriented so that the sun is on the opposite side of the shade to the telescope, but somewhat away from the line of sight. A small amount of sunlight can scatter from the edges of the shade directly into the telescope. Since the photon rate from an earthlike exoplanet might be only a few photons per minute, it is desirable that the scattered sunlight is also near this level. We have built an analytical model of the performance of starshade edges for both specular and Lambertian surfaces and derived requirements for properties such as reflectivity and radius of curvature. A computer model was also developed to show the appearance of the sunlight from the starshade and assess the contrast with the exoplanet. A commercial electromagnetism code was also used to investigate aspects of the results. We also constructed a scatterometer with which various test edges were measured and derived the likely performance if used in a starshade. We discuss these models and give the principal results.

Starshade mechanical design for the Habitable Exoplanet imaging mission concept (HabEx)

An external occulter for starlight suppression – a starshade – flying in formation with the Habitable Exoplanet Imaging Mission Concept (HabEx) space telescope could enable the direct imaging and spectrographic characterization of Earthlike exoplanets in the habitable zone. This starshade is flown between the telescope and the star, and suppresses stellar light sufficiently to allow the imaging of the faint reflected light of the planet. This paper presents a mechanical architecture for this occulter, which must stow in a 5 m-diameter launch fairing and then deploy to about a 80 m-diameter structure. The optical performance of the starshade requires that the edge profile is accurate and stable. The stowage and deployment of the starshade to meet these requirements present unique challenges that are addressed in this proposed architecture. The mechanical architecture consists of a number of petals attached to a deployable perimeter truss, which is connected to central hub using tensioned spokes. The petals are furled around the stowed perimeter truss for launch. Herein is described a mechanical design solution that supports an 80 m-class starshade for flight as part of HabEx.

Occulting ozone observatory ability to discover and locate single and multiple Earth-like planets in habitable zones

We present a study of the Occulting Ozone Observatory performance in observations of potential Earth-like planets around nearby solar-like stars. We use Monte Carlo techniques to simulate planetary systems and with assumptions about the signal-to-noise performance of the instrument we determine the significance of planetary system parameter determinations--for example, can we conclude that a particular planets semi-major axis (SMA) is in the stars habitable zone? In addition to studying the dependence of the SMA determination on the number of observations and detections, we present results on the ability to disentangle and analyze the data from two-planet systems.

Starshade design for occulter based exoplanet missions

We present a lightweight starshade design that delivers the requisite profile figure accuracy with a compact stowed volume that permits launching both the occulter system (starshade and spacecraft) and a 1 to 2m-class telescope system on a single existing launch vehicle. Optimal figure stability is achieved with a very stiff and mass-efficient deployable structure design that has a novel configuration. The reference design is matched to a 1.1m telescope and consists of a 15m diameter inner disc and 24 flower-like petals with 7.5m length. The total tip-to-tip diameter of 30m provides an inner working angle of 75 mas. The design is scalable to accommodate larger telescopes and several options have been assessed. A proof of concept petal is now in production at JPL for deployment demonstrations and as a testbed for developing additional elements of the design. Future plans include developing breadboard and prototype hardware of increasing fidelity for use in demonstrating critical performance capabilities such as deployed optical edge profile figure tolerances and stability thereof.

Error budgets for the Exoplanet Starshade (Exo-S) probe-class mission study

Exo-S is a probe-class mission study that includes the Dedicated mission, a 30 m starshade co-launched with a 1.1 m commercial telescope in an Earth-leading deep-space orbit, and the Rendezvous mission, a 34 m starshade intended to work with a 2.4 m telescope in an Earth-Sun L2 orbit. A third design, referred to as the Rendezvous Earth Finder mission, is based on a 40 m starshade and is currently under study. This paper presents error budgets for the detection of Earth-like planets with each of these missions. The budgets include manufacture and deployment tolerances, the allowed thermal fluctuations and dynamic motions, formation flying alignment requirements, surface and edge reflectivity requirements, and the allowed transmission due to micrometeoroid damage.