Ian J. E. Jordan

UMBRAS: a matched occulter and telescope for imaging extrasolar planets

We describe a 1-meter space telescope plus free-flying occulter craft mission that would provide direct imaging and spectroscopic observations of Jovian and Uranus-sized planets about nearby stars not detectable by Doppler techniques. The Doppler technique is most sensitive for the detection of massive, close-in extrasolar planets while the use of a free-flying occulter would make it possible to image and study stellar systems with planets comparable to our own Solar System. Such a mission with a larger telescope has the potential to detect earth-like planets. Previous studies of free-flying occulters reported advantages in having the occulting spot outside the telescope compared to a classical coronagraph onboard a space telescope. Using an external occulter means light scatter within the telescope is reduced due to fewer internal obstructions and less light entering the telescope and the polishing tolerances of the primary mirror and the supporting optics can be less stringent, thereby providing higher contrast and fainter detection limits. In this concept, the occulting spot is positioned over the star by translating the occulter craft, at distances of 1,000 to 15,000 kms from the telescope, on the sky instead of by moving the telescope. Any source within the telescope field-of-view can be occulted without moving the telescope. In this paper, we present our current concept for a 1-m space telescope matched to a free-flying occulter, the Umbral Missions Blocking Radiating Astronomical Sources (UMBRAS) space mission. An UMBRAS space mission consists of a Solar Powered Ion Driven Eclipsing Rover (SPIDER) occulter craft and a matched (apodized) telescope. The occulter spacecraft would be semi-autonomous, with its own propulsion systems, internal power (solar cells), communications, and navigation capability. Spacecraft rendezvous and formation flying would be achieved with the aid of telescope imaging, RF or laser ranging, celestial navigation inputs, and formation control algorithms.

A starshade for JWST: science goals and optimization

The James Webb Space Telescope will be an extraordinary observatory, providing a huge range of exciting new astrophysical results. However, by itself it will not be capable of directly imaging planets in the habitable zone of nearby stars, one of the most fascinating goals of astronomy for the coming decade. In this paper we discuss the New Worlds Probe (NWP) concept whereby we use an external occulter (or starshade) to cast a shadow from the star onto the telescope, therefore canceling the direct star light while the light from a planet is not affected. This concept enables JWST to take images and spectra of extrasolar planets with sufficient contrast and inner working angle to be able to discover planets down to the size of the Earth in the habitable zone around nearby stars. JWSTs instruments are appropriate to achieve low resolution spectroscopy (R ≅ 40) of these planets, and address a series of fundamental questions: are there planets in the habitable zone around nearby stars? What is the composition of their atmosphere? What are the brightness and structures of exozodiacal disks around nearby stars? What is the mass and composition of currently known giant planets? In this paper we study the starshade optimization for JWST given the instrumental constraints, and show that the modest optical quality of the telescope at short wavelength does not impact the possibility of using a starshade. We propose a solution to enable imaging and spectroscopy using target acquisition filters. We discuss possible time allocation among science goals based on exposure time estimates and total available observing time. The starshade can be launched up to 3 years after JWST and rendezvous with the telescope in orbit around L2.

Imaging planets about other stars with UMBRAS II

In this paper we discuss operational considerations for the free-flying occulter. Operations consist of maneuvering the Solar-Powered Ion-Driven Eclipsing Rover (SPIDER) between targets, alignment with the space-based telescope line of sight to the target, and stationkeeping target-to-target maneuvers need to be optimized to conserve propellant. A reasonable balance needs to be determined between target observation rate and the number of targets that are observable during mission lifetime. Velocity matching of the SPIDER with the telescope is essential to mission performance. An appropriate combination of solar electric and cold-gas thrusters provides the ability to match velocities using positional information derived from comminution and ranging between telescope, occulter and any metrology stations. Desirable features of using an external coronagraphic vehicle include the ability to obtain coronagraphic data with any instrument on the telescope-- imaging, spectroscopic, or interferometric.

Apodized Square Aperture Plus Occulter Concept for TPF

The standard approach to achieving TPF-level starlight suppression has been to couple a few techniques together. Deployment of a low- or medium-performance external occulter as the first stage of starlight suppression reduces manufacturing challenges, mitigates under-performance risks, lowers development costs, and hastens launch date for TPF. This paper describes the important aspects of a conceptual 4-metre apodized square aperture telescope system utilizing a low-performance external occulter. Adding an external occulter to such a standard TPF design provides a benefit that no other technique offers: scattered and diffracted on-axis starlight is suppressed by orders of magnitude before reaching the telescope. This translates directly into relaxed requirements on the remainder of the optical system.

The Advantages of Multiple Coronagraphic Vehicles in Occulter Missions

Free-flying external coronagraphs for space telescopes have been studied since the 1960s, but cost/ benefit analysis has not proved convincing for skeptics. A single space vehicle carrying an occulting screen can increase the contrast between a star and its orbiting extrasolar planets for a suitably designed space telescope. However, a number of advantages ensue when replacing the single occulter with a fleet of lighter-mass occulters. Target observation rates can increase in proportion to the number of coronagraphic free flying vehicles devoted to a mission. At the same time, mass requirements for individual vehicles is dramatically reduced. Mission lifetimes can at the same time potentially increase, and the risk of independent catastrophic failures onboard an occulting vehicle eliminating all science productivity for the mission are effectively vanish. Perunit vehicle costs are substantially lower than for production of a single unique craft.

Direct imaging and spectroscopy of habitable planets using JWST and a starshade

A starshade with the James Webb Space Telescope (JWST) is the only possible path forward in the next decade to obtain images and spectra of a planet similar to the Earth, to study its habitability, and search for signs of alien life. While JWST was not specifically designed to observe using a starshade, its near-infrared instrumentation is in principle capable of doing so and could achieve major results in the study of terrestrialmass exoplanets. However, because of technical reasons associated with broadband starlight suppression and filter red-leak, NIRSpec would need a slight modification to one of its target acquisition filters to enable feasible observations of Earth-like planets. This upgrade would 1) retire the high risk associated with the effects of the current filter red leak which are difficult to model given the current state of knowledge on instrument stray light and line spread function at large separation angles, 2) enable access to the oxygen band at 0.76 μm in addition to the 1.26 μm band, 3) enable a smaller starshade by relaxing requirements on bandwidth and suppression 4) reduce detector saturation and associated long recovery times. The new filter would not affect neither NIRSpecs scientific performance nor its operations, but it would dramatically reduce the risk of adding a starshade to JWST in the future and enhance the performance of any starshade that is built. In combination with a starshade, JWST could be the most capable and cost effective of all the exoplanet hunting missions proposed for the next decade, including purpose built observatories for medium-size missions.

Impact of observing constraints and unplanned events on observatory efficiency and long-range plan stability

In an era of increasing pressure to do more with less and make the most out of every budget dollar, HST science operations have steadily been able to give its customers more by increasing observatory efficiency. While original mission goals for observatory efficiency were targeted at less than 35%, HST now consistently achieves weekly schedules which are greater than 50% efficient. Furthermore, special concentration on continuous viewing opportunities and science campaigns (i.e. -- the Hubble Deep Field) has yielded efficiencies exceeding 60%. More than fourteen years of applied operational experience and system analysis by HST ground, flight, instrument, and user support systems personnel have resulted in the success. However, these efficiency levels could be even higher were it not for the variety of constraints and unplanned events which affect how and when the observatory can be used. Certain known spacecraft and instrument constraints impact efficiency with little effect on long range plan stability since they can be accounted for in advance. For this class of concerns, planning scenarios can be developed and analyzed to see what efficiencies might be achieved without these constraints. Unpredictable events such as spacecraft safings and anomalies, targets of opportunity and quick turnaround directors discretionary science reduce the overall stability of an observatorys planned use as well as its efficiency. In this paper we will describe various constraints and unplanned events, show their effects on HST observatory efficiency and stability, and discuss specific efforts of the HST Long Range Planning group to minimize their impact.

Planning and scheduling at STScI: from Hubble to the James Webb Space Telescope

While HST’s planning and scheduling processes are mature, JWST’s–with a planned 2018 launch–are still in development. The STScI science, engineering, software, and operations teams are working together to get the JWST planning and scheduling systems up and running in the next few years. Here, we review the improvements made to HST’s planning and scheduling processes over the past three decades, as well as the current state of the observing program. Also, differences between the two telescopes are discussed, as well as how they affect the creation of the JWST planning and scheduling system.

Testing JWST's proposal planning system: early progress

The observer program implementation, planning, and scheduling subsystems are undergoing software development for the James Webb Space Telescope front-end ground segment and are being tested in an integrated fashion. This part of the ground system leverages what was developed and fine-tuned for the Hubble Space Telescope over previous decades. This paper will describe the testing design, methods, results, plus the current capabilities and elements still to be developed for these subsystems through the time of publication. We will point out elements from Hubbles systems, from an operations perspective, which have been preserved for the new telescope, and those which require redevelopment.

Architecture impacts on planning and activity scheduling in external occulter missions

Architecture choices impact planning and scheduling of activity sequences for two widely separated spacecraft envisioned to be part of an astrophysics mission to observe extra-solar-planets. The two spacecraft consist of a large space telescope and an external occulter, separated by tens of thousands of kilometres. The science need is to maintain alignment at the tens of milliarcseconds level (~ metres) or less on given target stars after moving one of the spacecraft tens of thousands of kilometres. Doing this efficiently presents operational and architectural design challenges that rely on appropriate choice of navigation, propulsion, and alignment technologies, vehicle configuration, and activity scheduling strategies—an extensive combination of which may potentially be chosen from for such a mission. Challenges inherent in the general system architecture are described with emphasis on potential problems and the need for sound and appropriate integration of architecture planning, subsystem choice, and activity scheduling.