The Design Reference Mission (DRM) tool was developed to support the Exo-Starshade (Exo-S) Science and Technology Definition Team for modeling both the Dedicated (30-m starshade, 1.1-m telescope) and Rendezvous (34-m starshade, 2.4-m telescope) missions. The DRM describes the sequence of observations to be performed and estimates the number of planets that will be detected and characterized. It is executed with a MATLAB-based tool developed for the Exo-S Study.

In the DRM tool, a hierarchy of observations is used to fill up the allocated time, starting with a known RV framework.
The Exo-S DRM is anchored by a set of observations of known radial velocity giant planets that has a high-probability of success. These observations form about 1/3 of the DRM. Their timing is predetermined by the known orbital parameters. In many cases, there are large gaps in time between the observations, and these are populated with observations of either Earth-twin candidates or other high-priority targets. With remaining time, lower-priority targets, e.g., those with high completeness for super-Earths or mini-Neptunes, are scheduled.

The figure shows how the observation strategy works. An initial RV target is picked based on the date of observation, observational completeness, and solar and Earth pointing exclusion angles. Observations of the next tier of targets are then scheduled as shown in the middle layer of the figure. Paths to the N closest unscheduled targets (typically N=3–10 is used) in this tier are calculated as well as a path from the Nth star to the next RV. A weighting function to determine the most desired path based on completeness, detection time, and slew time is applied, where the weight per path segment is the time derivative of completeness, given by

W= C/(tdet + tslew)

for observational completeness C, detection integration time tdet, and starshade slew time tslew. The path with the highest weighting function is selected. Time tslew is chosen to make efficient use of propellant. Targets are observable over a window that begins up to 83º from the Sun and extends to as small as 28º (Dedicated) or 40º (Rendezvous) from the Sun. Time tslew and propellant are computed every 0.1º over the window of opportunity of a given target. It is found empirically that waiting to reach the target ≈0.5º, or about a half-day, after it enters the observing window is below the knee in the propellant curve of the rocket equation, saving significant fuel with just few percent increase in slew time over the minimum. This strategy is then applied to the next tier of targets, with the higher-ranking targets serving as the fixed path end-points for each observation segment. The process stops when all of the available time is used. It is also subject to propellant constraints. This approach is nearly optimal for each observational tier. Future advances to the DRM will be required to globally optimize the full program.

This work was done by Rachel E. Trabert, P. Douglas Lisman, and Stuart B. Shaklan of Caltech for NASA’s Jet Propulsion Laboratory.

The software used in this innovation is available for commercial licensing. Please contact Dan Broderick at This email address is being protected from spambots. You need JavaScript enabled to view it.. Refer to NPO-49558.


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Design Reference Mission Tool for Exoplanet Starshade Mission Study

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This article first appeared in the February, 2016 issue of NASA Tech Briefs Magazine.

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