Issue 15 - May 2015 Probe Study Final Reports Released
By Keith Warfield

After a year and a half, the direct imaging probe studies have been completed and their results publicly released in two final reports. In 2013, NASA commissioned two Science and Technology Definition Teams (STDTs) to examine the feasibility of developing exoplanet direct imaging missions with compelling science, ready to start in 2017, and targeted at $1B. One STDT—chaired by Karl Stapelfeldt (GSFC)—addressed a space telescope with a coronagraph design (Exo-C), while the other STDT—led by Sara Seager (MIT)—examined the starshade mission concept (Exo-S). Engineering support for the studies was provided by JPL. Both teams successfully met their objectives.

The coronagraph team developed a concept using a 1.4-meter off-axis telescope and a single science instrument consisting of a coronagraph with an integral field spectrograph (IFS) for spectral measurements. To contain costs, the concept borrows many design features from the successful Kepler mission. The mission operates for three years (with fuel for five years) in an Earth-trailing heliocentric orbit, and can image as close as 160 milliarcseconds to the target star. With a three-year mission, Exo-C can search 150 nearby stars for previously undetected planets (including 15 stars where super-Earth-size planets are detectable), image around 200 circumstellar disks, and measure spectra of 20 newly detected and previously identified planets. Use of Kepler heritage systems wherever possible and prior ExEP investments in coronagraph technology enable the mission to meet the FY17 project start date. The mission is also expected meet the $1B cost target.

The starshade STDT developed two concepts for the study: a stand-alone mission including both the starshade and telescope spacecraft in a shared launch (referred to as the “Dedicated” option), and a starshade-only concept with a starshade that launches independently to work with an existing space telescope (referred to as the “Rendezvous” option). The starshade Dedicated mission uses a commercial 1.1-meter on-axis telescope and a 30-meter starshade, each carried by commercially available spacecraft. The two spacecraft are co-launched into a heliocentric, Earth-leading, drift-away orbit and will operate for three years (with fuel for five years). The starshade Rendezvous mission adopted the Wide-Field Infrared Survey-Astrophysics Focused Telescope Assets (WFIRST-AFTA) space telescope for study and assumes WFIRST-AFTA is in orbit around the Earth-Sun L2 Lagrange point and is carrying a transponder for formation flying. The starshade Rendezvous mission joins up with the telescope sometime after the telescope has begun its primary science mission. Exoplanet science can be conducted with either a purpose-built direct imaging camera/IFS instrument, or—depending on the telescope’s sensor payload—with existing imaging and spectroscopy instruments. Both options can image to within 100 milliarcseconds of the target star, enabling detections down to Earth-size planets in the habitable zone. Spectral characterization of Earths is possible with the Rendezvous mission option, reaching R=70 for the most favorable candidates. The Dedicated option is close to—but above—the $1B target, while the Rendezvous option easily meets the cost goal.

The final reports provide valuable direct imaging mission design and technology development information and will likely feed into many of the concepts going into the next Astrophysics Decadal Survey. The reports can be downloaded for the ExEP website.

The Exo-C report is available in pdf format at URL:

The Exo-S report is available in pdf format at URL: