The EXoplanet Climate Infrared TElescope (EXCITE): A balloon-borne mission to measure spectroscopic phase curves of transiting hot Jupiters

TL;DR

EXCITE targets spectroscopic phase curves of hot Jupiters from a long-duration balloon to bridge space-based capabilities and ground-based limitations. It combines a 0.5 m near-infrared telescope with a two-channel, slit-less spectrograph spanning 0.8–3.5 μm, enabling full-orbit phase-resolved spectra and longitudinal brightness mapping that constrain atmospheric dynamics with $F_p/F_fstar(t,lambda)$ and eclipse depths $D_e(lambda)$. The paper details the as-built payload, including gondola/ACS, cryogenic receiver, spectrograph, H2RG detector, and data-reduction tools like ELDiP and ExoSim2, and reports results from the 2023 and 2024 Fort Sumner campaigns, along with refurbishments for an Antarctic LDB flight. EXCITE is poised to double the number of published spectroscopic phase curves, provide empirical constraints to 3D General Circulation Models, and advance exoplanet atmosphere characterization from a near-space platform. Overall, the work demonstrates the practicality of near-space phase curve observations and establishes EXCITE as a pathfinder for balloon-based exoplanet science.

Abstract

The EXoplanet Climate Infrared TElescope (EXCITE) is a balloon-borne mission dedicated to measuring spectroscopic phase curves of hot Jupiter-type exoplanets. Phase curve measurements can be used to characterize an exoplanet's longitude-dependent atmospheric composition and energy circulation patterns. EXCITE carries a 0.5 m primary mirror and moderate resolution diffraction-limited spectrograph with spectral coverage from 0.8--3.5 um. EXCITE is designed to fly from a long-duration balloon (LDB). EXCITE will observe through the peak of a target's spectral energy distribution (SED) and through spectral signatures of hydrogen and carbon-containing molecules. In this paper, we present the science goals of EXCITE, detail the as-built instrument, and discuss its performance during a 2024 engineering flight from Fort Sumner, New Mexico.

Figures (29)

• Figure 1: Surface gravities ($\log$ g$_{\text{p}}$) and dayside temperatures ${\langle T_{\text{day}}\rangle}$ of potential EXCITE targets and their K$_{s}$-band magnitudes. nagler2019observing These targets are available to observe during a single Antarctic LDB flight. Measuring a diverse set of targets in this parameter space allows for spectral classification of hot Jupiters. nagler2019observing Targets with lower surface gravity are good candidates for transit spectroscopy since the limb of the atmosphere is larger. Targets with higher dayside temperature tend to have large secondary eclipse depths since the thermal emission of the exoplanet is large. The X's indicate WASP-43b stevenson2014thermal and WASP-103b, kreidberg2018global the first two exoplanets for which phase-resolved spectroscopy has been published.
• Figure 2: Atmospheric transmittance as a function of wavelength for observations from an LDB platform (red), a commercial aircraft (green), or at the height of Mauna Kea on the ground (blue). The transmittance profiles are binned to a spectral resolution of 50.
• Figure 3: A diagram of the EXCITE gondola and science instrument. The science instrument is located in the middle of the gondola. The nested frames and other components are labeled. The outer frame connects to the hat/rotator and rotates in the azimuth direction shown with the blue arrows. Roll rotations occur at the middle frame and rotate in the direction shown by the orange arrows. The inner frame actuates the science instrument in the elevation direction shown with the green arrows.
• Figure 4: A CAD model of the EXCITE science instrument showing the optical path. Light collected by the telescope (yellow arrows) reflects off the primary and secondary mirrors to a piezo-actuated tip/tilt mirror. This mirror reflects incident light to an ambient temperature dichroic. Here, the light is split into a $0.6\--0.8$ µ m transmitted component (blue arrow) and a $0.8\--3.5$ µ m reflected component (red arrow). The transmitted beam is collected by an optical camera (FGC) which provides pointing feedback to the tip/tilt stage, enabling high line-of-sight stability ($\sim 50$ mas rms) at the telescope focus. The reflected component is the science beam. It passes through the entrance window of the cryostat and into the spectrograph. The spectrograph collimates the beam, disperses the light with a prism, focuses the dispersed beam, and splits it into short ($0.8\--2.5$ µ m) and long ($2.5\--3.5$ µ m) wave channels that are then incident on the detector. A zoomed-in region below the instrument shows the "transfer box." The transfer box opto-mechanically couples the cryogenic receiver to the telescope, and acts as the optical bench for the fine guidance system (FGS). Its design is described by McClelland. mcclelland2022generative A baffle (not shown) is incorporated into the transfer box and limits optical stray light on the FGC.
• Figure 5: The EXCITE payload with its hat removed. The starboard side of the payload is displayed; the stern direction is towards the left, and the bow direction is towards the right. The port side of the payload is obscured. The reaction wheel (bottom) stabilizes in azimuth. The roll motors are located at the bow and stern, where the middle frame is attached to the outer frame. The elevation actuators use both coarse stepper motors and frameless motors. They are located on the port and starboard sides where the telescope and inner frame attach to the middle frame. The roll encoder is located on the stern frameless motor, and the elevation encoder is located on the port side and cannot be seen. The gyroscopes mount to the telescope baffle tube and are covered by aluminized Mylar.