NASA's CUTE Mission employs a new design, enabling observations of exoplanets to be made using a small spacecraft for the first time.

 

In Figure 1, offer an artist's conceptualization depicting the CUTE mission in its on-orbit setting. Operating from a 560 km sun-synchronous orbit since September 2021, CUTE showcases its instrumental prowess in studying extreme exoplanets with precision.

Among the 5,500 exoplanets discovered thus far, a significant number orbit their parent stars in close proximity. These close-in planets offer a distinct opportunity to closely observe phenomena crucial to the understanding of our solar system's development and evolution, such as atmospheric mass loss and interactions with the host star. Launched in September 2021, NASA's Colorado Ultraviolet Transit Experiment (CUTE) mission introduces a groundbreaking design, utilizing a small spacecraft for the first time to delve into these processes. CUTE employs unique spectral diagnostics, enabling the tracking of escaping atmospheres from close-in, ultra-hot, giant planets. Its dedicated mission architecture ensures the necessary survey duration for a comprehensive characterization of atmospheric structure and variability on these intriguing worlds.

Atmospheric escape plays a crucial role in shaping the structure, composition, and evolution of various planets, impacting both our solar system's terrestrial planets and the short-period planet population identified by NASA's Kepler mission. This phenomenon, observed in escaping exoplanet atmospheres since 2003, particularly in the hydrogen Lyman-alpha line (121nm), is pivotal in understanding the habitability of temperate, terrestrial exoplanets. However, challenges arise in obtaining high-quality Lyman-alpha transit measurements due to contamination from neutral hydrogen in the interstellar medium and Earth's upper atmosphere.In a contrasting approach, the near-ultraviolet (NUV; 250 – 350 nm) flux from a host star provides a more accessible alternative, with two to three orders of magnitude higher intensity than Lyman-alpha. This allows for more accurate transit light curve measurements against a smoother stellar surface intensity distribution.

Motivated by the understanding of atmospheric escape's significance, Dr. Kevin France and a team from the University of Colorado Laboratory for Atmospheric and Space Physics designed the CUTE mission (Fig 2). Proposing the concept to NASA through the ROSES/Astrophysics Research and Analysis (APRA) Program in February 2016, the project received funding in July 2017. CUTE introduces pioneering technologies for a small space mission, including a novel rectangular Cassegrain telescope (20cm × 8cm primary mirror) and a miniature low-resolution spectrograph operating between approximately 250 – 330 nm.The rectangular telescope, tailored to the 6U CubeSat form factor, provides about three times the collecting area of a traditional circular aperture telescope. The compact spectrograph meets mission spectral resolution requirements, utilizing scaled-down component technology adapted from the Hubble Space Telescope.

In Figure 2, present a visual depiction of the CUTE science instrument, featuring a distinctive rectangular telescope and a compact spectrograph. This ensemble is strategically mounted to the spacecraft bus, showcasing the innovative design elements that contribute to the mission's capability for precise Near-Ultraviolet (NUV) measurements within the challenging thermal and pointing environment of a CubeSat.

This innovative instrument design empowers CUTE to achieve NUV measurements with a precision comparable to larger missions, even within the more demanding thermal and pointing conditions typical of a CubeSat. Moreover, the CUTE instrument's NUV bandpass allows it to capture data on iron and magnesium ions from expansive atmospheric layers that are beyond the reach of ground-based instruments. The CUTE science instrument is seamlessly integrated into a 6U Blue Canyon Technologies spacecraft bus, which provides essential functionalities such as power, command and data handling, attitude control, and communications.

Leveraging this CubeSat platform, CUTE can efficiently observe multiple transits of a given planet. The spectrogram captured by the CUTE instrument is recorded on a UV-optimized commercial off-the-shelf charge-coupled device (CCD), on-board data processing is executed, and the resulting data products are transmitted to a ground station located at the University of Colorado.

CUTE deployed from payload dispenser.

In Figure 2, we present an image showcasing the CUTE science instrument mounted to the spacecraft bus. This instrumental ensemble comprises a distinctive rectangular telescope and a miniaturized spectrograph, pioneering technologies crucial to the success of the CUTE mission.

Launched as a secondary payload on NASA's LANDSAT-9 mission on September 27, 2021, CUTE entered a Sun-synchronous orbit with a 560 km apogee. Approximately two hours post-launch, CUTE deployed from the payload dispenser (Fig 3) and subsequently unfurled its solar arrays. The amateur radio community detected spacecraft beacon signals during the first orbit, establishing communications with the ground station at the University of Colorado the next day. On-orbit commissioning of the spacecraft and instrument concluded in February 2022, initiating science operations.

As of February 2024, CUTE actively collects science and calibration data (Fig 4), observing 6 to 11 transits across seven different exoplanetary systems. The mission's capacity to observe more targets is primarily constrained by data downlink efficiency. CUTE's light curves and transit spectroscopy unveil extended Near-Ultraviolet (NUV) atmospheres on certain planets (Fig 5) and indicate potential time variability in the atmospheric transmission spectra of others. Notably, observations of the ultra-hot exoplanet, Jupiter WASP-189b, reveal a highly extended atmosphere, with magnesium ions observed to be gravitationally unbound—a sign of active escape of heavy elements in this system. CUTE data are systematically archived by the NASA Exoplanet Science Institute (NExScI).

Introducing Figure 4, we present flight data from CUTE, revealing both raw CCD observations (top) and calibrated one-dimensional spectra (bottom). This visual insight offers a glimpse into the instrumental capabilities of CUTE and showcases the meticulous process of data collection and calibration that underpins its scientific observations.

Introducing Figure 5, we present the CUTE Near-Ultraviolet (NUV) transit light curve of the ultra-hot exoplanet, Jupiter WASP-189b. Constructed from three distinct transit visits to the planet, this detailed curve provides valuable insights into the atmospheric characteristics and potential time variability of this intriguing celestial body.

CUTE has marked a significant by successfully showcasing the efficacy of a non-circular telescope and miniature spectrograph design for small space missions. This innovative approach has since been embraced by numerous NASA and international mission designs, including NASA's recently launched Monitoring Activity from Nearby sTars with uv Imaging and Spectroscopy (MANTIS) mission. By demonstrating sub-1% Near-Ultraviolet (NUV) precision, CUTE has proven that the level of precision achieved by large UV astronomy missions can be replicated by a CubeSat.

Moreover, the mission's success is attributed in part to its commitment to student training and early-career mentorship. Over 20 early career students and professionals have engaged in CUTE activities, spanning various domains from science and engineering to operations, contributing to a rich and diverse learning experience.