CUTE mission: Innovative design enables observation of extreme exoplanets from a small satellite

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Of the approximately 5,500 exoplanets discovered so far, many have been found orbiting close to their parent star. These nearby planets provide a unique opportunity to observe in detail phenomena crucial to the development and evolution of our solar system, including atmospheric mass loss and interactions with the host star. NASA's Ultraviolet Transit Experiment (CUTE) mission, launched in September 2021, uses a new design that makes it possible for the first time to investigate these processes using a small spacecraft.

CUTE provides a unique spectroscopic diagnostic that can detect the escaping atmospheres of nearby, extremely hot giant planets. Furthermore, CUTE's dedicated mission architecture allows for the research duration required to characterize atmospheric structure and variability on these worlds. Atmospheric escape is a fundamental process that affects the structure, composition and evolution of many planets. It has happened on all the terrestrial planets in our solar system, and is likely the driving force behind the short-lived planetary population characterized by NASA's Kepler mission. Atmospheric escape may ultimately be the deciding factor in predicting the habitability of temperate-terrestrial exoplanets. Escaping exoplanetary atmospheres were first observed at the Lyman-alpha hydrogen line (121 nm) in 2003. However, neutral hydrogen contamination in both the interstellar medium and Earth's upper atmosphere makes high-quality Lyman-alpha transit measurements for most planets External. In contrast, the near-ultraviolet (NUV; 250–350 nm) flux of the host star is 2 to 3 orders of magnitude higher than Lyman alpha, and light curves of the transition can be measured against a smoother stellar surface density distribution. .

This knowledge motivated a team led by Dr. Kevin France of the University of Colorado Laboratory for Atmospheric and Space Physics designed the CUTE mission. The team presented the CUTE concept to NASA in February 2016 through the ROSES/Astrophysical Research and Analysis (APRA) program and NASA funded the project in July 2017. The CUTE instrument pioneers the use of two technologies in a small space mission: a new, rectangular Cassegrain telescope (20 cm × 8 cm primary mirror) and a low-resolution miniature spectrometer operating from about 250 – 330 nm. The rectangular telescope was created to accommodate the unique instrument size of the 6U CubeSat form factor, a modification that provides nearly three times the assembly space of a conventional round aperture telescope. The compact spectrograph meets the mission's spectral resolution requirements and uses miniaturized component technology certified by the Hubble Space Telescope.

The new instrument design allows CUTE to measure the NUV with the same accuracy as larger missions, even in the most challenging thermal and point-blank environment encountered by the CubeSat. In addition, the NUV bandpass of the CUTE instrument makes it possible to measure iron and magnesium ions from very wide atmospheric layers that are inaccessible to ground-based instruments. The CUTE science instrument is built into a 6U Blue Canyon Technologies spacecraft bus that provides power, command, data processing, attitude control and communications. The CubeSat platform allows CUTE to monitor multiple transits of a given planet. The spectrogram from the CUTE instrument is recorded on a commercial UV-enhanced CCD (charge-coupled device). The data is processed on board and sent to a ground station at the University of Colorado.

CUTE was launched on September 27, 2021 as the second payload for NASA's LANDSAT-9 mission into a sun-synchronous orbit with an apogee of 560 km. CUTE was ejected from its cargo hold approximately two hours after launch and then deployed its solar panels. The spacecraft's signals were identified by the radio amateur community during its first orbit, and contact was made with the ground station at the University of Colorado the next day. The spacecraft and the instrument in orbit were completed in February 2022, and the mission has been conducting science activities since then.

CUTE NUV light curve of the extremely hot exoplanet Jupiter WASP-189b. This light curve was created based on
From three separate transits to the planet – Illustration: Sreejith, et al

As of February 2024, the CUTE program is actively collecting science and calibration data and has observed between 6 and 11 transits of seven different exoplanetary systems. Data downlink efficiency is the main factor limiting the number of targets observed over the course of the mission. CUTE light curves and transient spectroscopy reveal extensive NUV atmospheres on some planets (Figure 5) and possible temporal fluctuation in the atmospheric transport spectra of other planets. For example, observations of the extremely hot Jupiter exoplanet WASP-189b indicate the presence of an extremely extensive atmosphere. Magnesium ions were observed to be gravitationally unbound from the planet, which is evidence of the active escape of heavy elements in this system.

The CUTE program successfully demonstrated the use of a non-circular telescope and a miniaturized spectrometer design for small space missions, an approach that was later adopted by several NASA designs and international missions, including NASA's new observing activity of nearby sTars with the Ultraviolet Imaging and Spectroscopy mission ( MANTIS). . CUTE's demonstration of NUV's sub-1% accuracy showed that the accuracy achieved by large ultraviolet astronomy missions can also be achieved by a CubeSat. In addition, student training and early career mentoring were essential components of CUTE's mission success. To date, more than 20 students and early-career professionals have interned and participated in CUTE activities ranging from science, engineering and operations.

The CUTE small space telescope is being prepared for launch – Image: CUTE Team

source: NASA