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CUTE (Colorado Ultraviolet Transit Experiment)

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CUTE is a 4-year, NASA-funded project to design, build, integrate, test, and operate a CubeSat at LASP (Laboratory for Atmospheric and Space Physics) at the University of Colorado, Boulder, CO. The main science goal of CUTE is the study of the upper atmosphere of close-in planets by means of near ultraviolet transmission spectroscopy. Targets are therefore observed during planetary transits, meaning while the planet is passing in front of the star as seen from Earth. During transit, part of the stellar light passes through the planetary atmosphere, which leaves its fingerprint in the stellar spectrum. CUTE spectra will be used to extract and study the planetary atmospheric signature to improve our understanding of planet atmospheric escape. 1)

CUTE is a near-UV (2550 to 3300 Å) 6U CubeSat mission designed to monitor transiting hot Jupiters to quantify their atmospheric mass loss and magnetic fields. CUTE will probe both atomic (Mg and Fe) and molecular (OH) lines for evidence of enhanced transit absorption, and to search for evidence of early ingress due to bow shocks ahead of the planet's orbital motion. As a dedicated mission, CUTE will observe 100 spectroscopic transits of hot Jupiters over a nominal 7-month mission. This represents the equivalent of >700 orbits of the only other instrument capable of these measurements, the Hubble Space Telescope. 2)

CUTE efficiently utilizes the available CubeSat volume by means of an innovative optical design to achieve a projected effective area of ~28 cm2, low instrumental background, and a spectral resolving power of R~3000 over the primary science bandpass. These performance characteristics enable CUTE to discern transit depths between 0.1% and 1% in individual spectral absorption lines. We present the CUTE optical and mechanical design, a summary of the science motivation and expected results, and an overview of the projected fabrication, calibration, and launch timeline. 3)

CUTE is the first NASA-funded UV/O/IR astrophysics CubeSat, and second overall (the first being the University of Iowa X-ray mission HaloSat). CUTE leverages a compact optical design in a 6U CubeSat form factor to provide a high-efficiency NUV (Near UltraViolet) spectrograph dedicated to monitoring the spectral properties of hot Jupiter atmospheres during transit.

The typical transit depth of hot Jupiters at visible wavelengths is ~1%; however, the atmospheres of short-period planets may be inflated to several planetary radii, resulting in transit depths of 3% to 10% in specific spectral tracers.

Observing time resources on orbiting observatories are insufficient to monitor enough transiting systems to statistically characterize the interplay between short period, massive planets, and their host stars. Broadband visible/NIR light curves, such as those generated by TESS or Kepler, are not sensitive to atmospheric tracers. The observation time necessary to establish a complete transit lightcurve for multiple systems, with multiple visits to each system to check for variability, is too costly for a shared flagship resource, such as HST. The time to completely map a transit for a single system is determined not by the sensitivity of the observatory, but rather by the length of time the planet is in-transit and the observational time available (set by the orbit of the spacecraft).

As a dedicated satellite, CUTE will carry out the first survey of NUV spectral lightcurves of short-period exoplanets (Figure 1).

The spectral resolving power of CUTE is comparable with the G230L mode of HST-COS (R~3000) over a bandpass that covers critical atomic and molecular tracers, such as MgI, MgII, FeII, and OH, which are inaccessible from the ground. A compact design that maximizes throughput makes CUTE sensitive to transit depths of >1.2% in MgII to >3σ confidence in a single transit for the median planet in the preliminary CUTE target list, and >0.7% in the continuum. Folded over multiple transits, the continuum sensitivity reaches down to <1% transit depths for all targets, and <0.1% for HD 209458b, respectively.

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Figure 1: Transparent rendering of the CUTE spacecraft (image credit: LASP)

 


 

Spacecraft

CUTE is a 6U CubeSat whose bus and primary subsystems (including the attitude control system, GPS receiver, communication systems, electrical power system, and solar array) are provided by Blue Canyon Technologies (BCT).

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Figure 2: Final CUTE design. The top and sides of CUTE are not displayed to illustrate the orientation and fit of the science instrument. The BCT CubeSat bus will be delivered including the sun sensors, GPS patch, S-band transmitter, an ultra high frequency (UFH) antenna (not shown), and star tracker and attitude control and determination system (ADCS), image credit: LASP

To this end, CUTE houses a near-ultraviolet (~250 – 330 nm) spectrograph based around a novel rectangular Cassegrain telescope; the spectrograph sensor is an off-the-shelf Teledyne e2v CCD. To achieve desired spectral signal-to-noise ratio (SNR), dark current is reduced by cooling the CCD to a temperature of 50 °C with a thermoelectric cooler (TEC). The TEC is driven by a constant current buck converter with an H-bridge topology for bidirectional current control. The packaging of the CCD imposes a maximum time rate of change of temperature of 5 K/min. A cascaded software control loop (discussed here) was developed that constrains this time rate of change within allowable bounds while simultaneously driving the CCD temperature to a desired setpoint. Criteria for sizing a TEC to the application and initial laboratory results are discussed, as well as digital filtering methods employed and possible solutions to integral wind-up. 4)

CUTE is planned to have a 1-year nominal mission life time and will launch in late September 2021. During the one year nominal mission, CUTE will point at 10 known hot Jupiters and observe more than 10 transits per each. The resulting lightcurves will inform the CUTE team and the exoplanet community at large about the shape, size, and composition of these exoplanetary systems. Data will be hosted by the NASA Exoplanet Archive and available to the public roughly six months after observation.

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Figure 3: CUTE was assembled and tested at the University of Colorado, Boulder and the Laboratory for Atmospheric and Space Physics (LASP). Dr. Kevin France is the Principle Investigator of the CUTE mission at LASP (image credit: LASP)

 

Development status

• August 16, 2021: CUTE was successfully delivered to Vandenberg Air Force base in late July and is currently being integrated into NASA's Landsat-9 mission for launch in mid-September 2021. The CUTE engineering team worked extremely hard to get CUTE launch-ready and are excited to see the mission move forward. Stay tuned for updates about the launch and CUTE's flight status! 5)

• May 2019: NASA funded CUTE for science and operations between launch and 2023. As the CUTE mission is expected to last no more than 3 years in space before deorbiting into the Earth's atmosphere, this funding will provide support to produce excellent science with CUTE data. 6)

 

Launch: CUTE was launched as a rideshare payload on a United Launch Alliance (ULA) Atlas-5 vehicle AV-092) from Vandenberg Space Force Base (SLC3E), California on 27 September 2021 at18:12 UTC. The primary payload on this mission was Landsat-9 of NASA/USGS. 7)

Orbit of Landsat-9: Sun-synchronous near-circular orbit, altitude = 705 km, inclination = 98.2º, period = 99 minutes, repeat coverage = 16 days. The Landsat-9 satellite will be in coplanar orbit with Landsat-8, 180º apart. This reduces the repeat coverage to 8 days.

Secondary (or rideshare) payloads: Four CubeSats for NASA's 37th Educational Launch of Nanosatellites (ELaNa 37) mission and the Mission Manifest Office at the U.S. Space Force Space Systems Command will begin separating from Centaur at approximately T+plus 2 hours, 14 minutes. The CubeSats are carried on the Evolved Expendable Launch Vehicle Secondary Payload Adapter (ESPA) Flight System, or EFS. It is a ring positioned below the Landsat-9 and above the top of Centaur carrying the four science and national security CubeSats. 8)

ULA's four-meter-diameter metallic payload fairing, built at the company's facility in Harlingen, Tex., protects Landsat-9 and the CubeSats during ascent through Earth's atmosphere. The longest of available Atlas V four-meter fairings will be used, known as the Extra Extended Payload Fairing (XEPF).

• CUTE (Colorado Ultraviolet Transit Experiment), a 6U CubeSat astronomy mission of the University of Colorado at Boulder, Boulder, Colorado.

• CuPID (Cusp Plasma Imaging Detector), a 6U CubeSat space weather science mission of Boston University, Boston Massachusetts.

• CM1 (Cesium Mission 1) of CesiumAstro with two 6U CubeSats (CM1 and CM2), a technology demonstration mission.

Orbit of the secondary payloads: After deployment of Landsat-9, the Centaur stage will reignite its engine two times to maneuver into a different orbit for separation of four small CubeSat rideshare payloads. The CubeSats will be delivered into a near-circular sun-synchronous orbit of 550 km altitude with an inclination of 97.6º.

 


 

Sensor complement (NUV Imaging Spectrograph)

Using near-ultraviolet (NUV) transmission spectroscopy from 255 to 330 nm, CUTE will characterize the composition and mass-loss rates of exoplanet atmospheres by measuring how the NUV light from the host star is changed as the exoplanet transits in front of the star and passes through the planet's atmospheres. Transit lightcurves created from CUTE observations will provide constraints on the composition and escape rates of these atmospheres, and may provide the first concrete evidence for magnetic fields on extrasolar planets.

CUTE contains a novel rectangular reflector that increases collection area by a factor of 3 over a standard circular aperture. It was designed by Co-I Brian Fleming and has been fabricated by Nu-Tek of Minnetonka, MN (Minnesota). With the exception of the grating, all mirrors will be coated in aluminum (Al) with a protective overcoat of magnesium fluoride over (MgF2). MgF2 helps prevent the formation of an oxide layer that can absorb UV light.

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Figure 4: Ray trace of the CUTE telescope (image credit: LASP)

Here's how light travels through the CUTE instrument, as shown in Figure 4.

• Light travels in from the left and is collected by a 20 cm x 8.5 cm rectangular Cassegrain telescope. The rectangular design provides more than three times the collecting area of conventional circular apertures that fit into a 1U tall spacecraft.

• The incident beam is reflected off a small fold mirror (1st Fold Mirror) and passes through the spectrograph entrance slit, which has a projection of 23 arcminutes (23') on the sky.

• Light is dispersed by an ion-etched diffraction grating (from Horiba J-Y) that builds off the heritage of the COS (Cosmic Origins Spectrograph) on the Hubble Space Telescope.

• The dispersed beam is then reflected off of a second fold mirror (2nd Fold Mirror) before being recorded on a UV-optimized 2048 x 515 pixel e2v CCD array (Detector).

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Figure 5: The CUTE science instrument. The CUTE primary acts as its own optical bench, supporting the secondary mirror in front of it and the spectrograph housing behind it. Two flexures mount the telescope to the bottom of the CUTE CubeSat bus. A heat strap comes off the back of the CCD detector and is attached to a radiator on the side of the CubeSat bus (image credit: LASP)

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Figure 6: Internal close-up rendering of the CUTE spectrograph enclosure. A cover closes the enclosure and acts as the CCD mount. A shutter is used to block light from the CCD. The cylindrical fold mirror sits on a tip/tilt stage that will be used to align the spectrograph. The CCD is mounted on the outside of a panel that closes the spectrograph enclosure. A lamina emergent mount (LEM) will help absorb any thermal expansion and contraction that occurs on the CCD mount, and a heat strap is mounted to the LEM and attached to the side of the CubeSat bus (image credit: LASP)

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Figure 7: Solid render of the closed spectrograph enclosure with the CCD and a lamina emergent mechanism (LEM) heat strap mount. The flexure on the left has a vertical cylinder nested inside the horizontal cylinder that is bonded into the telescope. The width of the telescope is only 20 cm. The flexures are used to mount the science instrument to the bottom of the CubeSat (image credit: LASP)

 


1) Brian T. Fleming, Kevin France, Nicholas Nell, Richard Kohnert, Kelsey Pool, Arika Egan, Luca Fossati, Tommi Koskinen, Aline A. Vidotto, Keri Hoadley, Jean-Michel Desert, Matthew Beasley, and Pascal Petit "The Colorado Ultraviolet Transit Experiment (CUTE): a dedicated CubeSat mission for the study of exoplanetary mass loss and magnetic fields", Proceedings of SPIE 10397, 'UV, X-Ray, and Gamma-Ray Space Instrumentation for Astronomy XX, 103971A (29 August 2017); SPIE Optical Engineering + Applications, 2017, San Diego, California, United States, https://doi.org/10.1117/12.2276138

2) Brian T. Fleming, Kevin France, Nicholas Nell, Richard Kohnert, Kelsey Pool, Arika EganLuca Fossati, Tommi Koskinen,Aline A. VidottoKeri Hoadley, Jean-Michel Desert, Matthew Beasley, Pascal M. Petit, "Colorado Ultraviolet Transit Experiment: a dedicated CubeSat mission to study exoplanetary mass loss and magnetic fields," Journal of Astronomical Telescopes, Instruments, and Systems, Volume 4 (1), January-March 2018, URL: https://core.ac.uk/download/pdf/211376563.pdf

3) Kevin France, "Exploring extreme exoplanets," Nature Astronomy, Vol. 4, 1112 (2020), Published: 6 November 2020, https://doi.org/10.1038/s41550-020-01239-2

4) "Nicholas DeCicco, Nicholas Nell, Kevin France, Stefan Ulrich, Arika Egan, Brian Fleming, Rick Kohnert, "Design and Implementation of a Thermoelectric Cooling Solution for a CCD-based NUV Spectrograph, "Proceedings of the 33rd Annual AIAA/USU Conference on Small Satellites, August 3-8, 2019, Logan, UT, USA, paper: SSC19-WKIV-07, URL: https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=4374&context=smallsat

5) "CUTE delivered to NASA for mid-September launch!," LASP, 19 August 2021, URL: https://lasp.colorado.edu/home/cute/cute-delivered-to-nasa-for-mid-september-launch/

6) "Funding renewed for post-flight operations," LASP, May 2019, URL: https://lasp.colorado.edu/home/cute/funding-renewed/

7) "NASA Launches New Mission to Monitor Earth's Landscapes," NASA Press Release 21-126, 27 September 2021, URL: https://www.nasa.gov/press-release/nasa-launches-new-mission-to-monitor-earth-s-landscapes

8) "Landsat 9: Payloads stacked atop Atlas V for launch," ULA, 16 September 2021, URL: https://blog.ulalaunch.com/blog/landsat-9-payloads-stacked-atop-atlas-v-for-launch
 


The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).

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