The L'TES Instrument

Jan 14, 2021

L’TES is Lucy’s Thermal Emission Spectrometer, which detects far infrared radiation emitted by the asteroids.


Mass: 7.7 kg
Power: 17.6 W
FOV: 10 mrad
Spectral Range: 6-75 μm
Spectral Resolution: 10 cm-1

Though the Trojan asteroids are many hundreds of millions of kilometers from the Sun, sunlight still heats them up, causing them to emit far infrared radiation. L’TES detects this radiation, using a telescope with diameter 15.2 cm (5.98 in) to focus the incoming energy onto a small detector. In this way, L’TES acts like a remote thermometer. L’TES is technically not an imager, though it can take temperature measurements at various points on an asteroid, which can be combined to produce a “picture” of the surface properties. While Lucy’s other spectrometer (LEISA on L’Ralph) will examine the asteroids’ surface composition, L’TES will examine the physical properties of the regolith by measuring thermal inertia. Thermal inertia is a measure of how slowly an object heats up or releases heat. Smaller particles have low thermal inertia; for instance, sand on a beach heats up quickly during the day and cools down quickly at night. Larger particles have high thermal inertia; compared to the sand, a sidewalk heats up slowly during the day and cools down slowly at night. By measuring the temperature at different times of day on the asteroid, the science team can measure the thermal inertia and thus deduce how much dust, sand, or rock is present in the regolith. L’TES could also detect any differences in thermal inertia present on a single asteroid, something earthbound telescopes would not be able to detect.

Because L’TES is a point spectrometer and not an imager, it produces a substantially smaller amount of data during a typical encounter than the other instruments. (Its comparative simplicity has earned it the nickname “The Little Instrument that Could” from Victoria Hamilton, the Instrument Deputy Principal Investigator.) Depending on the settings, L’TES can produce a spectrum every 0.5, 1, or 2 seconds.

Animated gif of Bennu's surface temperature
A map of asteroid Bennu’s surface temperatures using data from OSIRIS-REx’s OTES and OVIRS spectrometers. Credit: NASA/Goddard/University of Arizona/The Open University/University of Tennessee.

L’TES is built by a team at Arizona State University, under the leadership of Phillip Christensen (Instrument Principal Investigator) and Victoria Hamilton (Instrument Deputy Principal Investigator). The instrument is based on OSIRIS-REx’s OTES (OSIRIS-REx Thermal Emission Spectrometer). L’TES retains most of the hardware from its predecessor, though the electronics were replaced with the electronics design from EMIRS (Emirates Mars Infrared Spectrometer), an instrument on the Emirates Mars Mission. Like its predecessor, inside L’TES is a large diamond that serves as a beamsplitter. (Perhaps a reference to a certain Beatles song is appropriate here.)