The Thermal Infrared Imager on Hayabusa2
More Than Just a Camera: The Power of Heat Vision
When the Japanese spacecraft Hayabusa2 reached the asteroid Ryugu in June 2018, it wasn't just taking pictures of a dark, rocky space diamond. It was also feeling its warmth. Onboard was a special instrument known as the Thermal Infrared Imager (TIR), a camera designed not to see light, but to see heat 1 .
This revolutionary tool acted as Hayabusa2's thermal vision, allowing scientists to take the temperature of the asteroid from orbit and uncover the surprising, porous nature of this primitive celestial body.
TIR's findings were later confirmed in a stunning feat of space exploration when Hayabusa2 returned samples of Ryugu to Earth. The direct laboratory analysis revealed that the asteroid is made of incredibly dark and fragile material, with a density much lower than any meteorite found on Earth 2 . This article explores how the TIR instrument served as a crucial remote-sensing detective, piecing together the thermophysical clues of an asteroid that holds secrets to the early solar system.
To understand TIR's achievements, we must first understand what it was measuring: thermal inertia. This property tells scientists how quickly a surface heats up in sunlight and cools down in darkness.
A surface with low thermal inertia, like fine sand on a beach, experiences extreme temperature swings. It heats up rapidly at noon but cools down quickly at night.
A surface with high thermal inertia, like a solid boulder, changes temperature more slowly. It takes longer to warm up and longer to cool down.
By mapping these temperature variations, TIR could infer the physical nature of Ryugu's surface—whether it was covered in fine dust, compact rocks, or something in between . This was vital for both science and safety, helping the mission team select a suitable landing site for sample collection.
The TIR was a sophisticated piece of technology tailored for the challenges of observing Ryugu.
As an uncooled micro-bolometer array, it operated without the need for complex cooling systems, making it robust and reliable 1 . It captured thermal radiation in the 8 to 12 micrometer wavelength range, ideal for measuring the typical surface temperatures of Ryugu, which it could accurately record from about 150 K to 460 K 5 .
With a field of view of 16.7° by 12.7° and a resolution of 328 x 248 pixels, TIR could capture wide-area thermal maps of the asteroid from Hayabusa2's home position about 20 km away 1 . Its high sensitivity allowed it to detect temperature differences as small as 0.4 K, enabling it to spot subtle variations in surface material .
| Parameter | Specification | Significance |
|---|---|---|
| Detector Type | Uncooled micro-bolometer array | Reliable and does not require complex cryogenic cooling |
| Spectral Range | 8 - 12 μm | Optimal window for measuring planetary surface temperatures 1 5 |
| Temperature Range | ~150 K to 460 K | Covers the expected temperature variations on Ryugu 1 |
| Field of View | 16.7° x 12.7° | Allows for efficient wide-area mapping of the asteroid from orbit 1 |
| Spatial Resolution | 0.051° per pixel | Resolves fine surface details and temperature variations 1 |
| Temperature Accuracy | < 2 K (absolute), < 0.4 K (relative) | Provides highly reliable and precise temperature measurements |
One of TIR's primary missions was to create a global map of Ryugu's thermal inertia. This was a complex operation that involved meticulous observation and data processing.
From its orbital position, TIR continuously took thermal images of Ryugu throughout its 7.6-hour rotation period. This provided a complete dataset of how different areas of the asteroid heated and cooled over time .
A significant challenge was accurately projecting the two-dimensional thermal images onto a three-dimensional shape model of the asteroid. Scientists developed sophisticated software to correct for spacecraft pointing fluctuations, ensuring each temperature pixel was correctly mapped to a specific location on Ryugu's surface 7 .
The observed temperature profiles were then compared with theoretical predictions from a thermophysical model (TPM). Scientists ran the model with different values of thermal inertia and surface roughness, searching for the combination that best matched the actual temperature data recorded by TIR 7 .
The results were revealing. TIR data showed that Ryugu's surface temperatures were remarkably homogeneous across the sunlit side, a key clue that pointed to a surface covered in highly porous material 7 . The estimated average thermal inertia was about 300 ± 100 J m⁻² K⁻¹ s⁻⁰·⁵ 7 , which is relatively low.
This low thermal inertia was the smoking gun. It indicated that Ryugu's surface is not made of solid rock, but is instead composed of porous, fragmented debris, similar to a rubble pile. The heat from the Sun could not easily penetrate this material, causing it to heat up and cool down rapidly—just like fine sand . This led to the conclusion that Ryugu has a high microporosity, meaning the rocks and boulders themselves are filled with tiny pores and spaces.
| Thermal Inertia (J m⁻² K⁻¹ s⁻⁰·⁵) | Typical Surface Material | Implied Physical State |
|---|---|---|
| < 50 | Fine dust, loose regolith | Very fluffy, unconsolidated grains |
| 50 - 500 | Sand, porous rubble | Fragmented, porous materials (similar to Ryugu) |
| 500 - 1000 | Coarse sand, cemented regolith | More compacted, less porous |
| > 2000 | Solid rock, dense bedrock | Consolidated, low-porosity rock |
The true test of TIR's remote-sensing analysis came in December 2020, when Hayabusa2's capsule containing precious grains from Ryugu landed in the Australian outback. Laboratory analysis of these samples provided the "ground truth" that spectacularly validated the orbital data 2 .
Scientists measured the bulk density of the returned particles and found it to be only 1,282 ± 231 kg m⁻³, significantly lower than any known carbonaceous meteorite 2 . When compared to the grain density of CI chondrites (a type of meteorite similar to Ryugu), this low bulk density implies a microporosity of about 46% 2 . This means nearly half of the volume of these millimeter-sized particles is empty space, confirming TIR's inference of an exceptionally porous asteroid.
| Property | Ryugu Returned Samples | CI Chondrites (e.g., Orgueil) | Tagish Lake Meteorite |
|---|---|---|---|
| Average Bulk Density (kg m⁻³) | 1,282 ± 231 2 | ~2,110 2 | ~1,660 2 |
| Estimated Porosity | ~46% 2 | Lower | ~40% (most porous meteorite) 2 |
| Albedo (Visual Brightness) | Very dark (~0.02) 2 | Dark, but brighter than Ryugu | Dark |
| Physical Nature | Extremely fragile, high microporosity | More consolidated | Fragile, but less so than Ryugu |
TIR did not work in isolation. It was part of a powerful suite of instruments on Hayabusa2 that worked in concert to build a comprehensive picture of Ryugu 3 6 .
Measure surface temperature and thermal inertia 8
Role: Determine physical properties like porosity and rock abundance.
Capture high-resolution visible-light images 3
Role: Navigate, map topography, and study geological features.
Measure distance to the asteroid surface 3
Role: Create precise topographic maps and shape models.
The story of the Thermal Infrared Imager on Hayabusa2 is a testament to the power of remote sensing. By meticulously recording the heat emissions from a distant asteroid, TIR allowed scientists to correctly deduce that Ryugu is a fragile, porous rubble-pile asteroid, likely formed from the accumulation of debris after a catastrophic disruption of a larger parent body.
The subsequent laboratory analysis of the returned samples served as the ultimate confirmation, proving that TIR's readings were accurate. This synergy between remote sensing and sample return has provided an unprecedented look at a primitive building block of our solar system. The success of TIR not only advances our knowledge of asteroids but also paves the way for future missions to unravel the mysteries of other small celestial bodies.