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Astronomy

Thermal-IR imaging of a near-Earth asteroid

SpaceWire technology has been used in building a real-time imaging system for the Hayabusa2 mission.
17 June 2013, SPIE Newsroom. DOI: 10.1117/2.1201306.004931

The Japan Aerospace Exploration Agency/Institute of Space and Astronautical Science Hayabusa2 mission (see Figure 1) is due to be launched in 2014 with the aim of returning a sample from the C-type (rich in carbon) near-Earth asteroid 1999JU3, which is thought to contain organic or hydrated materials.1 A thermal emission map of the asteroid's surface can be derived from regional variations in measurements of thermal inertia. These measurements provide information about the physical properties of the surface (i.e., whether sand, pebble, or boulder-sized materials are present) and will be used to identify a suitable landing site.

Hayabusa2 is a follow-up to the Hayabusa probe that performed the first round trip, with sample return, to an asteroid in 2010. The Hayabusa2 spacecraft design builds on the heritage of Hayabusa, but includes several refinements. Although the primary goal of the mission is to return a sample of the asteroid, remote sensing of the body is also an important aspect. The physical properties and composition of the uppermost layer of the asteroid will be investigated in an active impact experiment where the small carry-on impactor instrument will form a crater (a few to several meters in diameter) and excavate parts of the subsurface. The crater and surrounding ejecta will be studied with an optimized set of remote sensing instruments that include multi-band telescopic and wide-angle imagers, a laser ranger (lidar), a near-IR spectrometer (to detect an absorption band at 3μm characteristic of water ice or hydrated minerals), and a thermal-IR imager (TIR). With these sensors, the mid-IR thermal emission and surface temperature profiles of the asteroid will be acquired, as well as their temporal variation with the asteroid's rotation.

Our TIR instrument is based on the design of the long-wavelength IR imager that was built for the Venus climate mission Akatsuki.2, 3 It has been adapted to meet the Hayabusa2 requirements and includes a non-cooled NEC 320A bolometer (which measures the power of incident electromagnetic radiation) array with 320 × 240 effective pixels. The scientific objectives of the TIR are given in Table 1 and its technical features in Table 2. The absolute temperature of the instrument will be measured and calibrated by taking several images with the instrument shutter closed. Temporal variation in the characteristics of the asteroid's surface units will be identified by making continuous interlaced measurements of the asteroid as it rotates, while the shutter is open and closed. The thermal emission data from our instrument will be a vast improvement on that from Hayabusa, where the asteroid's thermal emission was obtained through thermal radiometry using the temperature profile of the x-ray spectrometer's radiator.


Figure 1. An artist's impression of the Hayabusa2 probe's encounter with a near-Earth asteroid. (© Akihiro Ikeshita.)

We have incorporated an onboard automatic data processing function and a high-speed data recording capability into the TIR instrument. These digital electronics are necessary to deal with the round-trip communication time between the asteroid and Earth, and to exploit the limited data downlink capacity (32kb/s). Image operations, such as summation of multiple images, dark image subtraction, and compensation of dead pixels are processed onboard.

Table 1.Main science objectives of the thermal-IR (TIR) instrument on Hayabusa2, including the features to be observed and the appropriate spacecraft position. SCI: Small carry-on impactor. TD: Touchdown (<0.1km). HP: Home position (10–20km). LA: Low altitude (1–5km). CU: Close-up (0.1–1km).
Observation featureCharacterization objectivePosition
Boulders Parent body interior materials LA, CU, HP
Crater walls Interior materials and structures LA, HP
Regolith Sedimentary processes at microgravity HP, LA, CU
Overall features Comparisons with ground-based observations; heterogeneity from surface to interior HP, LA
Yarkovsky effect Evidence of trajectory and rotation change by thermal effect HP
Phase function Updating TIR emission phase functions HP
Satellites Orbiting satellite for gravity measurement HP
Shape Asteroid shape imaged day and night HP
Geology Ejecta, sediments, ponds, buried rocks LA, HP
SCI crater Difference of surface and interior LA, CU, TD
TD site Touchdown site description CU, TD
Table 2.Technical characteristics of the TIR instrument. MTF: Modulation transfer function. NETD: Noise-equivalent temperature difference.
Mass3.3kg
Power 22W (nominal)
Detector Non-cooled bolometer
Pixels (effective) 320×240
Field of view ±8°×±6°
Instantaneous field of view 0.877mrad
MTF (at the Nyquist frequency) >0.3
Temperature range 250–400K
NETD <0.5K (at 350K)
Absolute temperature resolution
Analog-to-digital converter 12bit
Data 0.15MB/image
Temperature calibration Shutter open/close

SpaceWire is a spacecraft communication network that is used as a standard for high-speed links onboard spacecraft to ease interconnection between sensor, memory, processing, and telemetry subsystems. We use SpaceWire to connect a processing module to the sensor interfaces, which allows us to accurately determine processing times on the TIR. To reduce storage capacity and operation time restrictions, we use SpaceWire with a dedicated compression algorithm, StarPixel, to achieve data compression. On average, TIR images can be compressed in a 30th of the processing time of a JPEG2000 image, with the same compression ratio and image quality.

The modularity of SpaceWire enables us to use units that have been developed for previous projects and that do not require additional modifications. This helps us to work on the short development timescale of Hayabusa2. The digital electronics comply with the European Space Agency's SpaceWire-D draft standard and can achieve deterministic signal processing. This allows images to be acquired simultaneously with the autonomous operation of the attitude control subsystem during the touchdown sequence of the spacecraft onto the asteroid's surface.

The results of the TIR development phase will be reported in the forthcoming formal release of the ECSS SpaceWire standard. The TIR will be used to investigate the processes that led to the formation of the asteroid. The data will also be used to distinguish whether the asteroid is a rubble pile of accreted impact fragments of a larger parent body, or whether it has a more primordial unconsolidated structure.


Hisashi Otake, Tatsuaki Okada
Institute of Space and Astronautical Science
Japan Aerospace Exploration Agency
Sagamihara, Japan
Ryu Funase
Department of Aeronautics and Astronautics
The University of Tokyo
Tokyo, Japan
Hiroki Hihara, Ryoichi Kashikawa, Isamu Higashino
NEC TOSHIBA Space Systems Ltd.
Tokyo, Japan
Tetsuya Masuda
NEC Corporation
Tokyo, Japan

References:
1. T. Okada, T. Fukuhara, S. Tanaka, M. Taguchi, R. Nakamura, T. Sekiguchi, S. Hasegawa, Thermal-infrared imager TIR on HAYABUSA-2 to investigate physical properties of C-class near-Earth asteroid 1999JU3, Lunar Planet. Sci. Conf. 42, p. 1498, 2012.
2. T. Fukuhara, M. Taguchi, T. Imamura, M. Nakamura, M. Ueno, M. Suzuki, N. Iwagami, LIR: Longwave Infrared Camera onboard the Venus orbiter Akatsuki, Earth Planets Space 63, p. 1009-1018, 2011.
3. M. Taguchi, T. Fukuhara, M. Futaguchi, M. Sato, T. Imamura, K. Mitsuyama, M. Nakamura, Characteristic features in Venus' nightside cloud-top temperature obtained by Akatsuki/LIR, Icarus 219, p. 502-504, 2012.