applied sciences

Article Thermography of and Future Applications in Space Missions

Tatsuaki Okada

Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo, Sagamihara 252-5210, Japan; [email protected]; Tel.: +81-50-3362-2471

 Received: 31 January 2020; Accepted: 19 March 2020; Published: 22 March 2020 

Featured Application: Activities in lunar, planetary, and asteroid science and exploration.

Abstract: The Near-Earth Asteroid is a C-type asteroid which preserves information about the ancient Solar System and is considered enriched in volatiles such as water and organics associated with the building blocks of life, and it is a potentially hazardous object that might impact Earth. is the asteroid explorer organized by the Japan Aerospace Exploration Agency to rendezvous with the asteroid and collect surface materials to return them to Earth. Thermography has been carried out from Hayabusa2 during the asteroid proximity phase, to unveil the thermophysical properties of the primitive Solar System small body, which offered a new insight for understanding the origin and evolution of the Solar System, and demonstrated the technology for future applications in space missions. Global, local, and close-up thermal images taken from various distances from the asteroid strongly contributed to the mission success, including suitable landing site selection, safe assessment during descents into the thermal environments and hazardous boulder abundance, and the detection of deployable devices against the sunlit asteroid surface. Potential applications of thermography in future planetary missions are introduced.

Keywords: thermography; uncooled micro-bolometer array; asteroid; planetary exploration; thermal inertia

1. Introduction The Thermal Infrared Imager TIR [1] (see Figure1, Table1) is a remote sensing instrument on Hayabusa2, the Japanese second sample return mission from the asteroid [2], which is based on a two-dimensional uncooled micro-bolometer array with 328 248 effective pixels, with 16.7 12.7 × ◦ × ◦ field of view, and covering 8–12 µm wavelength range using a germanium lens and a band pass filter [1]. The absence of a cooling system in the TIR contributes to its small, lightweight, and long-living design. The basic design of the sensor unit (TIR-S), the digital electronics (DE), and the onboard data processing algorithm is inherited from the Longwave Infrared Camera from the Akatsuki Venus mission [3], but the wide detection range of temperature, observation sequences and plans, programming of data processing, and calibration methods are optimized for the TIR [4]. As with a deep-space mission, the maximum downlink rate is limited to within 32 Kbps, and onboard processing is essential for its design [1,2]. are primitive bodies of the Solar System and probably the parent bodies of meteorites. Planetary formation started with fluffy dust in the solar nebula, followed by accumulation of dust to planetesimals, or re-accretion to rubble-piles from fragments by impacts. The formation and evolution processes will be indicated by the surface physical state of asteroids or planets, such as grain size, porosity, boulder abundance, and roughness, which are informed by thermal infrared observations. Thermophysical models of asteroids [5] have been investigated and constructed for

Appl. Sci. 2020, 10, 2158; doi:10.3390/app10062158 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 2158 2 of 13 Appl. Sci. 2018, 8, x FOR PEER REVIEW 2 of 13 interpreting thosethose groundground observations observations in in thermal thermal infrared infrared wavelengths, wavelength buts, but more more detailed detailed models models are neededare needed for quantitativefor quantitative analysis analysis using using data data from from a spacecraft a spacecraft [1,4 ].[1,4].

Figure 1. The flight flight model of the sensor unit of the thermal infrared imager TIR onboardonboard Hayabusa2 (TIR-S).(TIR-S). WhiteWhite squaressquares on on the the sensor sensor case case are are the the Velcro Velcro tapes tapes to attach to attach the multi-layer the multi-layer insulators insulators (MLI, the(MLI, thermal the thermal blankets, blanket not shown)s, not shown which) which cover thecover sensor the unitsensor to unit stabilize to stabilize the temperature the temperature during theduring operation. the operation A baffl. edA baffled sunshade sunshade (not shown) (not shown) is attached is attached in front in of front the germaniumof the germa lensnium to lens avoid to insolation.avoid insolation The aperture. The aperture of the lensof the is 42lens mm is in42 diametermm in diameter (diameter (diameter of lens is of 47 lens mm), is and47 mm) the, stepping and the motorstepping for motor the mechanical for the mechanical shutter is shutter seen at is the seen right at the hand right of thehand lens. of the The lens. sheet The heater sheet around heater thearound lens casethe lens is to case adjust is to the adjust temperature the temperature of the optics. of the Theoptics. scale The on scale the nameon the platename is plate 10 cm is long10 cm (1 long cm each).(1 cm (each).©Japan (©J Aerospaceapan Aerospace Exploration Exploration Agency Agency (JAXA)). (JAXA)). Table 1. Characteristic performance of TIR on Hayabusa2 [1]. Table 1. Characteristic performance of TIR on Hayabusa2 [1]. Items Performance Items Performance TotalTotal Mass Mass 3.28 3.28 kg kg(DE (DE is isnot not included) included) Total Power 18 W (nominal) (DE is not included) Total Power 18 W (nominal) (DE is not included) Uncooled bolometer array NEC 320A (VO) Detector Uncooled bolometer array NEC 320A (VO) Detector (anti-reflection coating) Pixels 344(anti-reflection260 (effective coating) 328 248) × × FieldPixels of View (FOV)344 × 260 (effective 16.7 12.7 328 x 248) ◦ × ◦ Field of ViewIFOV (FOV) 0.8916.7° mrad × 12.7° (0.051 ◦) IFOVAperture 42 mm (e0.89ffective), mrad lens (0.051°) diameter is 47 mm MTFAperture (@Nyquist Freq.)42 mm (effective), lens 0.5 diameter is 47 mm F-number 1.4 MTF (@Nyquist Freq.) 0.5 233–423 K (well calibrated), Temperature range F-number 150–460 K (detectable1.4 range) NETD 233–423 K (well<0.3 calibrated), Temperature range Absolute Temperature Range 150–460 K (detectable<3 K range) ANETD/D Converter 12 bit (15< bit 0.3 after summed) AbsoluteReference Temperature Temperature Range Shutter temperature (monitored,< 3K nominally at 301 K) Frame Rate 60 Hz A/D Converter 12 bit (15 bit after summed) Image Summation 2ˆm, m = 0 to 7 (1,2, ... ,128) ReferenceConsecutive Temperature shot numbers Shutter Programable temperature (max 128(monitored, images), currentnominally programed: at 301 K) 30 Frame Rate 60 Hz Image Summation 2^m, m = 0 to 7 (1,2, ... ,128) RemoteConsecutive thermal shot radiometry numbers is an often-usedProgramable technique (max in128 planetary images), missionscurrent programed: [6–11] to investigate 30 the physical state of a planetary surface layer, especially in its particle size distribution of regolith p and boulderRemote abundance,thermal radiometry which are is derived an often from-used thermal technique inertia, in planetaryρkcp, with missions density ρ [6, thermal–11] to conductivityinvestigate thek, physical and heat state capacity of a planetarycp. Thermal surface inertia layer, is an especially index of in thermophysical its particle size propertiesdistribution that of regolith and boulder abundance, which are derived from thermal inertia, √휌푘푐푝, with density 휌, Appl. Sci. 2020, 10, 2158 3 of 13 tend to have lower thermal inertia for more porous material, while higher thermal inertia for the 2 0.5 1 denser material. Typical examples show that for a low thermal inertia below 50 J m− s− K− (tiu, hereafter), the surface is composed of fine-grained materials such as lunar regolith, while for a high thermal inertia beyond 2000 tiu, the surface consists of non-porous dense rocks [1]. On the other hand, carbonaceous chondrite meteorites that are primitive and enriched in volatiles are likely analogues to C-type asteroids [12,13], which show an intermediate thermal inertia between 600 to 1000 tiu [14]. Thermal inertia can be derived from thermal measurements, since diurnal temperature profiles show a large peak-to-peak temperature difference for the case of lower thermal inertia, while a temperature change becomes much smaller and the peak temperature is delayed from the local noon for the case of higher thermal inertia. For the , Mars, large asteroids, and satellites, the surface is covered with fine-grained or graveled regolith that is ejected by hypervelocity meteoritic impacts and sedimented on the surface. However, an in-depth study is required for small bodies since their surface condition has been poorly understood [15] since most of the ejecta could be escaped and the basement might be exposed due to low gravity. Hayabusa2 is the first mission to investigate the surface physical state of a small body through thermography [4]. In past planetary missions, thermal radiometry has been mainly conducted along the track of the orbiting spacecraft from a low circular orbit. However, thermal imaging is requested for Hayabusa2 because the spacecraft was not orbiting the asteroid but staying at a distant stational position, so that the whole surface was imaged by asteroid rotation to determine the temperature distribution and its diurnal temperature profiles of each geologic site. In Section2, the Hayabusa2 and the asteroid Ryugu are introduced in more detail. In Section3, the Hayabusa2 results during the approaching phase are displayed. In Section4, the Hayabusa2 results during the proximity phase are shown, and in Section5, the technical applications in the future missions is introduced and discussed, and in Section6, the concluding remarks of this paper are described.

2. Hayabusa2 and Asteroid 162173 Ryugu Hayabusa2 [2] is the second asteroid mission organized by Japan Aerospace Exploration Agency (JAXA), inherited from the Hayabusa mission [16] which visited and returned samples from S-type asteroid 25143 Itokawa. Hayabusa2 is to explore a C-type asteroid, which is considered to be a parent body of volatile-rich carbonaceous chondrite meteorites and a key target to investigate the origin and evolution of the early solar system and the possible source materials of building blocks of life [17]. Ground-based and space-based observations [18] predicted that Ryugu is classified as a C-type asteroid in taxonomy, of rounded shape with a diameter of about 0.9 km, and with rotation in 7.63 h. It is basically dark material with geometric albedo below 0.045, and possibly covered with coarse regolith of centimeter-sized granules with average thermal inertia of 150 to 300 tiu. Before the arrival of the Hayabusa2 spacecraft at Ryugu, the asteroid was considered to be a desiccated carbonaceous chondrite meteorite after thermally and aqueously alteration, and the surface might be sedimented with coarse-grained regolith materials and dense boulders, as well as excavated by impact craters where the materials from the interior might be excavated and exposed. Hayabusa2 was launched on 3 December 2014 and arrived at the home position, 20 km earthward from Ryugu on 27 June 2018, and started remote sensing [19] using the optical navigation camera (ONC) [20], the laser altimeter (LIDAR) [21], the near infrared spectrometer (NIRS3) [22], and the TIR [1,4] to characterize the asteroid regarding its shape, spin state, geomorphology, spectral and thermal properties, geologic features, and gravity. The state and characteristics of Ryugu before and after arrival was basically consistent, but the asteroid shape is double top-shaped, and the surface is not covered with regolith but with boulders, and no flat and smooth terrains larger than 10 m scale were found for easy and safe landing [4,19–22]. Appl. Sci. 2020, 10, 2158 4 of 13

3. TIR Observations of Ryugu in Cruise and Approach Phases Appl.During Sci. 2018 a, 8 3.5-years, x FOR PEER long REVIEW cruise phase before arrival at Ryugu, images of deep sky backgrounds4 of 13 were taken several times per year as the dark frames for calibration, and the degradation of TIR was During a 3.5-years long cruise phase before arrival at Ryugu, images of deep sky backgrounds checkedwere taken by comparing several times the per temporal year as changesthe dark offrames its bias for levelscalibration and the, and standard the degradation deviations of T asIR wellwas as thechecked number by of comparing bad pixels the [4,23 temporal]. No substantial changes of degradation its bias levels has and been the foundstandard for deviations five years afteras well launch, as sothe that number the radiation of bad pixels tolerance [4,23 of]. No the substantial TIR against degradation the deep-space has been environment found for five has years been after proven. launch, so Hayabusa2that the radiation passed tolerance by Earth of forthe theTIR gravityagainst the assist deep to- changespace environment its trajectory has en been route proven to Ryugu. on 3 DecemberHayabusa2 2015. passed From 14by OctoberEarth for 2015,the gravity the spacecraft assist to change changed its itstrajectory attitude en to route point to EarthRyugu and on 3 the MoonDecember to observe 2015. themFrom with14 October the remote 2015, sensingthe spacecraft instruments changed until its attitude 21 December to point 2015. Earth During and the this period,Moon TIR to hasobserve taken them images with of the Earth remote and thesensing Moon instruments from the various until 21 distances, December so that2015. the During dependency this ofperiod, distance TIR on thermal has taken radiation images intensities of Earth and has been the Moon investigated from the [23 various]. The critical distances, distance so that above the the thresholddependency level of for distance detecting on Earth thermal and theradiation Moon intensities were obtained, has been which investigated was applied [23 to]. the The estimate critical of thedistance critical distanceabove the ( threshold1.5 104 km) level or for the detecting critical sizeEarth of anddiameter the Moon ( 0.06 were mrad) obtained, at 1 au whichfrom the was Sun − × − forapplied detecting to the Ryugu estimate by TIR of beforethe critical arrival distance [23]. (−1.5 × 104 km) or the critical size of diameter (−0.06 mrad)The at first 1 au light from event the Sun of for TIR detect foring the Ry observationugu by TIR ofbefore Ryugu arrival was [23 conducted]. on 6 June 2018 at 2.5 10The3 km first from light the event asteroid. of TIR for Ryugu the observation was successfully of Ryugu detected was conducted as was predictedon 6 June 2018 [23]. at A 2.5 series × ×3 of10 one-shot km from imaging the asteroid. of Ryugu Ryugu by was TIR successfully continued almostdetected once as was a day predicted until arrival[23]. A atseries the of asteroid one-shot (see Figureimaging2, Table of Ryugu2) and by a relationshipTIR continued of almost thermal once radiation a day until intensity arrival toat the distance asteroid from (see the Figure asteroid 2, Table was 2) and a relationship of thermal radiation intensity to distance from the asteroid was obtained, as well obtained, as well as to distance from the Sun and the solar phase angle (nearly Sun–Probe–Earth angle). as to distance from the Sun and the solar phase angle (nearly Sun–Probe–Earth angle). It was It was important that Ryugu was the only bright spot detected in the thermal images after subtraction of important that Ryugu was the only bright spot detected in the thermal images after subtraction of dark frames, which is different from optical images where numerous bright spots other than the target dark frames, which is different from optical images where numerous bright spots other than the body were found such as bright background stars, bad pixels, or by irradiation. In addition, thermal target body were found such as bright background stars, bad pixels, or by irradiation. In addition, imagesthermal by images TIR are by never TIR are saturated never saturated by overexposure by overexposure for most for planetary most planetary surfaces surfaces (< 460 (< K).460 TheseK). featuresThese havefeatures an have advantage an advantage of using of thermal using thermal images images for the for survey the survey of orbiting of orbiting satellites satellites around around Ryugu orRyugu possible or ejection possible of ejection materials of from materials Ryugu from if they Ryugu were if largerthey were than larger the critical than sizethe critical of diameter size of [23 ]. Suchdiameter satellites [23]. or Such ejection satellites have or not ejection beendetected have not throughbeen detected the asteroid through proximity the asteroid phase, proximity although phase, some ejectedalthough flows some were ejected observed flows on were the observed asteroid Bennuon the asteroid by the OSIRIS-Rex Bennu by the mission OSIRIS [24-Rex]. mission [24].

2018/06/10 2018/06/11 2018/06/12 2018/06/13

2018/06/15 2018/06/16 2018/06/17 2018/06/18

2018/06/19 2018/06/20 2018/06/22 2018/06/29

Figure 2. Thermal images of asteroid 162173 Ryugu taken by TIR during the Approach Phase (©JAXA). Figure 2. Thermal images of asteroid 162173 Ryugu taken by TIR during the Approach Phase (© JAXA). Appl. Sci. 2020, 10, 2158 5 of 13

Table 2. Daily observations of asteroid Ryugu by TIR during the Approach Phase.

Date of Observation Distance from Sun [au] Sun–Probe–Earth Angle [deg] Distance from Ryugu [km] 10 June 2018 0.9650 17.02 1550 11 June 2018 0.9655 17.09 1380 12 June 2018 0.9661 17.20 1150 13 June 2018 0.9667 17.28 1000 15 June 2018 0.9681 17.45 600 16 June 2018 0.9689 17.53 500 17 June 2018 0.9698 17.63 350 18 June 2018 0.9707 17.71 200 19 June 2018 0.9716 17.78 170 20 June 2018 0.9727 17.87 100 22 June 2018 0.9749 18.01 49 29 June 2018 0.9843 18.48 20

4. TIR Observations in Asteroid Proximity Phase The Asteroid Proximity phase started on 28 June 2018 at the home position for almost 1.5 years, and Hayabusa2 kept its position there during the phase. The operations were categorized into three types: (1) Keeping the position within safety boxed areas, for the global and oblique angle observations; (2) Controlled descent operations down to 1 km altitude, for high-resolution local thermal imaging; − and (3) Descent operations for sampling or lander release, for close-up thermal imaging.

4.1. Global Thermal Images of Ryugu The first global disc-resolved thermal image set of an asteroid in history was taken by TIR for one asteroid rotation on 30 June 2018, just after arrival at the asteroid. The surface temperature in the daytime was around 300 to 370 K at a solar distance of 0.986 au, at the Sun–Probe–Earth (SPE) angle of 18.6◦. Comparison with calculated thermal images [25,26] shows that the surface of Ryugu is colder than in the case of fine regolith surface where the thermal inertia is about 50 tiu and hotter than in the case of base rocks whose typical thermal inertia is higher than 1000 tiu [4,20]. It was interesting that the temperatures of large boulders with several tens of meters or larger size on Ryugu are almost equal to those of the surrounding surfaces [4]. This was inconsistent with the original idea before arrival, where the large boulders should have high thermal inertia larger than 1000 tiu and looks like “cold spots”, i.e., much colder than the surrounding [4]. It was also found that the diurnal temperature profiles of the surface of Ryugu are rather constant compared with the anticipated profiles for a simple thermal calculation where a one-layer homogeneous material and a flat surface without roughness being assumed [4]. The thermal inertia of most geologic sites on Ryugu ranged around 300 100 tiu [4,26], which was derived from the maximum temperature ± and its delay from the local noon. This implies that the surface physical state of Ryugu is consistent with very porous and rough surface, where boulders have high porosity (30% to 50 %) and the rest of the surrounding surface is dominated by fragments of porous rocks several centimeters larger than the thermal skin depth [4]. It is important that the surface physical state predicted from the global thermal images was confirmed later by the close-up thermal imaging by TIR and also by the optical surface imaging and thermal radiation measurements from the surface lander Mobile Asteroid Surface Scout (MASCOT) [27,28]. Most thermal images are taken from the low SPE angles but there are large solar phase angle observation campaigns to view the asteroid from the oblique direction. Figure3 shows an example of the oblique thermal image of Ryugu taken by TIR at SPE angle of 42◦, at rotation phase of 105◦E, where the largest crater Urashima was found at equator of the asteroid (center-left). The flat diurnal temperature profiles and the temperature gradient at the subsolar to the night side should be well investigated since they are influenced by the roughness effect [26]. Appl. Sci. 2020, 10, 2158 6 of 13 Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 13

Urashima Crater

Otohime Saxum

Figure 3. Pseudo-colorPseudo-color obliqueoblique thermal thermal image image of of Ryugu Ryugu on on 31 August31 August 2018 2018 taken taken by TIR by fromTIR from the dusk the duskside (SPE side angle(SPE angle of 42◦ ).of The42°). largest The largest crater crater Urashima Urashima (diameter (diameter of 270 m)of 270 is seen m) atis seen the equator, at the equator, and the andlargest the boulder largest Otohimeboulder Otohime is seen at is the seen south at the pole. south The pole. night The region night shown region as theshown blue as color the areablue incolor the arearight end in of the the right asteroid end is also of seen the in asteroid the thermal is image. also seen (Image: in hyb2_tir_20180831_130142_l1.fit, the thermal image. (Image: ©hyb2_tir_20180831_JAXA). 130142_l1.fit, © JAXA).

4.2. High Resolved Global and Local Thermal Images of Ryugu 4.2. High Resolved Global and Local Thermal Images of Ryugu HigherHigher-resolution-resolution globalglobal and and local local thermal thermal images images of Ryugu of Ryugu were were taken taken by TIR by from TIR 5 kmfrom altitude 5 km altitudeseveral times several during times the during asteroid the asteroid proximity proximity phase [4, phase26], and [4,2 on6], the and way on tothe the way lower to the altitude lower downaltitude to 1down km altitude to 1 km foraltitude the gravity for the measurement gravity measurement campaign campaign on 6 to 7 Auguston 6 to 7 2018, August and 2018, during and the during hovering the hoveringoperation ope at 3ration km altitude at 3 km after altitude the release after the of release MASCOT of MASCOT lander on lander 3 to 4 October on 3 to 4 2018 October [4]. 2018 [4]. A high-resolutionhigh-resolution global thermal image set of Ryugu was taken on 1 August 2018 during the MidMid-Altitude-Altitude Observation Campaign, from the altitude of 5 km, at the solar dis distancetance of 1.056 au, and from the SPE angle of 19.019.0°.◦. Figure Figure 44 showsshows aa thermalthermal imageimage atat rotationrotation phasephase ofof 120120◦°E.E. Its spatial resolution is 4.5 m per per pixel. pixel. In In this this image, image, surface surface geologic geologic features features such such as large boulders are distinguished from the surroundings more clearly than in the imagesimages atat thethe homehome position.position. LargeLarge boulders of tens of meters scale are clearly identified identified one by one in the thermal imagesimages and their temperatures are almost the same as the surroundings.surroundings. Despite temperature variation for each of the boulders, craters, or other geologic features, no “hot spots” nor “cold spots” spots” are are apparently apparently identified identified in the thermal images of this spatial resolution. Thermal images images show show almost almost homogeneous homogeneous temperature temperat distributionure distribution on a boulderon a boulder whose surface whose surfacehas both has an both apparently an apparently sedimented sedimented area and area an and apparently an apparently non-sedimented non-sedimented bare rockbare surfacerock surface area, areaindicating, indicating that the that sediments the sediments have have the same the same thermal thermal inertia inertia with with the surroundings. the surroundings This. T implieshis implies that thatthe sediments the sediments should should not be not fine be regolith fine regolith made made of soils of and soils granules and granules but probably but probably fragments fragments of porous of porousboulders boulders with the with same the thermal same thermal inertia inertia as the as huge the huge boulders, boulders whose, whose size mustsize must be larger be larger than than the thethermal thermal skin skin depth. depth [20., 26[20,2]. 6]. Appl. Sci. 2020, 10, 2158 7 of 13 Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 13

Equatorial Urashima Ridge Crater

Otohime Saxum

Figure 4. Thermal image of Ryugu taken by TIR on 1 August 2018 from the Mid-Altitude (5 km from Figure 4. Thermal image of Ryugu taken by TIR on 1 August 2018 from the Mid-Altitude (5 km from Ryugu), with the spatial resolution of 4.5 m per pixel. The largest crater Urashima is observed at Ryugu), with the spatial resolution of 4.5 m per pixel. The largest crater Urashima is observed at the the equator, and the largest boulder Otohime is seen at the South Pole. Boulders larger than 20 m equator, and the largest boulder Otohime is seen at the South Pole. Boulders larger than 20 m are all are all identified in this image set, showing almost the same temperature with the surroundings. identified in this image set, showing almost the same temperature with the surroundings. The The temperature scale is in Kelvin. (Image: hyb2_tir_20180801_152032_l2.fit, ©JAXA). temperature scale is in Kelvin. (Image: hyb2_tir_20180801_152032_l2.fit, © JAXA). 4.3. Close-up Thermal Images of Ryugu 4.3. Close-up Thermal Images of Ryugu Close-up thermal images of local sites of Ryugu have been taken by the TIR at the altitudes below 100 mClose during-up thermal the release images of landers of local and sites during of Ryugu sampling have activities. been taken Those by close-upthe TIR at thermal the altitudes images beloware the 100 first m data during sets the in planetaryrelease of missions. landers and Close-up during thermal sampling images activities. show theThose surface close physical-up thermal state imagesand a variety are the of first boulders data sets at centimeter in planetary scale. missions. As was Close seen- byup optical thermal navigation images show cameras the surfaceand the physicalsurface landers,state and it a was variety evident of boulders that the at surface centimeter of Ryugu scale. is As widely was seen covered by optical with several navigation centimeter- cameras to andmeter-sized the surface rocks landers, and boulders, it was evident but not thatwith the fine surface regolith of [20 Ryugu,27,28 ]. is The widely surroundings covered with in the several global centimeterand local- thermal to meter images-sized rocks correspond and boulders, to the roughbut not surface with fine terrains regolith that [20,2 are7,2 covered8]. The surroundings with boulders indown the global to the and tens-of-centimeter local thermal images scale in correspond the close-up to thermalthe rough images surface (see terrains Figure that5). are covered with bouldersThese down close-up to the thermal tens-of- imagescentimeter have scale proven in the that close the-up thermal thermal inertia images of large (see Figure boulders 5). and their surroundingsThese close are-up almost thermal the im same,ages have since proven the surroundings that the thermal consist inertia of rocks of large larger boulders than the and typical their surroundingsthermal skin depth are almost of centimeter the same, scale. since We the also surroundings recognized several consist boulders of rocks as “coldlarger spots”, than the remarkably typical thermalcolder by skin more depth than of 20 centimeterK compared scale. to the We surroundings also recognized [4,26]. several The “cold” boulders boulders as “ shouldcold spo bets the”, remarkablydense rocks colder with lower by more porosity, than and 20 Kprobably compared have to typical the surroundings thermal inertia [4,2 of6] carbonaceous. The “cold” boulderschondrite shouldmeteorites be theof 700 dense to 1000 rocks tiu [ with14]. lower porosity, and probably have typical thermal inertia of carbonaceousAs shown chondrite in Figure meteorites5, close-up of thermal 700 to images 1000 tiu also [14]. show that surface boulders have fine structure in them,As sho indicatingwn in Figure rugged 5, rocks close with-up verythermal low images thermal also inertia show of < that300 tiu.surface Their boulders porosity have should fine be structuremore than in30 them, % (probably indicating50%) rugged [4]. rocks These with low-density very low and thermal high-porosity inertia of boulders < 300 tiu. indicate Their porosity different − shouldorigin from be more terrestrial than 30rocks % or (probably even meteorites. −50%) [4]. Considering These low the-density planetary and formationhigh-porosity processes, boulders the indicateformation different started with origin flu ff fromy dust terrestrial in the solar rocks nebula, or and even bodies meteorites. grew by coagulation Considering and the accumulation planetary formationto form planetesimals. processes, the Continued formation accumulation started with resultedfluffy dust in larger in the bodies solar nebula, to become and the bodies parent grew body by of coagulationRyugu. It should and accumulation be a porous body to form because plane ittesimals. has never Continued experienced accumulation thermal alteration resulted nor in igneous larger bodiesprocesses, to become except forthe a parent low-level body consolidation. of Ryugu. AtIt should the innermost be a porous layer of body the body, because materials it has should never experiencedbe more consolidated thermal alteration due to higher nor igneous lithostatic processes stress and, except temperatures. for a low- Afterlevel impactconsolidation. fragmentation, At the innermostthe fragments layer are of the accreted body, together materials to should form the be present-daymore consolidated asteroid due Ryugu. to higher That lithostatic is why most stress of and temperatures. After impact fragmentation, the fragments are accreted together to form the present-day asteroid Ryugu. That is why most of surface materials are highly porous, except for Appl. Sci. 2020, 10, 2158 8 of 13

Appl. Sci. 2018, 8, x FOR PEER REVIEW 8 of 13 surface materials are highly porous, except for dense rocks from higher consolidated materials such asdense meteorites. rocks from They higher should consolidated be sediments materials of impact such fragments as meteorites. slightly consolidatedThey should by be thesediments lithostatic of stressimpact in fragments the deep slightly inside of consolidated the parent body.by the Itlithostatic cannot be stress ruled in outthe deep thatthe inside dense of the boulders parent mightbody. haveIt cannot originated be ruled from out thethat impacted the dense meteoroid boulders andmight have have survived originated during from the the impact impacted fragmentation meteoroid processesand have survived [4]. during the impact fragmentation processes [4].

Figure 5. Close-upClose-up thermal image taken by TIR on 25 October 2018, during the descent operation of TD1-R3TD1-R3 Campaign. The width of this image is 20 m across. Most of boulders boulders are the same temperature but a cold boulder (in a red circle) is seen as a “Cold Spot”. The temperature scale is in Kelvin Kelvin.. (Image: (Image: hyb2_tir_20181025_022416_l2.fit,hyb2_tir_20181025_022416_l2.fit, ©© JAXA)JAXA).. 5. Applications of Thermal Images to Future Space Missions 5. Applications of Thermal Images to Future Space Missions It has been proven that thermal infrared imaging does not only give a strong contribution to It has been proven that thermal infrared imaging does not only give a strong contribution to understanding the physical state of planetary surfaces but also shows potential applications to future understanding the physical state of planetary surfaces but also shows potential applications to future space missions such as optical navigation especially in the nighttime ore dawn/dusk regions, as well as space missions such as optical navigation especially in the nighttime ore dawn/dusk regions, as well a hazard detection. Here we show some examples of the applications. as a hazard detection. Here we show some examples of the applications. 5.1. Optical Navigation for Approaching the Asteroid 5.1. Optical Navigation for Approaching the Asteroid Thermal imaging is applicable to the optical navigation for approaching the target body during rendezvousThermal or imaging fly-by operations. is applicable After to the extracting optical darknavigation frame images,for approach the targeting the asteroid target Ryugubody during was a onerendezvous pixel-sized or fly body-by that operations. was taken After during extracting the first dark light frame event ofimages, Ryugu the on target 6 June asteroid 2018 at a Ryugu distance was of 2.5a one 10pixel3 km,-sized and body it became that was larger taken and during brighter the until first the light arrival event at of the Ryugu home on position. 6 June 2018 It was at a predicted distance × 3 thatof 2.5 the × asteroid10 km, andwas itable became to be detectedlarger and by brighter the TIR until at a distance the arrival within at the 1.5 home104 positionkm, comparing. It was × 4 thepredicted relationship that the of thermal asteroid radiation was able intensity to be detected to distance by the for TIR Earth–Moon at a distance observations within 1.5 in × 2015 10 [km,23]. Thermalcomparing images the relation have anship advantage of thermal in optical radiation navigation intensity compared to distance with foroptical Earth– imagesMoon observations by ONC, since in there2015 [ are23]. no Thermal other bright images spots have in an the advantage images such in optical as background navigation bright compared stars, bad with pixels, optical or irradiationimages by ofONC solar, since wind there particles are no or other galactic bright rays, spots and in therethe images is no needsuch as to background change the exposurebright stars, time bad to pixels, avoid overexposureor irradiation whenof solar the wind target particles is seen fromor galactic a single rays, pixel and until there a numerous is no need pixel-sized to change body. the exposure Onboard functionstime to avoid of dark overexposure frame subtraction when the and target binarization is seen from implemented a single pixel in TIR until allow a numerous optical navigation pixel-sized by databody. delivery Onboard to thefunctions guidance of and dark navigation frame subtraction control system and bina of therization spacecraft. implemented in TIR allow optical navigation by data delivery to the guidance and navigation control system of the spacecraft. 5.2. Detection of and 5.2. Detection of Meteoroids and Moons As was mentioned in 5.1, the fact that an object is seen as the only bright spot in thermal image enablesAs was detection mentioned of the in meteoroids 5.1, the fact or that the an orbiting object moonsis seen as which the only encounter bright nearbyspot in thermal the spacecraft. image Attemptsenables detection to detect of the the meteoroids meteoroid arounds or the theorbiting Sun–Earth moons Lagrange which encounter points were nearby conducted the spacecraft. in April 2016,Attempts using to the detect optical the imagers meteoroid thes ONCaround and the TIR. Sun The–Earth ONC Lagrange has higher points spatial were resolution conducted by one in orderApril of2016, magnitude using the and optical the lower imager detections the ONC limit and of TIR. the sizeThe ofONC meteoroids has higher by spatial about oneresolution order of by magnitude, one order of magnitude and the lower detection limit of the size of meteoroids by about one order of magnitude, but the ONC images have numerous bright spots other than the target. The TIR can detect meteoroids confidently if they exist within the distance for its detectable size; within 1.5 x 104 km for 1 km-sized Appl. Sci. 2020, 10, 2158 9 of 13

Appl. Sci. 2018, 8, x FOR PEER REVIEW 9 of 13 but the ONC images have numerous bright spots other than the target. The TIR can detect meteoroids bodyconfidently, 1.5 × 10 if2 theykm for exist 10 withinm-sized the body distance, or 1.5 for km its for detectable 10-cm sized size; body within, provided 1.5 x 10 4thatkm the for meteor 1 km-sizedoids havebody, similar 1.5 10 geometric2 km for 10 albedo m-sized and body, emissivity or 1.5 kmto Ryugu for 10-cm [23 sized]. For body,the case provided of S-type that asteroids the meteoroids which × have several similar times geometric higher albedo geometric and emissivity albedo, the to detection Ryugu [23 range]. For is the a little case bit of smaller, S-type asteroids but there which is no substantialhave several difference times higher in the geometric detection albedo, range. theA similar detection situation range ishas a littleoccurred bit smaller, during but the thereapproach is no andsubstantial proximity diff phaseserence for in thethe detectionsurvey of range.orbiting A moons similar around situation the has asteroid occurred for duringthe both the purposes approach of asteroidand proximity science phases and spacecraft for the survey safety. of orbiting It is difficult moons to around detect thenatural asteroid moons for thewith both optical purposes images of becauseasteroid scienceof saturation and spacecraft by the presence safety. It o isf ditheffi cultvery to bright detect main natural large moons asteroid with opticaland its images smearing because and bloomingof saturation effect by. theThermal presence images of the have very an bright advantage main of large searching asteroid moons, and its since smearing there andis no blooming need to changeeffect.Thermal the exposure images time have to avoid an advantage saturation of, searchingsmearing,moons, or blooming since thereeffect. is Unfortunately, no need to change TIR has the neexposurever found time any to meteor avoid saturation,oids at the smearing,Sun–Earth or Lagrange blooming points effect. nor Unfortunately, orbiting moons TIR around has never Ryugu found. any meteoroids at the Sun–Earth Lagrange points nor orbiting moons around Ryugu. 5.3. Target Maker Tracking 5.3. Target Maker Tracking Close-up thermal imaging during descent operations of Hayabusa2 was carried out using the TIR. AnClose-up opportunit thermaly arose imaging to release during and descent track a operationsTarget Marker of Hayabusa2 (TM) on 25 was October carried 2018 out (see using Figure the 6).TIR. A AnTM opportunity is a 10 cm-diameter arose to releasespherical and ball track wrapped a Target with Marker a recursive (TM) on reflection 25 October sheet, 2018 and (see it Figurehas been6). provenA TM is that a 10 the cm-diameter TIR clearly sphericaldetects the ball TM wrapped falling to with and a settling recursive on reflection the asteroid sheet, surface and itas has a “cold been sphere”.proven that The theTM TIR was clearly not clearly detects detected the TM by falling optical to images and settling at the onsolar the phase asteroid angle surface of > 2° as without a “cold asphere”. flashlight The, and TM if was the notTM clearly was seen detected, it tended by optical to be imagesdazzling at when the solar a flash phaselight angle was ofused.> 2◦ Thewithout first samplinga flashlight, of andthe asteroid if the TM surface was seen, material it tended was carried to be dazzling out on 22 when February a flashlight 2019, waswhere used. the TheTM was first detectedsampling again of the by asteroid the TIR surfaceas a “cold material sphere” was beside carried a 1 outm-sized on 22 boulder February nearby 2019, the where landing the site. TM wasThe knowndetected size again and byshape the of TIR TM as in a “coldan image sphere” provides beside a geometric a 1 m-sized scale. boulder In the nearbycase of theS-type landing asteroid, site. theThe TM known of low size andalbedo shape and of high TM in emissivity an image provides(dark ball) a geometric should better scale. be In detectedthe case of easily S-type as asteroid, a “hot sphere”.the TM of low albedo and high emissivity (dark ball) should better be detected easily as a “hot sphere”.

Figure 6. CloseClose-up-up thermal thermal image image taken taken by by TIR TIR on 25 October 2018, during the descent operation of TD1TD1-R3-R3 Campaign. The width ofof thisthis imageimage isis 66 mm across. across. The The Target Target Marker Marker (TM, (TM, in in the the red red circle) circle) of ofthe the diameter diameter of 10of cm10 cm was was clearly clearly detected detected in the in the thermal thermal image image as a as “Cold a “Cold Sphere” Sphere” beside beside a 1 m-sized a 1 m- sizedrugged rugged boulder. boulder. This thermal This thermal image shows image the shows surface the is surface covered is with covered several with tens-of-centimeter- several tens-of- centimeterto meter-sized- to meter boulders-sized larger boulders than larger the TM than and the not TM covered and not withcovered fine with or small fine or grained small grained regolith regolithmaterials. materials. A very coldA very boulder cold boulder (in yellow) (in yellow) is seen atis theseen bottom-right at the bottom in-right this thermal in this thermal image. (Image:image. (Image:hyb2_tir_20181025_024712_l2.fit, hyb2_tir_20181025_024712_l2.fit,©JAXA). © JAXA). 5.4. Deployable Satellite or Rover Navigation 5.4. Deployable Satellite or Rover Navigation An impact experiment using the Small Carry-on Impactor (SCI) took place to expose the subsurface An impact experiment using the Small Carry-on Impactor (SCI) took place to expose the materials by forming a 10 m-sized artificial impact crater [29,30] on 5 April 2019 and collect the ejected subsurface materials by forming a 10 m-sized artificial impact crater [29,30] on 5 April 2019 and materials on the asteroid surface that were sedimented around the artificial impact crater during the collect the ejected materials on the asteroid surface that were sedimented around the artificial impact crater during the second touchdown operation. Thermal images have successfully tracked the SCI moving from the spacecraft with the background against the asteroid surface after it was released at Appl. Sci. 2020, 10, 2158 10 of 13

Appl.second Sci. 2018 touchdown, 8, x FOR PEER operation. REVIEW Thermal images have successfully tracked the SCI moving from10 of the 13 spacecraft with the background against the asteroid surface after it was released at altitude of 500 m altitude(see Figure of 5007). TIRm (see detected Figure a 7) nutation. TIR detected of theSCI, a nutation and it wasof the found SCI, toand be it su wasfficiently found to small be sufficiently to keep the smallimpact to point keep onthe Ryugu impact within point aon few Ryugu tens ofwithin meters a few beside tens the of targetmeters position beside onthe Ryugu. target position The optical on Ryugu.camera wasThe alsooptical used camera but it was could also not used image but the it SCIcould and not the image asteroid the surfaceSCI and at the the asteroid same time surface without at theoverexposure same timebecause without of overexposure too higher reflectance because of SCItoo comparedhigher reflectance to the low ofreflectance SCI compared of the to surface the low of reflectancethe C-type of asteroid. the surface Thermal of the imaging C-type hasasteroid. an advantage Thermal ofimaging simultaneous has an advantage imaging of of the simultaneous SCI and the imagingasteroid surfaceof the SCI without and th anye asteroid change surface of exposure without time. any This change capability of exposure is useful time. and applicableThis capability to take is usefulan image and of applicable deployable to smalltake an satellites image inof viewdeployable of planetary small satellites surfaces, in or view to navigate of planetary the rovers surfaces, moving or toon navigate planetary the surfaces. rovers moving on planetary surfaces.

FigureFigure 7. ThermalThermal image image of of the Small Carry Carry-on-on Impactor (SCI) released from Hayabusa2 spacecraft takentaken by by TIR TIR on on 5 5April April 2019, 2019, at atthe the altitude altitude of of500 500 m. m.The Theshape shape and andthe attitude the attitude of the of SCI the was SCI recognizedwas recognized in thermal in thermal images images against against the background the background of asteroid of asteroid surface. surface. A movement A movement of SCI of with SCI itswith small itssmall nutation nutation was was observed observed in thisin this image. image. The The temperature temperature scale scale is is in in Kelvin. (Image: (Image: hyb2_tir_20190405_015705_l2.fit,hyb2_tir_20190405_015705_l2.fit, © JAXA).JAXA). 5.5. Operation at Nighttime 5.5. Operation at Nighttime During the proximity phase, optical images observed only the sunlit surface of Ryugu, while During the proximity phase, optical images observed only the sunlit surface of Ryugu, while thermal images observed almost the whole surface regardless of day and night. This capability is thermal images observed almost the whole surface regardless of day and night. This capability is important and applicable to operate and work on the planetary surfaces in the night and in the important and applicable to operate and work on the planetary surfaces in the night and in the shadowed area, if the surface temperature is higher than 160 K in the case of TIR on Hayabusa2 [31]. shadowed area, if the surface temperature is higher than 160 K in the case of TIR on Hayabusa2 [31]. For example, when a rover works on the polar regions of the Moon where there are many shadows of For example, when a rover works on the polar regions of the Moon where there are many shadows boulders, optical images need lights if it is used in the shadow, but suffers from overexposure from the of boulders, optical images need lights if it is used in the shadow, but suffers from overexposure from sunlit surface within the image. A thermal imager can be used more easily even in the dark side of the sunlit surface within the image. A thermal imager can be used more easily even in the dark side the Moon. Thermal images are also applicable for in situ resource use such as mining on near-Earth of the Moon. Thermal images are also applicable for in situ resource use such as mining on near-Earth asteroids where day and night changes rapidly because of fast rotation. asteroids where day and night changes rapidly because of fast rotation. 5.6. Site Selection and Safe Assessment 5.6. Site Selection and Safe Assessment TIR contributed to the Hayabusa2 mission purpose for providing information on the landing site TIR contributed to the Hayabusa2 mission purpose for providing information on the landing selection for sampling and the release of landers, as well as the safe assessment during the descent site selection for sampling and the release of landers, as well as the safe assessment during the descent operations [19]. TIR data is applicable to provide the global distribution of temperatures at any time operations [19]. TIR data is applicable to provide the global distribution of temperatures at any time and the highest temperature in a day for each site. After determining the thermal inertia derived from and the highest temperature in a day for each site. After determining the thermal inertia derived from the diurnal temperature profiles, surface physical states are mapped to find the preferable sampling the diurnal temperature profiles, surface physical states are mapped to find the preferable sampling sites with existence of fine or small particles or the sites with base rock surfaces, or to avoid the sites with existence of fine or small particles or the sites with base rock surfaces, or to avoid the hazardous sites with numerous boulders. In addition, the temperatures of local sites are predictable by hazardous sites with numerous boulders. In addition, the temperatures of local sites are predictable comparison with numerical studies regarding the time of descent operations to assess safe landing. by comparison with numerical studies regarding the time of descent operations to assess safe landing.

5.7. Guidance and Navigation Control of Spacecraft Appl. Sci. 2020, 10, 2158 11 of 13

5.7. Guidance and Navigation Control of Spacecraft In the Hayabusa2 mission, the spacecraft is basically kept on the day side of Ryugu in the field of view of the wide angle camera (except for the escape from the ejecta excavated by the SCI impact), because the optical images should be used for its navigation control all the time. As an application to future missions, such as European Space Agency’s Hera mission to asteroid Didymos [32], the spacecraft will be operated at the proximity of the asteroid, especially at the large solar phase angles (or SPE angles), where detailed navigation control is required even with the night-side area to precisely determine the center of the asteroid. Thermal image will be useful for the navigation and guidance control to include the night-side area. Precise positioning is needed to know the asteroid whole shape, which is easily detectable by TIR using the function of subtraction of dark frame images and binarization.

6. Concluding Remarks During the asteroid proximity phase of Hayabusa2, thermography was applied for the first time in planetary missions. Thermal infrared imaging by TIR has been conducted through the mission to investigate C-type near-Earth asteroid 162173 Ryugu, for achieving both scientific and mission objectives [4,19,23] (see Supplementary Materials). Much progress has been made in asteroid science, offering a new insight for understanding planetary formation scenario and hypothesizing a highly porous nature of small bodies in the current and probably ancient Solar System [4]. Global, local, and close-up thermal images by the TIR have unveiled the thermophysical properties of the surface of Ryugu and provided us with new technical information and experiences. The observations of TIR on Hayabusa2 mainly from the stational home position for asteroid one-rotation are very helpful for data interpretation even for the boulder-rich rough surface, and are achievable for the mission success. The limitation of this method using the TIR on Hayabusa2 or the instruments based on the same type of uncooled micro-bolometer array is the minimum detection temperature of 150 K or higher. This is mainly caused by the filtered wavelength range of 8–12 µm and the background thermal radiation from optics. The wavelength of typical commercial-based bolometer arrays ranges 8–14 µm, which is not substantially different from the current one. A two-dimensional detector to cover 20 µm or longer wavelengths would detect the thermal radiation from the colder planetary surface at a lower temperature such as the dark side of the Moon, any asteroid within the main belt, or the satellites of Jupiter. The thermal design to cool the optics might be feasible by passive radiative cooling or using a state-of-the-art active cooling system. Thermography has been proven during the Hayabusa2 mission as a potentially useful and applicable tool for future space and planetary missions, for applications such as (1) optical navigation for approaching the distant target body, (2) detection of meteoroids or small satellites (ejected rocks) for hazardous protection of spacecraft, (3) target marker tracking for precise navigation and guidance for smart landing onto the surface of the target bodies, (4) tracking or navigating deployable small robots or landers against the backgrounds of planetary surfaces, (5) operations in the night or in the dark area, such as the dark side of the Moon, (6) landing site selection for determining the best site for sampling and for safety operations in hazardous protect or in thermal conditions, and (7) guidance and navigation of spacecraft around the asteroid or planetary satellites, such as orbiting the spacecraft. Some of these demonstrated technology should be applied in the future missions to the Moon, Mars, asteroids, , and other possible targets.

Supplementary Materials: TIR raw data and temperature converted data with ancillary data are available or will be available online at the Hayabusa2 data site: darts.isas.jaxa.jp/pub/hayabusa2/tir_bundle/browse/. The timing of archival for each data is basically one year after acquisition of the data. Funding: This work is partially supported by the JSPS Kakenhi No. JP17H06459 (Aqua Planetology). Acknowledgments: The author appreciate all the members of Hayabusa2 project and the people who contributed to its related development, operations, and scientific studies for fruitful discussions and technical supports. The author would give special thanks to the members of TIR teams. Appl. Sci. 2020, 10, 2158 12 of 13

Conflicts of Interest: The author declares no conflict of interest.

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