Deimos by Mars imaging camera (MIC) on NOZOMI

A& Space Res.Vol.23, No. 11,pp. 191l-1914,19% 0 1999 COSPAR.Publishedby Elsevier Science Ltd. All rights reserved

Pergamon

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PLANNED OBSERVATION OF PHOBOS/DE~MOS BY MARS IMAGING CAMERA (MIC) ON NOZOMI A. Inada,

M. Kawamata,

The Graduate

S. Sumikawa

and T. Mukai

School of Sci. and Tech., Kobe Univ., Nada 657-8501 Kobe, Japan

ABSTRACT The large elongated orbit planned for NOZOMI around Mars, i.e. a periapsis of 150 km and an apoapsis of 15 RM (RM denotes the radius of Mars), will provide many occasions for encounters of NOZOMI with two Martian satellites, Phobos and Deimos, where NOZOMI is the former Planet-B meaning “Hope” in Japanese. We present a plan for imaging the two satellites by the Mars Imaging Camera (MIC) on board NOZOMI at such encounters during the mission lifetime of two years from October 1999. An Autonomous Tracking Mode is available for fly-by imaging of satellites. MIC scans the azimuth direction (orthogonal to the CCD line arrays) using the spacecraft spin at a rotation rate of 7.5 rpm, and has an image resolution of 80 arc second in both elevation and azimuth directions. The main science objectives of MIC, related to the two satellites, are (i) to study the size/spatial distributions of craters on both satellites, (ii) to examine the groove structure on Phobos, (iii) to image areas not yet seen areas of Deimos, and (iv) to derive its whole shape. We will, furthermore, search for the dust rings along the orbits of these two satellites in the forward scattering region of sunlight. The capability of MIC to execute these objectives are briefly summarized. Q 1999 COSPAR. Published by Elsevier Science Ltd.

INTRODUCTION NOZOMI was launched on July 4th 1998. In October 1999, it will enter into a large elongated orbit around Mars with a periapsis and an apoapsis of 150 km and 15 RM, respectively, where RM denotes the radius of Mars, NOZOMI is a spin-stabilized spacecraft, rotating at 7.5 rpm, and its orbital period around Mars is about 34 hours. This orbit intersects the orbits of the two Martian satellites, Phobos and Deimos. Consequently, we can expect many opportunities for NOZOMI to pass nearby the two satellites. The Mars Imaging Camera (MIC) on board NOZOMI has three linear CCD arrays with the filters of Red (630 - 680 nm), Green (520 - 580 nm), and Blue (440 - 480 nm), respectively. The pixel arrays are parallel to the rotation axis, and MIC takes two-dimensional images by using NOZOMI’s spin motion. We can select the image size from a combination of 256, 512 and 1024 pixels, and 256, 512 and 1024 lines. The field of view (FOV) of MIC is 54.2 degrees (along the array) x 360 degrees (around the spin axis). MIC has two types of modes for imaging, i.e, a hardware mode and a software-control mode. The hardware mode has three sub-types, i.e. Command Mode usually used, One Shot Mode for MIC testing, and Auto Imaging Mode for taking images of a bright object automatically. In the software-control mode, there is an Autonomous Tracking Mode. In this mode, the software of MIC deduces the position of an object relative to the spacecraft and operates to take images every NOZOMI’s rotation at the specified time. Image size is set to be 512 x 512 pixels. The Autonomous Tracking Mode will be used in the case of high relative velocity during the encounters with PhobosfDeimos. Phobos is an irregularly shaped satellite, and its size and density are still uncertain, e.g. 13.3 x 11.1 x 9.3 4 km (Duxbury, 1989b): 1.9 f 0.1 g/ cm3 (Avanesov et ab. 1991, Thomas1998), and 12.97 x 11.42 x 9.14 km, 1911

1912

A. Inada er al.

date

0

100

300 Revolution

200

400

500

Fig. 1. Opportunities to observe Phobos/Deimos occur near the crossing points of ascending/descending node of NOZOMI (solid curves) with the orbits of Phobos/Deimos (dotted lines}.

Fig.. 2. The coordinate system of Fhobos/Deimos, where the x axis is always toward Mars. The trailing side is from east longitude 0 to 180 degrees, and the leading side is from 180 to 360 degrees.

2.2 g/cm3 (Hagiwara et al. 1994). The value of density is a key parameter to estimate the interior structure of the Martian satellites. The largest crater Stickney on the leading side and the grooves from it dominate the surface of Phobos. The spectral observations show that the bottom of and the ejecta from Stickney is ‘biuer’, in contrast with ‘redder’ of the remainder (see, e.g. Murchie and Erard 1996). The rims of the craters on the redder unit are brightened. However, there are no brightened rims on the bluer unit. An impact to produce the crater caused the grooves, which is not unique on Phobos because asteroids Gaspra and Ida have similar structure on them. (Thomas et al. 1994, Sullivan et al. 1996). It is shown that most of the grooves on Phobos are not related to the ejecta of Stickney (Thomas 1998). Deimos is an irregularly shaped small body like Phobos. Batson et csl. (1992) have estimated its mean ellipsoidal radii of 7.5 x 6.2 x 5.4 km and Thomas (1998) has derived its mean density of 1.9 g/cm3. Since the Viking’s orbit was always inside the orbit of Deimos (Duxbury 1989a), its camera could not take images of the hemisphere of Deimos away from Mars. The images taken by Mariner 9 lack coverage of some areas. As a result, the imaging of the unseen region of Deimos is strongly needed to estimate a precise model for its shape and to deduce its radii and its density. Deimos has a smooth surface and “streamers”, which are features of gradational changes in albedo. (Thomas et al. 1996) It was thought from the evidence of no grooves on it that Deiomos had no ~at~trophic impacts. Recently, however, Thomas et al. (1996, 1998) have suggested that a 10 km diameter crater existing in the southern pole had a significant influence on the surface structure of Deimos as Stickney had on Phobos. That is, the ejecta deposition and seismic shaking simultaneously occurred in the crater production event. Consequently, the significant numbers of craters originally existed might be erased completely. Unfortunately, we have not enough data to conclude that Deimos has no grooves since some grooves may be on the unobserved region or on the area imaged previously only at low resolution. OPPORTUNITIES When and how many times MIC will titude of NOZOMI. While NOZOMI’s of Mars and ascending or descending NOZOMI has well defined the chances

take images of Phobos and Deimos depend on the orbit and the atorbit will shift with time gradually, the distance between the center node of NOZOMI’s orbit changes as shown in Figure 1. As a result, to encounter periods with Phobos and Deimos.

Observation

6Q

1ZQ

18Q

east longitude

240

300

(deg)

Fig. 3. Observable region by MIC on Phobos. a; the region seen from less than 500 km. b; 1000 km, c; 2000 km, d; 3000 km, e; more than 3000 km

of PhobodDeimos

381)

by MIC

0

6Q

120

180

east longitude

240

300

360

(deg)

Observable region by MIC on Fig. 4. Deimos. A; the region seen from less than 2000 km. B; 3000 km, C; more than 3000 km, D; night area during two observable occasions by MIC.

From Figure 1, it is found that the ascending node of NOZOMI will be far from Mars when NOZOMI will arrive at Mars (at revolution (rev.) = 0). On the other hand, its descending node will be near Mars at that time. The position of the ascending node will become closer to Mars with a time, and consequently it will cross the satellites’ orbits. NOZOMI has two short periods to approach Deimos at about rev.140 3 10 and 520 f 10, while NOZOMI has a long period to approach Phobos from rev.250 to 400 in the case of nominal schedule of orbits. NOZOMI will keep its high gain antenna toward the Earth during the observational phase around Mars and MIC is fixed on its side. Therefore, even when NOZOMI encounters Phobos or Deimos, the FOV of MIC sometimes will not include the satellites. Taking into account this condition, we calculate how many times MIC will be able to image Phobos and Deimos. When NOZOMI is less than 3000 km from the center of the target satellite, MIC has only 5 opportunities for Deimos. On the other hand, 71 times are available for Phobos. The relative velocity of NOZOMI to Deimos is slower than that to Phobos. This implies that MIC has a longer time to image Deimos during one encounter, although the opportunities of encounter are few. Since there are many chances for MIC to pass nearby Phobos, the images of Phobos will be obtained at various conditions, i.e. different sun angles and different camera angles. It is expected that the highest spatial resolution of Phobos will be about 70 m/pixel from the distance of 180 km, while that of Deimos will be about 600 m/pixel from 1500 km. It should be noted that the total number of images obtained during one encounter is limited by an available on board memory (about 1 Mbyte}. OBSERVABLE

REGIONS

In the calculations to derive the observable region on the satellite by MIC, we assumed that the satellite is a sphere. The observable region is defined on the luminous hemisphere as the region which can be seen by MIC. Figure 2 shows our coordinate system for Phobos/Deimos. Figure 3 shows the observable region by MIC on Phobos from the distance of 3000 km or less during the first two years of the mission lifetime. The distance of 3000 km leads to the spatial resolution of MIC about 1200 m/pixel. It is found from Figure 3 that MIC images will be able to cover the entire surface of Phobos. The images of Phobos will be able to supply the data of the size/spatial dist~butions of craters on its surface. Furthermore, we can expect that MIC will take the images of the southern hemisphere of the leading side (see the definition in Figure 2) in high resolution. These images are suitable for studying the

grooves structure

in detail.

Figure 4 shows the observable region by MIC on Deimos. Unfortunately, due to a shortage of encounter chances, MIC will not be able to image the whole surface of Deimos. Most of the not-yet-seen regions by MIC shown in Figure 4 is in the night area during the time of encounter, although the whole body of Deimos is in the FOV of MIC. However, MIC will be able to observe a part of unknown regions in the anti-Mars hemisphere. As a result, the large crater in the southern polar region of Deimos would be examined with relatively good resolution. SUMMARY It should be noted that our plan is based on NOZOMI’s nominal orbit for two years, The extension of the mission lifetime may produce more opportunities to encounter with ~hobos~Deimos. A possibility for such an extension will be discussed after NOZOMI goes into orbit around ?Mars. In that case, MIC will take images of Phobos from more different view and the observable region of Deimos would increase from those predicted in this study. The spatial resolution of images of both satellites expected in the current nominal schedule is not high, compared with the Viking images. The alternation of the orbit will make more frequent encounters of NOZOMI with the satellites, and yield more close encounters to them. Referring to our expected images of both satellites densities. We expect furthermore that M1C data more information for its surface roughness albedo of Deimos will give us useful information to derive

by MIC, we will try to estimate their volumes and their of Phobos taken from different solar angles will provide and color. In addition, the data of the unknown region its shape and volume.

References Batson,

R. M., K. Edwards, T. C. Duxbnry, Geodesy and Cartography of the Martian Satellites, in Mars edited by H. H. Kieffer, B. M. Jakosky, C.W. Snyder, M. S. ~~atthews~ pp. 1249-1256 (1992)

Duxbury, T. C., J, D. Callahan, Pole and Prime Meridian J., 86, pp. 1722-1727 (1981) Phobos

Expressions

and Deimos Control

for Phobos

Networks,

Duxbury,

T. C., J. D. Callahan,

Duxbury, Duxbury,

T. C., The Figure of Phobos, Icarus, 78, pp. 169-180 (1989b) T. C., An Analytic Model for the Phobos Surface Planet. Space Sci., 39, pp. 355-376 (1991)

Hagiwara, Y., Y, Kotake, F. Yamamizu, The Shape and Gravitatio~lal the Geodeitic Society of Japan, 40, pp. 221-232 (1994)

Icarus,

and Deimos, Astronomjcal

Potential

‘77, pp. 275-286 (1989a)

Field of Phobos,

Jona

of

Mukai, T., T. Akabane, T. Hashimoto, H. Ishimoto, S. Sasaki et at., Observations of Mars and its Satellites by the Mars Imaging Camera. (MIC) on Planet-B, Earth, Planets and Space, 50, pp. 183-188 f1998) Murchie, S., S. Erard, Spectral Properties Icarus, 123, pp. 63-86 (1996)

and Heterogeneity

of Phobos

from Measurements

by Phobos

2,

Ninomiya, K., T. Hshimoto, A. M. Nakamura, T. Mukai, M. Nakamura et al., “Mars Imaging Camera (MIC) on Board PLANET-B”, Third TAA International Conference on Low-Cost Phmetary Missiom, Pasadena, U.S.& in press (1998) Simonelli, D. P,, MEyz, Wise, A. .%&ala, D. Adinolfi, J_ Veverka, P. C. Thomas, P. Helfenstein, Photometric Properties of Phobos Surface materials From Viking Images Icarus, 131, pp. 52-77 (1998) Sullivan, R., R. Greeley, R. Pappalardo, E. Asphaug, J. M. Moare et al, Geology of 243 Ida Icarus, 120, pp. 119-139 (1996) Thamas, P. C., J. Veverka, D. Simonelli, P. Helfenstein, B, Carcich et al., The Shape of Gaspra, Icarus, 107, pp. 23-36 (1994) Thomas, P. C., D. Adinolfi, P. Helfenstein, D. Simonelf, 3. Veverla, The Surface De&as: Contribution of Materials and Processes to Its Unique Appearance, Icarus, 123, pp. 536-556 (1996) Thomas, P. C., Ejecta Emplacement Veverka, J., M. Belton, K. Klaasen,

on the Martian Satellites! Icarus, 131, pp. 78-306 (1998) C. Chapman, Icarus, 107, pp. 2-17 (1998)