4. Science instruments and targets

The basic designs of 5 cameras are shown in Fig. 4, while the basic specifications are summarized in Table 1.

4.1 1-micron Camera (IR1)

IR1 is designed to image the dayside of Venus at 0.90 micron wavelength and the nightside at 0.90, 0.97 and 1.01 micron wavelengths, which are located in the atmospheric windows (Taylor et al., 1997). These windows allow radiation to penetrate the whole atmosphere. On the dayside IR1 visualizes the distribution of clouds illuminated by sunlight. Although the dayside disk at this wavelength appears almost flat, small-scale features with contrasts of ~3% are thought to originate in the middle and lower cloud region (Belton et al., 1991); tracking of such cloud features provides the wind field in this region over the hemisphere. We expect to determine the wind vectors with accuracy of the order of a few m s^-1. On the nightside, IR1 measures the thermal radiation mostly from the surface and a little from the atmosphere. The 0.97 micron radiation is slightly absorbed by H₂O vapor, and thus the comparison of this radiance with other wavelengths allows the estimation of H₂O content below the cloud. The measurements at 0.90 and 1.01 micron will yield information about the surface material (Barnes et al., 2000; Hashimoto and Sugita, 2003), and are expected to find out hot lava at active volcanoes by utilizing the temperature dependence of the 0.90 to 1.01 micron radiance ratio and the high sensitivity of the radiance itself to temperature in this wavelength region (Hashimoto and Imamura, 2001).

As an imaging instrument, IR1 has many features in common with IR2. These cameras share electronics for 16 bit A/D conversion since the detector arrays in these cameras are electronically nearly identical. Each of both cameras consists of a large baffle which eliminates stray light from the sun, F/4 optics with a focal length of 84.2 mm and a 1040 x 1040 pixels detector array (1024 x 1024 area is used) with a pixel pitch of 17 micron. The optics and the detector array altogether yield an effective field of view of 12 degrees, giving the pixel resolution of ~16 km from the apoapsis (13 Rv) and ~6 km from the distance of 5 Rv.

IR1 utilizes a 1.01 micron band-pass filter, a 0.97 micron filter, and two 0.90 micron filters, one of which has a low transmission for observing the dayside disk. The detector array is a Si-CSD(charge sweeping device)/CCD which is cooled down to 260 K to achieve a signal-to-noise ratio of ~300 on the dayside and ~100 on the nightside. The total weight excluding the electronics is 2.3 kg and the electronics weighs 3.7 kg. The power consumption is 9.4 W.

4.2 2-micron Camera (IR2)

The atmospheric windows IR2 utilizes are at 1.73, 2.26, and 2.32 micron: the first two are nearly absorption free, while the last one contains a CO absorption band. At these wavelengths, IR2 is most sensitive to infrared radiation originating from altitudes 35-50 km. To track cloud motions, a series of 2.26-micron images are exclusively used. As the inhomogeneity of the Venusian cloud layer is thought to occur predominantly at altitudes 50-55 km (Belton et al., 1991), the IR2 observations should yield wind maps in this region. As CO is photochemically produced above the cloud and subsequently transported to the deeper atmosphere (such sinks are not yet precisely located), distribution of CO should give us additional information about the circulation of the atmosphere. We will extract the CO distribution at 35-50 km altitudes by differentiating images taken at 2.26 and 2.32 micron (Collard et al., 1993). To study cloud microphysics, cloud opacities at 2.26 and 1.73 micron, together with the IR1 1.01-micron and 0.90-micron images, will be analyzed with the aid of radiative transfer calculation. Such analysis gives us information on the spatial and temporal variations in the cloud particle size and density (Carlson et al., 1993).

IR2 employs two additional wavelengths, 2.02 micron (a prominent CO₂ absorption band) and an astronomical H-band centered at 1.65 micron. At 2.02 micron, we expect to detect variations of cloud-top altitude as intensity variations of reflected sunlight. The sensitivity to the cloud height may not be compared to what LIR would achieve but the horizontal resolution is superb. The H-band aims at observing the zodiacal light. Each of these 5 wavelengths can be selected by rotating a 6-position filter wheel which has a blank position as well for protection and acquisition of dark frames.

The PtSi sensor IR2 utilizes is not ultra sensitive in 2-micron region but has a number of advantages: it is very stable, uniform, and durable against energetic radiation of over 30 kRad. To suppress the thermal electrons in the detector, it is cooled down to 65 K by a single-stage stirling cooler driven at 50 W. The cooler consists of a compressor, a cold head with a cold tip, and a driver electronics unit. Although the cold tip of the cooler is attached to the detector mount, heat is removed via conduction from the lens and lens housing also, making these components cooled to approximately 170 K. Combined with a cold filter fixed in front of the detector housing, we expect to achieve the signal-to-noise ratio over 100 when imaging the Venusian nightside.

In order to fabricate observations of the zodiacal light, the camera optics is designed to suppress the instrumental background as well as the stray light. The large baffle of the camera is very useful for the interplanetary dust (IPD) observations, because it enables us very wide coverage in the solar elongation angle from 180 arc-degree (anti-solar direction) to 30 arc-degree. The device is specially designed for the camera to realize precise measurements of the instrumental zero level. Stability of the zero level is essentially important for the IPD observations, because the target is extending beyond the instantaneous field of view of the camera. The architecture of the device is based on the similar technology of 512 x 512 PtSi IR CSD which was applied for astronomical observations (Ueno, 1996). Insensitive (or dark) pixels, which monitors the instrumental zero level steadily and places 8 lines in each edge of the 1024 x 1024 sensitive pixels, were employed for the device (corresponding to 1040 x 1040 pixels in total). Balanced type FDAs (floating diffusion amplifier) are also important to maintain the precise zero level of the signal and to suppress the interfering noise in the spacecraft. Expected sensitivity of IR2 in low noise mode will reach 1.5 x 10^-6 W m^-2 sr^-1 of surface brightness, or 13 magnitude of point source at H band with a single exposure of 2 minutes integration. The typical surface brightness of zodiacal light at 1AU from the sun (~4 x 10^-7 W m^-2 sr^-1) is within the detection limit after 2 x 2 pixels on-chip binning. The wide field of view with fine spatial resolution is a big advantage in removing the star light component, which contributes in the sky brightness at H band (Hauser et al., 1998). The wide coverage in solar elongation angle is also very important to observe rather inner part of the zodiacal light, which is key information to count the contribution of the isotropic component of the IPD cloud. The isotropic component is hard to estimate precisely by the former missions, because it is very difficult to distinguish the zodiacal light from the smooth background of the integrated extragalactic light. Recent observations by the star-tracker camera onboard the Clementine spacecraft shows rather abundant dust particle in isotropic component (Hahn et al., 2002). The determination of the amount of the isotropic component is very important for studying its origin and also has a big impact on the cosmological studies. Pointing toward the inner direction of the solar system is very effective for these studies, because the isotropic component gains its brightness at the inner part, while the light by extra-galactic origin is constant along the heliocentric distance. The cruising trajectory itself is also very unique because VCO will trek into the IPD cloud clumps at the beginning, and will change her heliocentric distance from 1.1 AU to 0.7AU. The camera has enough chance to trace the resonance structure produced by the Earth and Venus, since the cloud structure of the mean motion resonance is co-rotating around the sun together with the earth. The camera will thus give us a very unique opportunity to observe the IPD cloud, and will paint the complete three-dimensional distribution of the IPD cloud. The expected results on IPD observations with IR2 are radial dependence of the zodiacal light, accurate trace of resonance structures by Venus and the Earth, determination of the symmetrical plane of the IPD cloud, inner distribution and scale height, and precise measurements of the uniform component of IPD cloud.

We have developed a sophisticated lens support mechanism to satisfy the conflicting requirements.

(1) The lens elements need to be loosely supported at room temperature to allow contraction of lens housing when it is cooled to the operating temperature once in the space.

(2) The lens elements need to be tightly supported at room temperature so they could survive high level of vibration at the moment of spacecraft launch.

To achieve the desired performance, the lens support employs springs of optimized strength in both radial and lateral directions. Alignment errors of optical elements after a shaking/cooling cycle have been measured with a test model and we have found the mechanism very promising.

The entire system of IR2 is mounted on a cold plate for higher efficiency of cooling. The cold plate is bolted onto the northern or southern surface of the spacecraft with the camera pointing perpendicular to the north-south axis. The camera is located inside the spacecraft while the compressor and the cold head of the cooler are mounted on the other side of the cold plate, in other words, exposed to the space to effectively dispose heat. The weight of IR2 including the lens shade and the cold plate is slightly over 9 kg, the heaviest of 5 cameras onboard VCO.

4.3 Ultraviolet Imager (UVI)

UVI is designed to measure ultraviolet radiation scattered from cloud tops at ~65 km altitude in two bands centered at 283 nm and 365 nm wavelengths. The Venusian atmosphere shows broad absorption of solar radiation between 200 and 500 nm; SO₂ at the cloud top explains the absorption in the range between 200 nm and 320 nm, while the absorption above 320 nm should be due to another absorber that is not identified yet (Esposito et al., 1997). Identification of the absorber is important not only for atmospheric chemistry but also for the energy balance and dynamics of the atmosphere, because the species influence the albedo and the heating profile of the atmosphere. UVI will make clear the spatial distributions of these ultraviolet absorbers and their relationships with the cloud structure and the wind field. The tracking of cloud motions yields the wind vectors at the cloud top (Rossow et al., 1990). Furthermore, the vertical distributions of cloud particles and the haze layer above the main cloud will be studied with limb observations.

With the field of view of 12 degrees, the full disk of Venus can be captured in one image at distances >8.5 Rv. UVI uses a UV-coated backthinned frame transfer Si-CCD with 1024 x 1024 pixels and the pixel size of 13 micron. The spatial resolution is ~16 km at apoapsis and ~6 km from the distance of 5 Rv. The fullwell of the CCD is 10⁵e^- per pixel, and the singal-to-noize ratio is 120. The output is digitized with 12 bit A/D conversion. The total mass including the optics, CCD detector assembly and electronics is ~3.4 kg, and the power consumption is 9.4 W.

4.4 Longwave Infrared Camera (LIR)

LIR detects thermal emission from the cloud top in a rather wide wavelength region 8-12 micron to map the cloud-top temperature. Unlike other imagers onboard VCO, LIR is able to take images of both dayside and nightside with equal quality and accuracy. The cloud-top temperature map will reflect the cloud height distribution, whose detailed structure is unknown except in the high latitude observed by the Pioneer Venus (Taylor et al., 1980), as well as the atmospheric temperature distribution. The images will visualize the cloud height anomalies which originate from convection cells and various waves within the cloud layer. Furthermore, the tracking of blocky features in successive images will yield wind vectors including those on the nightside, which have been inaccessible in the previous missions. The cloud-top temperature is typically as low as 230 K; LIR has the capability to resolve temperature difference of 0.3 K for such a cold target, corresponding to a few hundred-meter difference in the cloud height. The accuracy of absolute temperature measurement is 3 K.

LIR consists of a sensor unit and an external power supply unit. The sensor unit includes optics, a mechanical shutter, an image sensor and its drive circuit, and attached with a baffle that keeps direct sunlight away from the optical aperture. The F-number of the Germanium lens module is 1.4 and the field-of-view is 12 degrees. The mechanical shutter driven by a stepping motor works not only as a light shutter but also as a calibration source. The image sensor is an uncooled micro-bolometer array with 320 x 240 pixels (240 x 240 area is used) and the pixel size of 37 micron. Since the sensor can work under room temperature, huge and heavy cryogenic apparatus which is usually necessary for infrared devices is unnecessary. This makes the instrument very light and small. The temperature of the bolometer is stabilized to 313 K by a Peltier cooler. The frame rate is 60 Hz, and several tens of images obtained within a few seconds will be accumulated in DE (described below) to increase the signal-to-noise ratio. The spatial resolution is 0.05 degree, which corresponds to ~70 km on the Venus surface when viewed from the apoapsis and ~26 km from the distance of 5 Rv. LIR weighs 3.7 kg and the power consumption is 29 W.

4.5 Lightning and Airglow Camera (LAC)

LAC is a high-speed imaging sensor which measures lightning flashes and airglow emissions on the nightside disk of Venus when VCO is located within the umbra (shadow region) of Venus. One of the major goals of LAC is to settle controversy on the occurrence of lightning in the Venusian atmosphere. Lightning observations will give us information on the charging mechanism, charge separation mechanism, physics of sulfuric acid clouds, mesoscale meteorology and its impacts on atmospheric chemical processes. If lightning discharge occurs in the upwelling cloud regions like as the Earth and Jupiter, we can monitor vertical convections inside the cloud layer via lightning detection. The 777.4 nm [OI] band associated with the excitation of atomic oxygen is expected to be a strong emission from the laboratory discharge experiment in a simulated Venusian atmosphere (Borucki et al., 1996). Possible lightning flashes were detected on the nightside disk of Venus at this wavelength by a ground-based telescope (Hansell et al., 1995).

We will also obtain information on the global circulation in the lower thermosphere by continuous observations of large-scale structures in the O₂ Herzberg II (542.5 nm) night airglow, whose production is the consequence of the recombination of atomic oxygen in downwelling. The Herzberg II bands of molecular oxygen are the strongest emissions among the visible Venusian airglows, and their integrated intensity in the 551-552 nm region, which includes rotational lines in the 0-10 band, is 270 R (Rayleigh) (Slanger et al., 2001). LAC also enables us to observe wave-like structures produced by gravity waves which might play important roles in the dynamical coupling between the lower and the upper atmosphere. Furthermore, LAC will measure the 557.7 nm [OI] and 630.0 nm [OI] emissions. Both of these atomic oxygen emissions were not detected by Venera 9 and 10 (Krasnopolsky, 1983); however, Slanger et al. (2001) discovered the 557.7 nm emission whose intensity is 150 R ± 20% using a ground-based telescope, while 630.0 nm emission was not detected. The origin of such variability in the 557.7 nm emission is also one of themes of LAC.

LAC is designed to detect lightning flashes with an intensity of 1/100 of standard lightning on the Earth when viewed from 1000 km altitude and to measure 100-R night airglow with a signal-to-noise ratio more than 10. LAC has a field-of-view of 16 degrees, and as the detector it uses a multi-anode avalanche photo-diode (APD) that has 8 x 8 matrix of 2-mm square pixels. We will measure lightning flashes at 777.4 nm [OI] with 4 x 8 pixels and airglow emissions at 542.5 nm [O₂ Herzberg II], 557.7 nm [OI] and 630.0 nm [OI] with 1 x 8 pixels, respectively, using rectangular interference filters. Airglow-free background images are also acquired at 545.0 nm with 1 x 8 pixels with the same kind of filter. A complex of these interference filters is placed on the detector. Individual lightning flash events are sampled at 50-kHz by pre-triggering, while airglow images are recorded continuously at intervals of 20 seconds, scanning the nightside disk of Venus by attitude maneuver or orbital motion of the spacecraft. The field-of-view of one pixel corresponds to about 35 km on the Venusian surface at 1000 km altitude and 850 km at 3 Rv altitude. The total weight of LAC is about 1.5 kg.

4.6 Sensor Digital Electronics Unit (DE)

DE is a science payload to interface and control four cameras; IR1, IR2, UVI, and LIR. For the nominal observation sequence which is repeated every 2 hours, the main satellite system controller (Data Handling Unit) triggers DE unit. DE, then, sequentially triggers detailed observation sequence of each camera, filter wheel and gain settings, exposure, and data transfer. DE is also responsible for (i) acquiring raw data from cameras, (ii) arithmetic data processing and data compression, and (iii) science and telemetry data formatting and packetting. The weight and the power consumption of DE are 4.6 kg and 20 W, respectively, including 512 Mbytes data recorder unit installed in a same box.

The arithmetic data processing includes (a) dark signal subtraction, (b) pixel gain collection, and (c) background region (off Venus) suppression to zero. Due to the limited data downlink capacity, i.e. the 6-7 hours of tracking per day and the telemetry rate of 4-32 kbps, data volume must be reduced by the combination of following approaches; (a) partial cancellation of imaging opportunities depending on the bandpass filters and/or cameras, (b) increase of the observation interval to 4 hours or more, (c) data slicing, (d) pixel binning, and (e) data compression. Since the derivation of wind vectors from successive cloud images at different layers requires high fidelity data acquisition, VCO will use the JPEG2000 (Boliek et al., 2000) lossless data compression technique based upon discrete wavelet transform as far as possible. However, in the epochs of low telemetry rate (4 kbps at < 2.2 AU), lossy compression will also be adopted. The data compression efficiency is strongly dependent on the high frequency component of images; since VCO will acquire the finest Venusian images ever observed from spacecraft or the ground, the data compression efficiency is difficult to estimate at this moment.

4.7 Radio Science (RS)

RS, in its atmospheric occultation mode, provides vertical profiles of atmospheric temperature, sub-cloud H₂SO₄ vapor density and ionospheric electron density. In the experiment, the spacecraft sequentially passes behind the atmosphere and the planetary disk as seen from the tracking station on the Earth, and then reemerges in the reverse sequence. The Venus atmosphere causes ray bending of the radio wave, thereby causing the time-dependent Doppler frequency shift due to the orbital motion of the spacecraft. The frequency variation observed at the tracking station is processed off-line assuming the spherical symmetry of the atmosphere to yield the vertical profile of the refractive index of the atmosphere, which is further converted to the neutral density profile below ~90 km and the electron density profile above (Fjeldbo et al., 1971). The neutral density profile yields the profiles of atmospheric pressure and temperature on the assumption of hydrodynamic equilibrium. The vertical resolution is diffraction-limited to the diameter of the first Fresnel zone, which is typically 1 km. The temporal variation of the signal power will also be analyzed to retrieve the vertical profile of H₂SO₄ vapor, which absorbs the radio wave (Jenkins et al., 1994). When the spacecraft moves into superior conjunctions with the sun, the structure of solar corona will also be studied.

RS will be conducted in one-way mode using X-band (8.4 GHz) downlink stabilized by an onboard ultra-stable oscillator (USO) with the frequency stability on the order of 10^-13. The downlink signal will be recorded by an open-loop receiver at the Usuda Deep Space Center of Japan (Imamura et al., 2005). During the occultation the spacecraft must perform attitude maneuvers to compensate for the ray bending, which is as large as ~20 degrees. Due to the 8-degrees tilt of the spacecraft orbit relative to the Venus’ ecliptic plane and the 3.4-degrees difference of the ecliptic planes between Venus and the Earth, broad latitude regions will be sounded with emphasis on the low latitude.

The temperature profiles contain information on the static stability, energy balance, and various wave activities in the atmosphere. The H₂SO₄ vapor profiles are indispensable for the study of cloud physics. It should also be noted that the temperature field is related to the zonal wind field through cyclostrophic balance; the meridional distribution of zonal wind can be obtained from the temperature distribution by integrating the thermal wind equation with an appropriate lower boundary condition (Newman et al., 1984). Furthermore, the residual-mean meridional circulation (Andrews et al., 1987) can be diagnosed from the temperature and zonal wind distributions with the aid of radiative transfer calculation.