Venus Orbiter Mission Research Articles

A Short Review on India’s Interplanetary Missions

India's space program has witnessed transformative achievements in interplanetary exploration, propelling the nation into the forefront of space science and technology. This comprehensive overview delves into key milestones, objectives, and outcomes of India's interplanetary missions, beginning with the trailblazing Chandrayaan-1 that altered lunar studies by detecting water traces. Mangalyaan-1's success in orbiting Mars on its maiden attempt underscored India's prowess in Martian exploration. Despite a setback in the Chandrayaan-2 lunar landing, it provided ground-breaking lunar data. The recently launched Aditya-L1 focuses on solar observation, capturing high-energy X-rays and solar disc images. Future missions, including Chandrayaan-3, Mangalyaan-2, and the Venus Orbiter Mission (Shukrayaan), exemplify India's commitment to advancing planetary exploration. These endeavours, marked by cutting-edge technology and diverse scientific objectives, highlight India's growing prominence in global space exploration and contribute significantly to humanity's understanding of the cosmos.

Electrostatic Solitary Waves in the Venusian Ionosphere Pervaded by the Solar Wind: A Theoretical Perspective

Electrostatic solitary waves (ESWs) in the Venusian ionosphere that are impinged by the solar wind are investigated using a homogeneous, collisionless, and magnetized multicomponent plasma consisting of Venusian H+ and O+ ions, Maxwellian Venusian electrons and streaming solar wind protons, and suprathermal electrons following κ − distribution. The model supports the propagation of positive potential slow O+ and H+ ion-acoustic solitons. The evolution and properties of the solitons occurring in two sectors, viz., dawn-dusk and noon-midnight sector of the Venus ionosphere at an altitude of (200–2000) km, are studied. The theoretical model predicts positive potential solitons with amplitude ∼(0.067–56) mV, width ∼(1.7–53.21) m, and velocity ∼(1.48–8.33) km s−1. The bipolar soliton electric field has amplitude ∼(0.03–27.67) mV m−1 with time duration ∼(0.34–22) ms. These bipolar electric field pulses when Fourier transformed to the frequency domain occur as a broadband electrostatic noise, with frequency varying in the range of ∼9.78 Hz–8.77 kHz. Our results can explain the observed electrostatic waves in the frequency range of 100 Hz–5.4 kHz in the Venus ionosphere by the Pioneer Venus Orbiter mission. The model can also be relevant in explaining the recent observation of ESWs in the Venus magnetosheath by the Solar Orbiter during its first gravity assist maneuver of Venus.

Long-duration Venus lander for seismic and atmospheric science

An exciting and novel science mission concept called Seismic and Atmospheric Exploration of Venus (SAEVe) has been developed which uses high-temperature electronics to enable a three-order magnitude increase in expected surface life (120 Earth days) over what has been achieved to date. This enables study of long-term, variable phenomena such as the seismicity of Venus and near surface weather, near surface energy balance, and atmospheric chemical composition. SAEVe also serves as a critical pathfinder for more sophisticated landers in the future. For example, first order seismic measurements by SAEVe will allow future missions to deliver better seismometers and systems to support the yet unknown frequency and magnitude of Venus events. SAEVe is focused on science that can be realized with low data volume instruments and will most benefit from temporal operations. The entire mission architecture and operations maximize science while minimizing energy usage and physical size and mass. The entire SAEVe system including its protective entry system is estimated to be around 45 ​kg and approximately 0.6 ​m diameter. These features allow SAEVe to be relatively cost effective and be easily integrated onto a Venus orbiter mission. The technologies needed to implement SAEVe are currently in development by several funded activities. Component and system level work is ongoing under NASA’s Long Lived Insitu Solar System Explorer (LLISSE) project and the HOTTech program.. LLISSE, is a NASA project to develop a small Venus lander that will operate on the surface of Venus for 60 days and measure variations in meteorology, radiance, and atmospheric chemistry. LLISSE is developing a full-function engineering model of a Venus lander that contains essentially all the core capabilities of SAEVe thus greatly reducing the technology risk to SAEVe.The SAEVe long duration Venus lander promises exciting new science and is an ideal complimentary element to many future Venus orbiter missions being proposed or planned today.

The upper atmosphere of Venus: Model predictions for mass spectrometry measurements

Venus is the closest and most similar planet to Earth in the whole solar system, yet, basic measurements are missing. The atmospheric composition has been studied to some extent with the Pioneer Venus Orbiter mission and the more recent Venus Express, however, the concentration of many species and their isotopic ratios are not yet known with satisfying accuracy. Especially when it comes to the heavy noble gases as Kr and Xe, there is little to no collected data in this field. These elements form a key link to the reconstruction of the atmospheric evolution, which would deliver crucial information on the formation of Earth-like bodies. Knowing the abundances of the noble gases may allow us to answer the question why Venus did not evolve to a potentially habitable planet like its neighbour Earth.This paper presents a meta-study of existing observations of Venus' upper atmosphere and exosphere and an exospheric model of the thermal and hot atmosphere of Venus above the homopause. Mass spectra for possible future Venus missions are predicted, as for example the EnVision mission of ESA. The main focus are the heavy noble gases and requirements for the trajectories are proposed. In addition, mass spectra and recommended integration times for the Venus flyby of ESA's JUICE mission are derived.

Venus looks different from day to night across wavelengths: morphology from Akatsuki multispectral images

Since insertion into orbit on December 7, 2015, the Akatsuki orbiter has returned global images of Venus from its four imaging cameras at eleven discrete wavelengths from ultraviolet (283 and 365 nm) and near infrared (0.9–2.3 µm), to the thermal infrared (8–12 µm) from a near-equatorial orbit. The Venus Express and Pioneer Venus Orbiter missions have also monitored the planet for long periods but from polar or near-polar orbits. The wavelength coverage and views of the planet also differ for all three missions. In reflected light, the images reveal features seen near the cloud tops (~ 70 km altitude), whereas in the near-infrared images of the nightside, features seen are at mid- to lower cloud levels (~ 48–60 km altitude). The dayside cloud cover imaged at the ultraviolet wavelengths shows morphologies similar to what was observed from Mariner 10, Pioneer Venus, Galileo, Venus Express and MESSENGER. The daytime images at 0.9 and 2.02 µm also reveal some interesting features which bear similarity to the ultraviolet images. The nighttime images at 1.74, 2.26 and 2.32 µm and at 8–12 µm reveal features not seen before and show new details of the nightside including narrow wavy ribbons, curved string-like features, long-scale waves, long dark streaks, isolated bright spots, sharp boundaries and even mesoscale vortices. Some features previously seen such as circum-equatorial belts (CEBs) and occasional areal brightenings at ultraviolet (seen in Venus Express observations) of the cloud cover at ultraviolet wavelengths have not been observed thus far. Evidence for the hemispheric vortex organization of the global circulation can be seen at all wavelengths on the day- and nightsides. Akatsuki images reveal new and puzzling morphology of the complex nightside cloud cover. The cloud morphologies provide some clues to the processes occurring in the atmosphere and are thus, a key diagnostic tool when quantitative dynamical analysis is not feasible due to insufficient information.

Initial performance of the radio occultation experiment in the Venus orbiter mission Akatsuki

After the arrival of Akatsuki spacecraft of Japan Aerospace Exploration Agency at Venus in December 2015, the radio occultation experiment, termed RS (Radio Science), obtained 19 vertical profiles of the Venusian atmosphere by April 2017. An onboard ultra-stable oscillator is used to generate stable X-band downlink signals needed for the experiment. The quantities to be retrieved are the atmospheric pressure, the temperature, the sulfuric acid vapor mixing ratio, and the electron density. Temperature profiles were successfully obtained down to ~ 38 km altitude and show distinct atmospheric structures depending on the altitude. The overall structure is close to the previous observations, suggesting a remarkable stability of the thermal structure. Local time-dependent features are seen within and above the clouds, which is located around 48–70 km altitude. The H2SO4 vapor density roughly follows the saturation curve at cloud heights, suggesting equilibrium with cloud particles. The ionospheric electron density profiles are also successfully retrieved, showing distinct local time dependence. Akatsuki RS mainly probes the low and middle latitude regions thanks to the near-equatorial orbit in contrast to the previous radio occultation experiments using polar orbiters. Studies based on combined analyses of RS and optical imaging data are ongoing.Graphical abstract.

The thermal structure of the Venus atmosphere: Intercomparison of Venus Express and ground based observations of vertical temperature and density profiles

The Venus International Reference Atmosphere (VIRA) model contains tabulated values of temperature and number densities obtained by the experiments on the Venera entry probes, Pioneer Venus Orbiter and multi-probe missions in the 1980s. The instruments on the recent Venus Express orbiter mission generated a significant amount of new observational data on the vertical and horizontal structure of the Venus atmosphere from 40 km to about 180 km altitude from April 2006 to November 2014. Many ground based experiments have provided data on the upper atmosphere (90–130 km) temperature structure since the publication of VIRA in 1985. The "Thermal Structure of the Venus Atmosphere" Team was supported by the International Space Studies Institute (ISSI), Bern, Switzerland, from 2013 to 2015 in order to combine and compare the ground-based observations and the VEx observations of the thermal structure as a first step towards generating an updated VIRA model. Results of this comparison are presented in five latitude bins and three local time bins by assuming hemispheric symmetry. The intercomparison of the ground-based and VEx results provides for the first time a consistent picture of the temperature and density structure in the 40 km–180 km altitude range. The Venus Express observations have considerably increased our knowledge of the Venus atmospheric thermal structure above ∼40 km and provided new information above 100 km. There are, however, still observational gaps in latitude and local time above certain regions. Considerable variability in the temperatures and densities is seen above 100 km but certain features appear to be systematically present, such as a succession of warm and cool layers. Preliminary modeling studies support the existence of such layers in agreement with a global scale circulation. The intercomparison focuses on average profiles but some VEx experiments provide sufficient global coverage to identify solar thermal tidal components.The differences between the VEx temperature profiles and the VIRA below 0.1 mbar/95 km are small. There is, however, a clear discrepancy at high latitudes in the 10–30 mbar (70–80 km) range. The VEx observations will also allow the improvement of the empirical models (VTS3 by Hedin et al., 1983 and VIRA by Keating et al., 1985) above 0.03 mbar/100 km, in particular the 100–150 km region where a sufficient observational coverage was previously missing. The next steps in order to define the updated VIRA temperature structure up to 150 km altitude are (1) define the grid on which this database may be provided, (2) fill what is possible with the results of the data intercomparison, and (3) fill the observational gaps. An interpolation between the datasets may be performed by using available General Circulation Models as guidelines.An improved spatial coverage of observations is still necessary at all altitudes, in latitude–longitude and at all local solar times for a complete description of the atmospheric thermal structure, in particular on the dayside above 100 km. New in-situ observations in the atmosphere below 40 km are missing, an altitude region that cannot be accessed by occultation experiments. All these questions need to be addressed by future missions.

Solar zenith angle‐dependent asymmetries in Venusian bow shock location revealed by Venus Express

AbstractIt has been long known that the Venusian bow shock (BS) location is asymmetric from the observations of the long‐lived Pioneer Venus Orbiter mission. The Venus Express (VEX) mission crossed BS near perpendicularly not only in the terminator region but also in the near‐subsolar and tail regions. Taking the advantage of VEX orbit geometry, we examined a large data set of BS crossings observed during the long‐lasting solar minimum between solar cycles 23 and 24 and found that the Venusian BS asymmetries exhibit dependence of solar zenith angle. In the terminator and tail regions, both the magnetic pole‐equator and north‐south asymmetries are observed in Venusian BS location, which is similar to the Pioneer Venus Orbiter (PVO) observation near terminator. However, in the near‐subsolar region, only the magnetic north‐south is observed; i.e., the BS shape is indented inward over magnetic south pole and bulged outward over magnetic north pole. The absence of the magnetic pole‐equator asymmetry in the near‐subsolar region suggests that the magnetic pole‐equator asymmetry is mainly caused by the asymmetric wave propagation rather than the ion pickup process. The evident magnetic north‐south asymmetry in solar minimum, which is not observed by PVO, suggests that even during the low long‐lasting solar minimum, the ion pickup process is very important in Venusian space environment.

Characterizing the low‐altitude magnetic belt at Venus: Complementary observations from the Pioneer Venus Orbiter and Venus Express

AbstractUsing Venus Express, Zhang et al. (2012b) identified strong magnetic field enhancements at low altitudes over the north polar region of Venus as giant flux ropes. Strong fields at low altitudes were also observed during the Pioneer Venus Orbiter mission, but at low latitudes near the subsolar and midnight regions. We examine the possibility that the Venus Express observations are not giant flux ropes, but part of a low‐altitude magnetic belt that builds up in the subsolar region, passes over the terminator, and extends to the nightside. Our analysis indicates the magnetic belt is dominantly horizontal over the dayside and gains a radial component nightward. The peak magnetic field strength of the belt and the altitude at which it peaks also varies around the planet, with the lowest altitude and strongest field strength in the subsolar region, consistent with the idea of the belt forming on the dayside. Zhang et al. (2012b) also noted the fields in the polar region had a bias in the +By direction in Venus Solar Orbital coordinates. The multifluid magnetohydrodynamic simulation we present shows an asymmetry in the plasma flow from the subsolar region to the poles due to the oxygen ion and proton mass ratio. This causes the magnetic field to preferentially accumulate in the north for a +By interplanetary magnetic field direction, providing an explanation for this bias.

Mirror‐mode‐like structures in Venus' induced magnetosphere

The solar wind interaction with Venus creates an induced magnetosphere around the planet. It is shown that within the space bound by Venus' bow shock and ionopause, there is a rich occurrence of mirror‐mode‐like structures in the magnetic field data. The dayside magnetosheath and nightside magnetosheath/wake regions are investigated separately. It is shown that the probability to observe mirror mode structures is much higher at the dayside, where it is also strongly dependent on the angle between the solar wind magnetic field and the bow shock normal. In Venus' wake the chance to observe these structures is low, most likely because of the fully developed turbulence in this region, which will decrease temperature anisotropies. The results stand in contrast to the very low occurrence rate claimed from data taken by the Pioneer Venus Orbiter mission.

Venus O + pickup ions: Collected PVO results and expectations for Venus Express

Observations of oxygen pickup ions by the plasma analyzer on the Pioneer Venus Orbiter (PVO) Mission arguably launched broad interest in solar wind erosion of unmagnetized planet atmospheres, and its potential evolutionary effects. Oxygen pickup ions may play key roles in the removal of the oxygen excess left behind from the photodissociation of water vapor by enabling direct escape, additional sputtering of oxygen when they impact the exobase, and escape as energetic neutrals produced in charge exchange reactions with the ambient exospheric oxygen and hydrogen. Although the PVO observations were compromised by an ∼8 keV energy limit for O + detection, a lack of ion composition capability, and the limited sampling and data rate of the plasma analyzer which was designed for solar wind monitoring, these measurements provide our best information about the extended O + exosphere and wake at Venus. Here we show the full picture of the spatial distribution and energies of the O + ion observations collected by the plasma analyzer during PVO's ∼5000 orbit tour. A model of O + test particles launched in the circum-Venus fields described by an MHD simulation of the solar wind interaction is used to help interpret the PVO observations and to anticipate the expanded view of Venus O + escape that will be provided by the ASPERA-4 experiment on Venus Express.

Current status of the PLANET-C Venus orbiter design

The ISAS (Institute of Space and Astronautical Science) working group is currently studying the feasibility of a Venus orbiter mission [Proposal Report of feasibility study of Venus Climate Orbiter (in Japanese), ISAS working group, 2001], called PLANET-C project, in which a 3 axis stabilized spacecraft will be launched in early 2008, and it will arrive at Venus in 2009. Primary purpose of the mission is to observe Venus climate so as to understand mechanism of the global circulation and three-dimensional motion of the Venus atmosphere in the lower stratum. In order to achieve the above scientific objectives, the spacecraft will carry several types of optical cameras mainly with near-infrared wavelengths. An accurate orientation for the attitude control is required in the spacecraft design. Furthermore, thermal control of the cameras is critical, because of large heat input from camera hoods and strict requirement of low temperature of detecting head. This paper describes characteristics of the PLANET-C spacecraft. And also depicted are the mission outline and the operation plan of the attitude orientation.

Spatial and temporal variations of Venus haze properties obtained from Pioneer Venus Orbiter polarimetry

The spatial and temporal variations of the polarization of light scattered by Venus, as observed by the Pioneer Venus Orbiter between 1978 and 1990, is analyzed in terms of spatial and temporal variations of Venus upper haze properties. Special attention is given to choosing maps with sufficiently accurate geometrical information. This selection involves multiple‐scattering calculations for a number of reasonable models for the Venus atmosphere. For a set of observations in four wavelength bands of the Venus disk as a whole or a particular region thereof, observed values of Stokes parameters I and Q were used to obtain the average observed relative Stokes parameter qobs. Subsequently, the haze particle column density and the cloud top pressure were iteratively adjusted until values of qobs at two wavelengths were reproduced. For the planet as a whole, it was found that the haze particle column density decreased gradually during the Pioneer Venus Orbiter mission, whereas little significant temporal variability was found for the cloud top pressure. Similar long‐term behavior was derived for selected regions of Venus. Regions at higher latitudes exhibited higher values of both the haze particle column density and the cloud top pressure than regions at lower latitudes. Something similar holds for regions with low solar elevations as compared to regions in which the Sun was close to the zenith.

Feasibility study for Venus atmosphere mission

A Venus orbiter mission is going to be proposed as one of the future Japanese planetary missions. The primary objective is to reveal the mechanism of the atmospheric general circulation. The mission will reveal also ionospheric circulation, atmospheric escape processes and geologic activities. The spacecraft will be launched in February–April of 2007 by a M-V rocket of ISAS and arrive at Venus in September of 2009. The spacecraft will be inserted into an elliptical equatorial orbit with a period of 21.3 hours. The outline of the mission is presented in this paper.

Theory of kilometer‐size density waves in the nightside Venus ionosphere

Quasi‐sinusoidal density waves were frequently observed during the end of the Pioneer Venus Orbiter (PVO) mission when the orbiter was at low periapsis [Brace, this issue]. These waves occur at altitudes ∼145–155 km and have wavelengths ∼1 km. It is suggested that a radial, ambipolar electric field E0, directed downward, is established in the Venus ionosphere during electron pressure enhancements above ∼160 km. This field generates an electron E × B drift VE; the ions move radially and do not E × B drift because they are unmagnetized (i.e., νin » Ωi). This drift is shown to drive a collisional drift wave instability for sufficiently large values of VE, nominally, VE > υi where υi is the ion thermal velocity. For parameters typical of the nightside Venus ionosphere, this instability generates plasma fluctuations with wavelengths ∼1 km, consistent with observations.

Remote sensing of Venus atmospheric dynamics

Aspects of our current understanding of the dynamics of the atmosphere of Venus are reviewed and critically discussed. We consider how this information was obtained, noting the importance of remote sensing in the ultraviolet and in the near and thermal infrared portions of the spectrum. It is proposed that a great deal could be done to clarify the remaining uncertainties and outstanding mysteries with a relatively simple multispectral imaging device on a hypothetical low-cost Venus orbiter mission. The basic parameters of such a project are described and its timely implementation advocated.

A search for the solar roots of the most disturbed interplanetary field intervals of solar cycle 21

During the course of the Pioneer Venus Orbiter mission, fairly continuous interplanetary plasma and magnetic field data were obtained which span the interval from prior to the last solar maximum to the current solar minimum recovery. Within this nearly complete solar cycle interval, several periods of exceptional disturbance of the interplanetary field stand out. We have examined the available solar data to determine what features, if any, distinguish these periods. Neither flare nor coronal mass ejection reports show particularly unusual behavior. However, these periods appear to occur in conjunction with marked changes in the interplanetary sector structure. This suggests that heliospheric current sheet reconfiguration is an indicator of the level of interplanetary disturbance distinct from the more traditional solar activity data.

Synchrotron emission and gamma-ray bursts

The parameter space within which synchrotron emission models for gamma-ray burst spectra are valid is evaluated in the cases of thermal and power-law particle distributions. The spectra emitted by noncooling and cooling thermal and power-law particle distributions are modeled numerically over relativistic and nonrelativistic regimes. The structure of the radiation is defined near the fundamental and the continuum up to the 1000th harmonic. Comparisons are made with burst event data collected during the Apollo 16, Venera, SMM, Pioneer Venus Orbiter and Hackucho missions. A two-component synchrotron model is proposed for gamma-ray burst events. The electron-positron distribution is projected to have a thermal component and a nonthermal power-law tail.

5. Solar Research from Space, 1982-1984 (Some Highlights)

During this reporting period, the satellites ISEE 3 (now ICE), IMP 8, Helios I, Hinotori, Nimbus 7 (ERB), P78-1, Pioneer Venus Orbiter (PVO), Spacelab 1 and Solar Maximum Mission (SMM) continue to provide solar observations. We review briefly some recent developments, selected by the author as most significant, concerning: coronal mass ejecta, high-energy neutral emissions, X-ray and microwave bursts, the solar constant, and solar flare periodicity. While the emphasis is on new work, some significant results, obtained in earlier spacecraft experiments and only recently reported, are also mentioned. The repair, in space, of SMM in 1984 April, resulting in reactivation of three of the four pointed experiments is briefly described and a status report of the SMM 2 mission is given. We conclude with a summary of important future programs.

An Interplanetary Targeting and Orbit Insedion Maneuver Design Technique

The paper describes a tradeoff in selecting a planetary encounter aimpoint and a spacecraft propulsive maneuver strategy in the Pioneer Venus Orbiter Mission. The method uses parametric data spanning a region of acceptable targeting aimpoints in the delivery space and the geometric considerations. Real-time maneuver adjustments accounted for known attitude control errors, orbit determination updates, and late changes in a targeting specification.