September 30, 2024

Prologue

In a historic move, on September 18, 2024, the Union Cabinet of the Government of India has accorded approval for two significant space science missions, viz. the Venus Orbiter Mission (VOM) to study different facets of Venus including its surface and atmosphere, and the Chandrayaan-4 mission, which is meant to collect lunar samples and bring back the same to the Earth. These missions will be major stepping stones to achieve Honb’le Prime Minister’s Space Vision 2047, which envisions India being one of the most impactful space-powers with the Bharatiya Antariksha Station in orbit by year 2035, and Indians having landed on the Moon with indigenous technologies by year 2040. These would necessitate achieving a few technologies like heavy-lift-off launch vehicles, human-rated vehicles, docking technology, high-capacity landers, re-entry technology, to name a few. Technology, along with science, will shape up the country’s space programme roadmap, which has been discussed systematically with the scientists from the national institutes and academia.

Speaking about the scientific aspects, India’s planetary exploration programme is driven by the motivations of comparative planetology to study the similarities and dissimilarities between the planets and natural satellites, and exploring the diversities of the Sun-planet interactions. This programme comprises several space missions to study the Moon, Mars, Venus and other members of the solar system, which will, eventually, bring out the detailed perspective of the solar system. In this context, India has already sent three space missions to Moon in its Chandrayaan series in years 2008, 2019 and 2023 respectively, and one mission to the Mars, as the Mars Orbiter Mission, in year 2014. These missions have created significant scientific impact globally, and materialized several International collaborations in planetary science.

Venus Orbiter Mission (VOM)

The importance of the recently-approved Venus Orbiter Mission lies in the fact that globally it is going to impact the Venusian science. Despite sending several space missions globally, Venus retains its enigma. Early missions in the 1960s and 1970s by NASA and the Soviet Union revealed Venus's scorching surface temperature and dense atmosphere. These missions provided initial insights into the planet's atmospheric composition, surface features, and magnetic environment. Later missions in the 1970s and 1980s, such as Pioneer Venus and Vega, expanded our understanding of Venus's atmosphere, including its composition, circulation, and interaction with the Sun. These missions also collected data on the planet's surface and its geological history. More recent missions, like Venus Express and Akatsuki, have focused on studying the planet's atmospheric dynamics, climate evolution, and surface features. These missions have provided valuable information on the planet's unique characteristics and its potential for habitability in the past.

However, these missions to Venus had limited and narrow spatial coverage either South-polar region or in equatorial belt. Hence it is difficult to build global maps of many phenomena, including winds, waves, and chemical abundances. Venus Orbiter Mission would provide uniform coverage of Venus, thus providing a unique global dataset for future science missions.

The Venus Orbiter Mission will explore the planet's atmosphere, surface, and its interaction with the Sun. Key scientific objectives include examining dust in the Venusian atmosphere, mapping its surface topography in high resolution, studying the solar X-ray spectrum near Venus, analyzing Venusian airglow, and investigating sub-surface characteristics. Additionally, the mission will serve as a technology demonstration for ISRO, testing aerobraking and thermal management techniques in the harsh Venusian environment.

For the Venus Orbiter mission (VOM), sixteen Indian payloads, two Indian and international collaborative payloads, and one international payload have been recommended by Experts Review Committee under well-defined broad science themes viz., surface/sub-surface, atmosphere, ionosphere and solar wind Interaction with an aim to explicate the outstanding science questions as well as the gap areas which need further investigations.

The scientific payloads / experiments have been recommended for the Venus Orbiter Mission, as described below.

1. S-Band Synthetic Aperture Radar for Venus mission (VSAR)

   The SAR Instrument operates at S Band frequency of 2.5 GHz (12cm) with four different polarisations viz., Circular-Pol, Full-Pol, Single- and Dual-Pol. The major science objectives of SAR Instrument are to search for recent and active volcanism and detection of volcanic hotspots, Characterizing Impact craters and associated parabolic ejecta and impact melts and the global mapping of Venus at a spatial resolution of 20-30m an order of magnitude better than the available Magellan data. The SAR instrument is also proposed to work in radiometer mode to measure the brightness temperature with spatial Resolution of - 10 km/pixel. Thus SAR instrument will measure dielectric constant, surface roughness parameters, brightness Temperature and topography.

2. Venus Surface Emissivity and Atmospheric Mapper (VSEAM)

   Venus Surface Emissivity and Atmospheric Mapper is a hyper spectral imaging spectrometer operating in the NIR to SWIR spectral range (0.78-1.7 µm) with about 200 bands, selectable by tele-command with spectral resolution of about 10 nm and spatial resolution about 200m@500km. The measured parameter is spectral radiance in the hyper spectral bands. The major scientific objectives of VSEAM instrument are identification of active volcanic hotspots based on thermal anomaly and atmospheric studies, study of the cloud structure using various spectral channels. H2O and aerosol mapping.

3. Venus Thermal Camera (VTC)

   Venus Thermal Camera, on board Venus Orbiter Mission aims at understanding the atmospheric dynamics and Venusian clouds. Venus thermal camera will map the thermal emission from the cloud top at 8-12 μm wavelengths. The spatial resolution will be 0.5 km from 500 km orbital altitude. Radiance is measured to derive the further parameters to investigate the following major science objective, to study the spatial and temporal variation of the thermal characteristics of Venusian atmosphere and address the Planetary Scale atmospheric features.

4. Venus Cloud Monitoring Camera (VCMC)

   The scientific objectives of VCMC instrument are understanding the atmospheric circulation dynamics of the Venusian atmosphere with first-time near-pole observations and the diverse wave characteristics and induced angular momentum forcing in the Venusian atmosphere and to explore the unambiguous signatures of lightning using synergistic measurements. The proposed camera will be operating two UV wavelengths 283 & 365 nm and one visible wavelength 700 nm. This camera will have a spatial Resolution of 0.5 km for UV channels & 1km for visible lightning channel from 500 km.

5. Lightning Instrument for VEnus (LIVE)

   LIVE is a low frequency electric field analyser operating in range from ~100 Hz to ~30-40 kHz. It will be used for measurement of the field emissions due to plasma and lightning in Venus atmosphere LIVE will have four frequencies viz., 100 Hz, 730 Hz, 5400 Hz and 30000 Hz, from which 100 Hz is a whistler mode. LIVE’s science objectives are to detect lightning in the Venusian environment and to understand the electrostatic model of Venusian clouds through lightning signal characteristics and estimate the received lightning energy and its rate in the atmosphere of Venus. The instrument will capture the lightning generated voltage signal and flash rate with a sensitivity better than ~100 μV/m.

6. Venus Atmospheric SpectroPolarimeter (VASP)

   The proposed instrument is a near infrared spectropolarimeter in the wavelength range of 0.9 - 1.7 μm which is sensitive to linear polarization. The major science objectives of the instrument are to investigate the correlation of cloud Top Altitude with clouds microphysical Properties and to study the process of global circulation on Venus. The instrument measures the spectral radiance with a spectral resolution and spatial sampling of ~2-4 nm and ~8 km respectively.

7. Solar occultation photometry for vertical profiling of Aerosols and thin clouds in Venusian atmosphere (SPAV)

   The measurement of vertical distribution of aerosols and haze layers in the Venusian mesosphere is proposed based on solar occultation technique in which the attenuated solar irradiance is measured along the Line of Sight (LoS) during each sun set and sun rise for all orbits, using a 2-channel photometer (at wavelengths 500 nm and 850 nm). The science objectives of SPAV are to understand the dynamical and physical processes such as transport and diffusion and the influence of the atmospheric thermodynamics in regulating the vertical distribution of aerosols in the mesospheric region. The solar irradiance measurements can be used to retrieve the altitude profiles of aerosol extinction at a vertical resolution of ~1-2 km.

8. Narrow band oxygen Airglow detection in Venusian Atmosphere (NAVA)

   NAVA payload employs novel photometric technique which has been successfully augmented into a CCD based instrument to measure Venusian airglow emissions (both day and night) at OI 557.7 nm (green) and OI 630.0 nm (red). NAVA aims at to investigate the causes of the spatio-temporal variations in Oxygen green (557.7 nm) and red (630.0 nm) line emission intensities, how does the night side ionosphere of Venus sustain ionization and to whether the electron precipitation through deep ionospheric holes responsible for the sustenance of ionosphere and nightside airglow emission. The temporal resolution of the measured intensities of airglow emissions vary from 1 to 5 seconds.

9. VEnus THermosphere Ionosphere composition Analyser (VETHICA)

   The VETHICA is a quadrupole mass spectrometer based payload, which can be operated in the neutral as well as in the ion mode. The scientific objectives of VETHICA payload are to study the altitude-latitude distribution of neutral and ion composition in the Thermosphere-ionosphere-exosphere region of Venus and to investigate the dynamics of Venusian plasma environment through ion neutral interactions. The mass resolved partial pressure of different species in the mass range of 1 to 300 amu in the Venusian atmosphere are measured with 1 amu resolution.

10. Venus Advanced Radar for Topside Ionosphere and Subsurface Sounding (VARTISS)

    VARTISS is a low frequency radar sounder that operates in two modes; ionospheric mode and subsurface mode. In ionospheric mode it sweeps frequencies between 0.1 MHz and 10 MHz and can probe the topside ionosphere of Venus continuously and at all solar zenith angles. The science objectives of the VARTISS instrument in the ionospheric mode are to study the structure of the Venusian ionosphere viz., peak density and altitude on the solar zenith angle and quantifying any deviations, night side ionosphere, characterization of the Venusian ionopause and its variation with solar wind and solar transient events and studying different perturbations such as gravity waves, tides and planetary waves present in the ionosphere. The science objectives of the subsurface mode are to Investigate the vertical structure and stratigraphy of geological units including active volcanic hotspots and lava flows and detection of buried features and structures. The measured reflected echoes can be converted to electron density profiles with a vertical and horizontal resolution of ~5-10 km and resolution: 80-100 km respectively. The subsurface profiles are measured with a vertical and horizontal resolution of 10m & 25m and of ~3km respectively.

11. Venusian Electron temperature and Density Analyser (VEDA)

    The scientific objectives of VEDA instrument are to understand the variability of ionopause altitude vis-à-vis solar forcing (SZA, dynamic pressure etc and to provide realistic inputs of Te for models. This sensor measures the electron density in the range of 100-5x10⁵ cm^-3 and electron temperature in the range 700-8000K.

12. Retarding Potential Analyser (RPA) for the observation of Venusian ionosphere

    Retarding potential analyzer (RPA) is a plasma diagnostic tool which uses a series of electrostatic grids to measure the ion energy distribution. The science objectives of the RPA instrument are the systematic understanding of the Venusian ionospheric and upper atmospheric plasma variability and dynamics and Venus-Solar wind interaction and its implications for plasma transport and atmospheric escape.The measured parameters are electron density, electron temperature, ion density, ion temperature, ion drift velocity and ion composition. The measured ion/electron density are in the range: 10 – 10⁶ cm^-3 and temperatures in the range 100 K – 5000 K &100-25000 K.

13. Venus Ionospheric Plasma wave detectoR (VIPER)

    The science objectives of the VIPER instrument are to sample the plasma and magnetic environment around Venus and to characterize plasma waves. The measured parameters are electron and ion densities, electron and ion plasma temperature and background magnetic field Varying electric and magnetic fields. The ion/electron densities are in the range of 10² –10⁶ cm^-3, electric field resolution of 1Hz and magnetic field resolution of 0.1 nT.

14. Venus Radiation environment monitor (VeRad)

    The objective of the VeRad instrument are to study the impact of Supra thermal and high energy solar energetic particles (SEPs) on the Venus atmosphere and investigate their role in the sustenance of ionosphere on the nightside. The measured charged particles spectrum lies in the energy range of 100 keV – 100 MeV.

15. Solar Soft X-ray Spectrometer (SSXS) for Venus Orbiter

    The primary scientific objective of Solar Soft X-ray Spectrometer (S3) onboard Venus orbiter is to measure the solar irradiance in the soft X-ray region entering in to the Venus atmosphere. S3 sensor package consists of SDD detector, three position mechanism and front end electronics associated with the detector charge readout. The measured spectrum of the solar X-rays in the energy range of 1-15 keV and energy resolution of 200 eV @ 5.9 keV with time cadence of 1 second.

16. Venus Orbit Dust Experiment (VODEX)

    VODEX is an impact ionization dust detector made of thin sheet or foil of gold plate. The major scientific objectives of VODEX is to study abundance, flux and distribution of Interplanetary Dust Particles (IDPs) at Venus. The measured parameters are rise time, peak voltage and impact rate. The measured velocity and mass range of the IDPs are in the range of 1 to 40 km/s and in the mass range of 10^-18 to 10^-12 kg.

    Indian and International Collaborative payloads

17. Venus Ionospheric and Solar Wind particle AnalySer (VISWAS)

    VISWAS has two components namely Plasma Analyser (PA) which will be developed by SPL,VSSC and Venusian Neutrals Analyser (VNA) by IRI, Sweden. The scientific objective is to study the solar wind interaction with Venus to understand the loss of Venus upper atmosphere/ionosphere (ions as well as non-thermal neutrals) and the role of different escape mechanisms and characteristics of plasma in different plasma boundaries. The measured parameters are particle count rates (differential flux) for different energy, angular and mass bins (for ions & Energetic Neutral Atoms(ENAs)). The measured ions, electrons and ENA energy ranges will be in the range of 10 eV - 30 keV, - 10 eV- 5 keV and 10 eV to 10 keV respectively.

18. Radio Anatomy of Venus Ionosphere (RAVI)

    The science objective of RAVi instrument is to study thermal structure in the Venus atmosphere above and below the clouds, Variation of ionosphere under quiet and disturbed solar conditions and to estimate H2SO4 contribution in the energetics of the Venus atmosphere. The normal spacecraft communication channel between the satellite and ground will suffice for the RO experiment. The measured parameter frequency residuals would be used to derive the electron density in the ionopsher with a resolution of >500 per cc and Temperature, Pressure and Number Density with 0.1% uncertainties at 50 km and upto 10 % uncertainties at 100 km. This instrument will have a science collaboration with a Germany team.

    International Payload

19. VIRAL (Venus InfraRed Atmospheric gases Linker)

    The scientific objectives of VIRAL payload are to retrieve the vertical profiles of atmospheric density, temperature Carbon dioxide, CO and HDO/H2O above the cloud top, H2O and SO2 in and above the clouds as well as particulate components and to measure mesospheric wind field through direct Doppler measurements. VIRAL employs solar occultation spectroscopy, covering the IR range from 2.3 to 4.3 μm, and achieving high vertical resolution with a footprint of < 1 km at the limb. The VIRAL instrument will provide the vertical profiles of Temperature and CO2 from 65 to 180 km with a 10% precision and CO2/CO with 1% precision, direct measurement of CO2 line doppler broadening and wind-induced Doppler shift of CO2 lines from 65 to 140 km. This instrument will be developed by IKI, RUSSIA .

Once the VOM would reach the science orbit, the operations of the science instrument would begin. Based on the observational requirements the science instruments will be operated and science data will be collected, sent back to Earth and will be received at ISRO Deep Space Network (IDSN) for the further analysis and data dissemination.

DSN32 station of Indian Deep Space Network (IDSN) will be used for TTC and science data collection from the spacecraft. In addition to the Indian DSN station, network support from external agencies (like NASA) will be required to have maximum possible contact time with the spacecraft. The IDSN antenna used for the VOM is given in Figure.1.

The science data, to be obtained from the instruments onboard the Venus Orbiter Mission will be received, processed and archived at the Indian Space Science Data Center (ISSDC) for dissemination and use by scientific community in India and abroad. The data from Venus orbiter mission instruments will be formatted and archived in PDS4 standard, in such a way that they are easily accessible by Internet through simple interfaces used elsewhere for similar kind of missions.

India’s Venus Orbiter Mission is planned to be launched in March 2028 and will cost approximately 1236 Crore Indian Rupees. LVM-3 has been identified as the candidate launch vehicle which will place the spacecraft in an Elliptical Parking Orbit (EPO) of 170 km x 36000 km, 21.5° inclination and Argument of Perigee (AOP) of 178°. Minimum energy requirement (expressed as incremental velocity, V) for the launch opportunity that exists in 2028 for placing a spacecraft in an elliptical orbit of 500 x 60000 km around Venus is as given below in Table 1.

Table-1. Attributes of the targeted launch window for the VOM

The launch configuration of VOM is presented in Figure 2.

After the cruise phase, Venus Orbit Injection (VOI) will be at 500 km x 60000 km. Aerobraking will be employed for over a period of 6 to 8 months from VOI to achieve the desired low altitude Science Orbit of 200 X600 km with an inclination of around 90º, to carry out proposed science studies for a period of 5 years. This inclined orbit will provide an excellent opportunity for the first time to probe the surface and atmosphere with high unprecedented spatial and temporal resolution and in-situ observation of ionosphere. The on-orbit configuration of VOM is portrayed in Figure 3.

The Venus Orbiter Mission is scheduled to launch in March 2028 at a cost of approximately 1236 Crore Indian Rupees. This ambitious mission is expected to create significant employment opportunities, foster skill development, and drive technological advancements in India. To ensure broad engagement and collaboration, ISRO has organized national-level discussions involving scientists, academics, and research students from across the country. These discussions have focused on the scientific rationale behind the mission, the potential contributions of national institutions, and the opportunities for research students to participate.

Chandrayaan-4 Mission

The Chandrayaan-4 mission will be India’s fourth mission to the Moon. The first two missions, viz. Chandrayaan-1 and 2 have studied the Moon’s surface, sub-surface and exosphere in a global scale, from orbiter platforms. Chandrayaan-3 has been the first-ever successful lunar soft-landing and robotic exploration in the Southern polar region of the Moon. It has conducted in-situ studies of the lunar surface, near-surface plasma, and recorded, for the first time, lunar ground vibrations in the Southern polar regions.

The Chandrayaan-4 mission will be a leap-frog progress in India’s lunar endeavour. While in the Candrayaan-3 mission ISRO demonstrated soft-landing in the lunar South Polar region, roving on the lunar surface, hopping of the lander module on the lunar surface, and bringing back the propulsion module from the lunar to the Earth-bound orbit, the Chandrayaan-4 mission will demonstrate taking off from the lunar surface after collecting surface samples, and bringing back the same to the Earth with protection of the collected samples against damage and contamination.

Although the Apollo and Luna samples revolutionized our understanding of the Moon’s origin, they were from largely similar geological areas and thus not representative of the entire Moon. To continue piecing together the complex origin and history of the Earth-Moon system, samples are needed from new locations. In December 2020, China’s lunar mission Chang’e 5 successfully brought samples from a geologically young region on the Moon, which revealed many facets of the Moon, especially the Moon’s complex thermal history.

In this context, India’s Chandrayaan-4 mission, planned for lunar sample return from the Southern polar region is of utmost significance. This is more so after India’s systematic exploration of the Moon through orbiter, lander and rover. Lunar surface exploration missions followed by sample return will be the next major step in lunar exploration for India, and will result in important contributions in understanding the Earth-Moon system.

Science enabled by returned samples is unique when compared to in-situ instruments and global measurements. Much of the science in Chandrayaan-4 is concentrated on analysing the samples on ground, however the cameras and sensors onboard the spacecraft modules will provide useful insights on the texture of lunar regolith around the landing site and geologic context for interpretation of analysed samples. Studying the returned samples, which are chemically and mineralogically diverse, involve steps ranging from classifying the samples, cataloguing, sample preparation all the way to sample characterisation.

Scientists widely use optical and electron microscopy techniques and various spectroscopic methods over a wide range of electromagnetic spectrum to understand the physical properties and internal structure of the samples. Scanning electron microscopes and electron microprobes enable chemical and mineralogical analysis at nanometer to micrometer scales. In addition, ion beam instruments and transmission electron microscopes provide detailed understanding of the samples in terms of composition. Such analysis are extremely useful to understand the origin and evolution of the Moon and Earth, as well as the inner solar system.

Moreover, the returned samples will take advantage of all the instrumentation and capabilities available on the Earth, not only at the time of return but also in the future.

The spacecraft would comprise five modules - Ascender Module (AM), Descender Module (DM), Re-entry Module (RM), Transfer Module (TM), and Propulsion Module (PM). The five modules are planned to be launched as two stacks; DM + AM in one stack and TM + RM + PM as the second stack, onboard two separate LVM3 launch vehicles. Figures 4 and 5 depict the stacks of the Chandrayaan-4, comprising different modules.

After two launches, the stacks will be docked together in elliptical Earth orbit to form an integrated stack. Subsequent to docking, the Integrated Stack will perform first set of Earth-bound maneuvers with PM propulsion system. Once the PM is depleted, it gets jettisoned from the Integrated stack. The integrated stack (DM + AM + TM + RM) performs all the maneuvers to achieve the lunar orbit, such that the orbit plane has the pre-determined landing site. In the final lunar orbit, DM+AM gets separated from TM+ RM. DM+AM then undergoes powered descent to achieve soft landing on the lunar surface.

After touch down, a robotic arm, also called the Surface Sampling Robot mounted on the DM will scoop ~ 2 – 3 kg samples around the landing site and transfer it to a container on the AM. In addition, a drilling mechanism will collect sub-surface samples and transfer it to another container in the AM. The containers with samples will be sealed to prevent contamination and leakage during its journey to Earth. Various phases of sample collection operations will be monitored through video cameras.

Once sample collection is completed, AM would ascend to the lunar orbit and dock with the parked TM+RM. Samples will be transferred from AM to RM. After sample transfer, the TM+RM will be undocked from AM.

Later, the TM + RM will perform maneuvers to return to Earth. At suitable entry corridor, RM would get separated from TM and performs ballistic re-entry into Earth’s atmosphere and finally landing onto Earth landmass along with Lunar Sample.

In the Chandrayaan-4 mission, preserving the samples in a pristine and uncontaminated state is of paramount importance. Sample recovery, contamination and safety protocols, preservation and curation will be worked out in an impeccable manner.

The Chandrayaan-4 mission aims to be entirely self-reliant, with all critical technologies developed domestically. Surface sampling, drilling mechanism, sample storage cartridge, sample transfer and docking are some of the new technologies which would be demonstrated in this mission. Indian industries will play a pivotal role in realizing this mission, which is expected to promote skill development, create significant employment opportunities and drive technological advancements. The total cost of the mission is estimated at 2104.06 Crore Indian Rupees, covering spacecraft development, two LVM3 launches, International network support, and various tests. Chandrayaan-4 will pave the way for India's future manned missions, lunar sample return, and scientific analysis.

Epilogue

To foster collaboration and knowledge sharing, both the missions will involve Indian academia through workshops and conferences. Additionally, for Chandrayaan-4, the establishment of facilities for sample curation and analysis will serve as valuable national assets.

The Venus Orbiter Mission and the Chandrayaan-4 mission will be two major vehicles for India’s journey to the goal of Vikshit Bharat , towards excellence in space science exploration.