Space Technology

Orbits are classified by altitude. LEO (Low Earth Orbit): 160–2000 km — used by ISS, Earth observation, Starlink; short period, high resolution. MEO (Medium Earth Orbit): 2000–35,786 km — navigation satellites (GPS at ~20,200 km, IRNSS partly). GEO (Geostationary): exactly 35,786 km above equator, period 24 hrs, satellite appears fixed — used by communication and weather (INSAT/GSAT). Geosynchronous orbits have a 24-hr period but may be inclined (not fixed over one point). Memory aid: 'Lower = faster & closer.' Polar/Sun-Synchronous orbits (SSO) cross poles at ~600–800 km, passing a location at the same local solar time daily — ideal for remote sensing (RESOURCESAT, CARTOSAT). Higher altitude = larger footprint but more signal delay.

Orbital velocity (to stay in circular orbit near Earth): v = √(GM/r) ≈ 7.9 km/s for LEO. Escape velocity (to break free of Earth's gravity): v = √(2GM/R) ≈ 11.2 km/s at Earth's surface. Key relation: escape velocity = √2 × orbital velocity. Escape velocity is independent of the object's mass and direction; depends on the planet's mass and radius. Memory aid: '11.2 to escape, 7.9 to orbit.' First cosmic velocity = orbital (7.9), second cosmic velocity = escape (11.2), third cosmic velocity ≈ 16.7 km/s (to escape the solar system from Earth). For the Moon escape velocity is only ~2.4 km/s due to low mass.

PSLV (Polar Satellite Launch Vehicle): 4-stage, alternating solid-liquid stages; workhorse for SSO/polar and small GEO payloads; launched Chandrayaan-1 and Mangalyaan. GSLV Mk II: 3-stage with indigenous Cryogenic Upper Stage (CUS); ~2.5-tonne GTO payload. LVM3 (earlier GSLV Mk III): heaviest, 3-stage (2 solid boosters S200 + liquid L110 core + cryogenic C25); ~4-tonne GTO / 8-tonne LEO; launched Chandrayaan-2, Chandrayaan-3, and crewed Gaganyaan. SSLV (Small Satellite Launch Vehicle): for small satellites up to ~500 kg to LEO, 'launch-on-demand.' Memory aid: cryogenic engines use liquid hydrogen (fuel) + liquid oxygen (oxidiser), giving high specific impulse for heavy GTO missions.

Chandrayaan-1 (2008, PSLV): India's first lunar mission; its Moon Impact Probe and the Moon Mineralogy Mapper (NASA payload) confirmed water/hydroxyl molecules on the lunar surface. Chandrayaan-2 (2019, LVM3): Orbiter (still functioning), Vikram lander (crash-landed), Pragyan rover. Chandrayaan-3 (2023, LVM3): Successful soft landing on 23 August 2023 near the lunar South Pole — making India the FIRST country to land near the South Pole and the 4th to soft-land on the Moon. Lander = Vikram, Rover = Pragyan; payloads ChaSTE (thermal), ILSA (seismic), RAMBHA (plasma). Memory aid: 'C-3 = South Pole, 23 Aug (now National Space Day).' Chandrayaan-1 → water; Chandrayaan-3 → South Pole soft landing.

Mars Orbiter Mission (MOM/Mangalyaan, 2013, PSLV): made India the FIRST country to reach Mars orbit on its first attempt and the first Asian nation to reach Mars; entered orbit 24 Sep 2014; notably low-cost. Aditya-L1 (2023, PSLV): India's first solar mission, placed in a halo orbit around the Sun–Earth Lagrange point L1 (~1.5 million km from Earth), giving continuous unobstructed view of the Sun. Key payload: VELC (Visible Emission Line Coronagraph) studies the solar corona. L1 is favoured because gravitational forces balance there, allowing a satellite to 'hover' with minimal fuel. Memory aid: 'Aditya = Sun god → L1 Sun-watcher.' Lagrange points: L1, L2, L3 unstable; L4, L5 stable.

Gaganyaan: India's first crewed spaceflight programme, aiming to send astronauts ('Gagannauts'/Vyomanauts) to LEO (~400 km) aboard LVM3 with indigenous crew escape system. NavIC (IRNSS): regional navigation system of 7 satellites (mix of GEO and inclined geosynchronous orbits) covering India and ~1500 km around it; uses L5 and S-band; civilian (SPS) and restricted services. GAGAN: GPS-aided augmentation for aviation. INSAT/GSAT: communication satellites in GEO for broadcasting, telephony, weather (INSAT carries VHRR/meteorological payloads). Memory aid: 'NavIC = regional GPS, 7 satellites; GAGAN = aviation booster.' Bhuvan is ISRO's geo-platform; NISAR is the upcoming ISRO–NASA radar imaging satellite.

Remote sensing acquires information about Earth without physical contact, using reflected/emitted electromagnetic radiation. Active sensors (RADAR, LiDAR) emit their own energy; passive sensors (most optical cameras) rely on sunlight. Four resolutions: (1) Spatial — smallest object detectable (pixel size); (2) Spectral — number/width of wavelength bands; (3) Radiometric — sensitivity to brightness differences (bit depth); (4) Temporal — revisit frequency over the same area. Memory aid: 'SSRT — Space, Spectrum, Radiance, Time.' Synthetic Aperture Radar (SAR) works day-night and through clouds (microwave penetrates cloud) — used in RISAT and the upcoming NISAR. India's main remote-sensing satellites: CARTOSAT (cartography/high spatial resolution), RESOURCESAT (natural resources), OCEANSAT (ocean colour), RISAT (radar).

Communication satellites sit in geostationary orbit so ground antennas can point at a fixed spot. They use transponders that receive an uplink signal, amplify and shift frequency, and retransmit (downlink). Common bands: C-band (lower frequency, rain-resistant), Ku-band, Ka-band (higher frequency, more bandwidth but rain fade). India's INSAT/GSAT series handle broadcasting, telecom, tele-education, telemedicine and disaster warning. Weather satellites are of two types: geostationary (INSAT, continuous view of one hemisphere) and polar-orbiting (global coverage, higher resolution). INSAT-3D carries an imager and sounder for atmospheric profiling. Memory aid: 'GEO for comms & continuous weather watch; Polar for global, sharper weather data.'

During floods and cyclones, SAR satellites (RISAT) map inundation even through cloud cover, while INSAT-3D tracks cyclone tracks and intensity — feeding IMD's early warnings (helping evacuate ahead of cyclones like Fani/Amphan). In agriculture, the FASAL and CHAMAN programmes use RESOURCESAT/multispectral data to estimate crop acreage and yield via vegetation indices (NDVI = (NIR−Red)/(NIR+Red); healthy vegetation reflects strongly in near-infrared). Forest cover assessment by FSI, groundwater prospecting, and the SVAMITVA scheme (drone + satellite property mapping) are other uses. Memory aid: 'Red = stress, NIR-high = healthy crop.' Bhuvan portal disseminates this geospatial data for governance.

James Webb Space Telescope (JWST, 2021, NASA–ESA–CSA): largest space telescope, observes mainly in infrared, positioned at the Sun–Earth L2 point (~1.5 million km, anti-sun side) to stay cold and shielded; successor to Hubble (which orbits Earth in optical/UV). Artemis programme (NASA): aims to return humans to the Moon, including the first woman and person of colour, and build the Lunar Gateway station. Mars rovers: Perseverance (2021, seeks signs of past life, carried Ingenuity helicopter). China's Tiangong is an operational space station; Chang'e missions explore the Moon (Chang'e-5 returned samples; Chang'e-6 returned far-side samples). Memory aid: 'JWST = infrared, L2, cold; Hubble = optical, Earth orbit.'

Space debris (defunct satellites, spent stages, collision fragments) threatens active assets at orbital speeds (~7–8 km/s). The Kessler Syndrome describes a cascading chain of collisions generating ever more debris, potentially rendering orbits unusable. ISRO's response: Project NETRA (Network for Space Object Tracking and Analysis) for debris monitoring, and the IS4OM centre; India targets a 'Debris-Free Space Missions' goal by 2030. Key treaties: Outer Space Treaty (1967) — space is the 'province of all mankind,' no national appropriation, no WMD in orbit; Moon Agreement (1979, poorly ratified); Artemis Accords (US-led principles, India signed in 2023). Memory aid: 'OST 1967 = the constitution of outer space.'

India opened space to private players: IN-SPACe (Indian National Space Promotion and Authorization Centre) is the single-window promoter/regulator; NSIL (NewSpace India Limited) is the commercial/PSU arm handling demand-driven launches and tech transfer; ISRO focuses on R&D. Indian startups: Skyroot Aerospace (Vikram-S, India's first privately built rocket, 2022) and Agnikul Cosmos (Agnibaan with 3D-printed semi-cryogenic engine). The Indian Space Policy 2023 formalised these roles and 100% FDI reforms followed. Globally, SpaceX pioneered reusable rockets (Falcon 9) and Starship; reusability slashes launch cost. Memory aid: 'IN-SPACe = promote/permit, NSIL = sell/commercialise, ISRO = research.'