Satellite Orbits and Space Debris: LEO, GEO, SSO and Kessler

The correct answer is C, 35,786 km. Only at this height does a satellite in a circular equatorial orbit take exactly one sidereal day to complete a revolution, which is the condition for appearing stationary above one point on the Earth. The figure is commonly rounded to about 36,000 kilometres, and both forms are accepted in answers.

Option A, 3,578 km, is the same digits with a misplaced decimal and is included to catch hasty reading. Option B, 20,200 km, is the altitude of the Global Positioning System satellites, which lie in medium Earth orbit and have a period of about twelve hours, not one day. Option D is far too high and is closer to the scale used for Lagrange point missions; India's Aditya-L1, for instance, sits about 1.5 million kilometres away at the Sun-Earth point L1. Remember the trio: low Earth orbit up to 2,000 km, medium Earth orbit beyond that, and geostationary at 35,786 km.

The correct answer is B, one sidereal day, 23 hours 56 minutes 4 seconds. A geostationary satellite must match the rotation of the Earth with respect to the distant stars, and that rotation takes one sidereal day. Matching the solar day of twenty-four hours would leave the satellite drifting slowly, because the solar day includes the extra turn needed to face the Sun again as the Earth moves along its orbit.

Option A, exactly 24 hours, is the answer most candidates give and is accepted loosely in general conversation, but in a careful question the sidereal figure is the right one. Option C, 12 hours, is roughly the period of a navigation satellite in medium Earth orbit and of a Molniya orbit. Option D, about 90 minutes, is the period of a satellite in low Earth orbit, such as a space station at around 400 kilometres. The rule behind all of this is that orbital period increases with orbital height.

The correct answer is B, sun-synchronous polar orbit. Such an orbit is near-polar, typically 600 to 900 kilometres high with an inclination of about 98 degrees, which makes it retrograde. Its plane precesses at the same rate at which the Earth revolves round the Sun, so the satellite crosses a given latitude at the same local solar time on every pass. Shadows and illumination are therefore comparable, which is what makes change detection possible. India's Cartosat, Resourcesat and Oceansat satellites use it.

Option A, geostationary orbit, gives a constant view of one hemisphere and suits communication and weather satellites such as INSAT, but it is far too high for detailed mapping. Option C, Molniya orbit, is a highly elliptical orbit inclined at about 63.4 degrees used to give long dwell time over high latitudes. Option D, graveyard orbit, is not a working orbit at all; it is a disposal orbit above the geostationary belt where retired satellites are parked.

The correct answer is B, Arthur C. Clarke. The British science fiction writer and engineer set out in 1945 how three satellites placed in a circular equatorial orbit at about 36,000 kilometres could relay radio signals to almost the whole inhabited world. The geostationary orbit is therefore also called the Clarke Orbit, and the ring of satellites in it is sometimes called the Clarke Belt.

Option A, Isaac Asimov, was a famous science fiction writer but is not connected with this proposal. Option C, Konstantin Tsiolkovsky, was the Russian pioneer who worked out the rocket equation and the theory of spaceflight decades earlier, but not the geostationary relay idea. Option D, Jules Verne, imagined a journey to the Moon in the nineteenth century, long before orbital mechanics of this kind were applied. Clarke is the name to remember with 1945 and 35,786 kilometres.

The correct answer is B, Medium Earth Orbit. Global Positioning System satellites orbit at about 20,200 kilometres, which lies in the medium Earth orbit band stretching from 2,000 kilometres up to the geostationary height of 35,786 kilometres. At that altitude each satellite takes roughly twelve hours to go round, and a constellation of them keeps several satellites visible from any point on the Earth, which is what a receiver needs to fix its position.

Option A, Low Earth Orbit, is used by space stations and remote sensing satellites; a navigation satellite there would sweep past too quickly to be useful. Option C, Geostationary Orbit, is used by communication and weather satellites, and India's NavIC does use geostationary and inclined geosynchronous orbits because it is a regional system, but the American GPS is a medium Earth orbit constellation. Option D, Sun-synchronous Orbit, is a specialised low Earth orbit for imaging.

The correct answer is A, Donald J. Kessler in 1978. Kessler, then working with the American space agency, published the argument that once the population of objects in low Earth orbit crosses a certain density, collisions between them will generate fragments faster than the atmosphere can drag the old debris down. Each collision then feeds the next one, and the cascade can continue even if no further launches take place.

Option B keeps the right person but the wrong year, which is the commonest error in this question, so the pairing of Kessler with 1978 should be memorised together. Option C, Hermann Oberth, was one of the founding theorists of rocketry and had nothing to do with debris modelling, and 1957 is the year of Sputnik 1. Option D, Wernher von Braun, led the development of the Saturn V rocket, and 1969 is the year of the first Moon landing. Both wrong options are built out of famous but unrelated space dates.

The correct answer is B, Microsat-R. In Mission Shakti an interceptor developed by the Defence Research and Development Organisation was launched from Dr. A. P. J. Abdul Kalam Island off the Odisha coast and destroyed India's own Microsat-R satellite in low Earth orbit at a height of about 283 kilometres. The low altitude was chosen on purpose so that the fragments would lose energy and re-enter the atmosphere quickly rather than linger in orbit.

Option A, Cartosat-2, is a working Indian remote sensing satellite and was never a target. Option C, RISAT-2B, is a radar imaging satellite launched a few months after the test. Option D, Fengyun-1C, is the Chinese weather satellite destroyed in China's own anti-satellite test of January 2007, which produced the largest cloud of trackable debris in history and is the usual distractor here. With Mission Shakti India became the fourth country after the United States, Russia and China to demonstrate this capability.

The correct answer is B, tracking and analysis of objects in space, including debris. NETRA stands for Network for Space Objects Tracking and Analysis. It is ISRO's programme to build radar, optical telescope and control centre capability for space situational awareness, so that India can detect, track and catalogue objects in orbit and warn its own satellites about possible close approaches instead of depending entirely on data from abroad. It works alongside IS4OM, the ISRO System for Safe and Sustainable Operations Management, set up at Bengaluru.

Option A is wrong because human spaceflight preparation comes under the Gaganyaan programme and its astronaut training facility. Option C is wrong because crop insurance support uses remote sensing data under separate schemes of the agriculture ministry. Option D is wrong because launching navigation satellites is a mission activity; NETRA is a surveillance and safety network, not a launch programme.

The correct answer is A, Iridium 33 and Kosmos 2251. In February 2009 the working American communication satellite Iridium 33 and the defunct Russian military communication satellite Kosmos 2251 struck each other over Siberia at a relative speed of several kilometres per second. Both were destroyed and the event created thousands of trackable fragments, proving that an accidental collision between whole satellites was no longer a theoretical worry.

Option B names two European Earth observation satellites that never collided. Option C pairs the targets of the Chinese and Indian anti-satellite tests, which were deliberate destructions of a country's own satellite, not an accident, and they happened in different years. Option D names the first Soviet and first American satellites of 1957 and 1958, which were in orbit at very different times and were tiny by comparison. Remember 2007 for Fengyun-1C, 2009 for Iridium-Kosmos and 2019 for Mission Shakti.

The correct answer is B, a few hundred kilometres above the geostationary belt. A communication satellite nearing the end of its life keeps a small reserve of fuel to raise itself out of the crowded geostationary ring into a higher orbit where it cannot drift back into a working slot or collide with an active satellite. Bringing such a satellite all the way down for re-entry would need far more fuel than raising it, which is why disposal upward is the accepted practice at that altitude.

Option A describes the opposite approach, used for low Earth orbit satellites, which are lowered so that atmospheric drag pulls them down for re-entry. Option C is wrong because the sun-synchronous band is a busy working region for imaging satellites and nothing is parked there deliberately. Option D is wrong because the Lagrange points are used for science missions such as space observatories, not for dumping retired hardware.

The correct answer is C, the Liability Convention, 1972. Its full name is the Convention on International Liability for Damage Caused by Space Objects, and it fixes responsibility on the state that launches or procures the launch of a space object. Liability is absolute for damage caused on the surface of the Earth or to aircraft in flight, and fault based for damage caused elsewhere, such as a collision between two satellites in orbit.

Option A, the Outer Space Treaty of 1967, is the foundational instrument that declares space free for exploration by all, bans weapons of mass destruction in orbit and bars national appropriation of celestial bodies; it states the principle of state responsibility but the detailed liability rules came later. Option B, the Rescue Agreement of 1968, deals with the return of astronauts and space objects. Option D, the Moon Agreement of 1979, covers activities on the Moon and other bodies and has very few parties.

The correct answer is C, 25 years. The long-standing guideline of the Inter-Agency Space Debris Coordination Committee, formed in 1993 and including ISRO among its member agencies, is that a spacecraft or upper stage leaving the low Earth orbit protected region should be de-orbited, or placed in an orbit from which it will re-enter, within twenty-five years after the mission ends. Along with this goes passivation, that is venting leftover propellant and discharging batteries so that the hardware cannot explode and create fresh fragments.

Option A, 5 years, and option B, 10 years, are shorter than the accepted figure, although newer national rules in some countries now push towards tighter timelines. Option D, 50 years, is far too long to be useful, because debris lingering that long in a crowded band keeps the collision risk high. For geostationary satellites the corresponding practice is not de-orbiting but raising the satellite into a graveyard orbit above the belt.