Which type of orbit is normally used by Earth observation and remote sensing satellites so that images are taken under similar lighting every day?

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, 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.