(a) A hypersonic missile is launched into space to counter the asteroid approaching the Earth and explode it in space.

(b) A spacecraft lands on another planet, after making several orbital motions.

(c) A missile is put into a stable orbit around the Earth and de-orbits over a target on the Earth.

(d) A spacecraft moves along a comet with the same speed and places a probe on its surface.

Ans: c

Explanation:

Fractional orbital bombardment systems (FOBS)  is a term coined by the West during the Cold War for a Soviet innovation that would exploit limitations in the US Ballistic Missile Early Warming (BMEW) radar system at that time.

The concept of the FOBS was to insert a thermonuclear warhead into a steeply inclined low altitude polar orbit, such that it would approach the continental United States (CONUS) from any direction, but primarily from the southern hemisphere.

The idea behind FOBS is, a warhead is put into a stable orbit and it deorbits over the target. The warhead would employ retrorockets to break itself out of orbit and fall on the target from a direction not covered by BMEW.

 The FOB system is distinguished from an ICBM delivery system in that an ICBM take a ballistic trajectory that reaches an altitude of approximately 800 miles (1,300 kilometers) and releases its warhead or warhead bus, which then descends on a ballistic trajectory to its target

A FOBS trajectory launches a warhead and its delivery vehicle into an orbit of around 100 miles (160 kilometers) but breaks from that orbit before it completes a full revolution of the Earth.  

FOBS , ICBM  &MOBS

The FOB system is distinguished from an ICBM delivery system in that an ICBM take a ballistic trajectory that reaches an altitude of approximately 800 miles (1,300 kilometers) and releases its warhead or warhead bus, which then descends on a ballistic trajectory to its 

Another concept explored by the Soviet Union was the multiple orbit bombardment system (MOBS), also called the nuclear-armed bombardment system (NABS), where a vehicle carrying either a single or several warheads would make one or more complete orbits of the Earth before it delivered its payload. MOBS are differentiated from FOBS in as their weapons complete one or more orbits before descending on its target.

A fractional orbital bombardment system has several advantages over traditional ballistic missiles: its range covers the entire Earth; it can execute an attack from a much larger number of directions; and the time required for payload delivery can be many minutes shorter than for a comparable ICBM payload. 

Fractional orbital bombardment systems are inaccurate delivery mechanisms. The large velocity of the payload during re-entry and subsequent delivery creates a significant degree of error in targeting.

Unlike error-prone bombardment systems, a hypersonic glide vehicle can maneuver and adjust its flight path precisely after re-entry into the atmosphere. It can sustain atmospheric flight for an extended period (a “glide”), or even exit and re-enter the atmosphere later (a “skip”). Non-ballistic atmospheric entry enables hypersonic glide vehicles to avoid interception by following unpredictable flight paths, and to fly below the ceilings of radar detection systems. 

A fractional orbital hypersonic delivery system combines these advantages with orbital bombardment to create the best of both worlds. A fractional orbital hypersonic delivery system can deliver payloads up to 10 minutes faster than ICBMs. The trajectories of fractional orbital hypersonic delivery systems are difficult to track, both because they stay closer to the Earth and therefore below the floor of missile-defense radars, and also because of the increased manoeuvrability of the hypersonic glide vehicle payload.

Hence, option c is correct.

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