Project Icarus (1967)

Walter Baade used the 48-inch telescope at Palomar Observatory to capture humankind's first image of asteroid 1566 Icarus on June 26, 1949. Icarus, it was soon found, is unusual because its elliptical orbit takes it from the inner edge of the Main Asteroid Belt beyond Mars's orbit to well within Mercury's orbit. Every 19 years Icarus and Earth pass within five million miles of each other at a relative velocity of about 18 miles per second. Baade detected Icarus during one of these close encounters.

MIT Professor Paul Sandorff taught the Interdepartmental Student Project in Systems Engineering in the Spring 1967 Term at Massachusetts Institute of Technology (MIT) in Boston. He noted that Icarus and Earth would pass each other at a distance of four million miles on June 19, 1968. He then asked his students to suppose that, instead of missing Earth on that date, Icarus would strike in the Atlantic Ocean east of Bermuda with the explosive force of 500,000 megatons of TNT. Debris flung into the atmosphere would cool the planet and a 100-foot wave would inundate MIT. Sandorff gave his class until May 27, 1967 to develop a plan for averting the catastrophe.

In 1967, the physical characteristics of Icarus were little known. For purposes of their study, Sandorff's students assumed that it measured 4200 feet in diameter and had a density of 3.5 grams per centimeter, yielding a mass of 4.4 billion tons. They acknowledged, however, that Icarus might be a defunct comet nucleus, in which case its density and mass would likely be considerably less.

In March 1967, the MIT students visited Cape Kennedy, Florida, to size up U.S. space capabilities. At the time, the first manned flight of the Apollo Command and Service Module (CSM) (bottom image above) had been postponed indefinitely following the January 27, 1967 Apollo 1 Fire and the Saturn V moon rocket (middle image above) had yet to fly. (Apollo 4, the successful first Saturn V test flight, would occur on November 9, 1967.) Nevertheless, the students wrote that "the awesome reality" of the Vertical Assembly Building (VAB), in which the Saturn V and Apollo would be prepared, and the twin Complex 39 pads, from which they would be launched, had "completely erased" any doubts they might have had about using Apollo/Saturn technology in their project.

Sandorff's students proposed to hijack Project Apollo, delaying NASA's first lunar landing by about three years. They would take over the first nine Saturn V rockets earmarked for the moon program, commence construction in April 1967 of a third Complex 39 launch pad, and add a high bay to the VAB, bringing the total to four. Three Saturn Vs would be used for flight tests, and the remainder would each launch toward Icarus one heavily modified unmanned Apollo CSM bearing a 44,000-pound nuclear bomb with a destructive yield of 100 megatons.

The Icarus CSM - which the MIT students dubbed the Interceptor - would comprise three modules: a drum-shaped propulsion module corresponding to the Apollo Service Module (SM), with attitude control thrusters and a Service Propulsion System (SPS) main engine; a drum-shaped payload module based on the SM structural design bearing the nuclear device; and a stripped-down Command Module (CM) containing Icarus detection sensors and an MIT-designed Apollo Guidance Computer. Unlike the two-module Apollo CSM, the three modules of the Interceptor would remain bolted together throughout its flight.

The first Project Icarus Saturn V (Saturn-Icarus 1) would leave Cape Kennedy on April 7, 1968, 73 days before the asteroid was due to collide with Earth. Its payload, Interceptor 1, would reach Icarus 60 days later, when Icarus was 20 million miles from Earth. At about the time Interceptor 1 was due to reach its target, the MIT Lincoln Laboratory's Haystack radar would detect Icarus for the first time.

Saturn-Icarus 2 would launch on April 22, 1968, 58 days before Icarus was due to strike. Interceptor 2 would reach its target 15.5 million miles and 10 days out from Earth. Saturn-Icarus 3 would lift off on May 6, 1968, 44 days before Icarus was due to arrive, and its Interceptor would reach Icarus when it was one week and 11 million miles out from Earth. Saturn-Icarus 4 would lift off on May 17, 1968, 33 days before Icarus arrival, and Interceptor 4 would reach the asteroid 28 days later, when Earth and Icarus were 7.7 million miles apart.

Saturn-Icarus 5 would leave Earth near dawn on the U.S. east coast on June 14, 1968, and Interceptor 5 would reach Icarus 1.4 million miles out from Earth, 22 hours before expected impact. By then, the asteroid would appear as a modest star in the pre-dawn sky near the constellation Orion. Saturn-Icarus 6 would lift off a few hours after Saturn-Icarus 5. Icarus would be about 20 hours and 1.25 million miles from impact when Interceptor 6 reached it.

As each Interceptor closed to within a quarter-million miles of Icarus, an optical sensor in its nose would spot the asteroid. Based on its data, the SPS and thrusters would adjust its course to ensure a successful interception.

As the Interceptor closed to a distance of 550 feet, a radar would detect Icarus and trigger the nuclear device, which would explode at a distance of from 50 to 100 feet. If the students' assumptions about Icarus's mass and density were correct, then each 100-megaton near-surface nuclear blast would excavate a bowl-shaped crater up to 1000 feet wide. The effect the explosions would have on Icarus's course was, of course, not known with precision; the students calculated that each blast would alter its velocity by between eight and 290 meters per second.

The MIT students acknowledged that an explosion might shatter Icarus; in that event, subsequent Interceptors would target the largest fragments. Data from each Interceptor as it approached Icarus and from Earth-based optical telescopes and radars would be used to target subsequent Interceptors as needed. Conversely, if fewer than six explosions were sufficient to deflect or destroy the asteroid, then the remaining Saturn V rockets and Interceptors would stand down.

All but one of the Interceptors would be joined at Icarus by a separately launched 540-pound Intercept Monitoring Satellite (IMS) based on the Mariner II design. Mariner II, the first successful interplanetary probe, had flown past Venus on December 14, 1962 (image below). In addition to data immediately useful for Project Icarus, the IMS would provide pure science data.

The first IMS would leave Earth atop an Atlas-Agena rocket on February 27, 1968. It would pass between 70 and 135 miles of Icarus at the time of the first explosion. This would place it outside of the zone of large high-velocity debris from the explosion, but within the zone of plasma, dust, and small debris so that it could gather data on Icarus's composition. A 50-pound foam-honeycomb bumper would shield the IMS during passage through the debris cloud.

No IMS would monitor the fifth interception (if it occurred) unless the sixth interception were called off. The IMS for monitoring the sixth (or fifth) interception would lift off on June 6, 1968, between the Saturn-Icarus 4 and 5 launches.

Professor Sandorff's class estimated that Project Icarus would cost $7.5 billion. It would, they calculated, stand a 1.5% chance of only fragmenting the asteroid. If this happened, then Icarus might cause even more damage to Earth than if it had been permitted to impact intact. The probability that the damage Icarus would cause would be reduced by their project's efforts was, however, 86%, and the probability that Project Icarus would succeed in preventing any part of the asteroid from reaching Earth was 71%.

Project Icarus, MIT Report No. 13, Louis A. Kleiman, editor, The MIT Press, 1968.