|Image credit: NASA.|
Every nine, 19, or 28 years, always during the month of June, Icarus and Earth reach their point of closest approach, during which they typically pass each other at a relative velocity of about 29 kilometers (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 the Massachusetts Institute of Technology (MIT). At the beginning of the course, he told his students that, on 19 June 1968, Icarus and Earth would pass each other at a distance of 6.4 million kilometers (four million miles) — that is, about 15 times the Earth-moon distance.
Sandorff then asked his students to suppose that, rather than miss Earth on that date, Icarus would instead strike 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 to some unknown degree and a 30-meter (100-foot) wave would inundate MIT. Sandorff gave his class until 27 May 1967 to develop a plan to avert the catastrophe.
In 1967, the physical characteristics of Icarus were poorly known. For purposes of their study, Sandorff's students assumed that it measured 1280 meters (4200 feet) in diameter and had an average density of 3.5 grams per centimeter, yielding a mass of 4.4 billion tons. For comparison, Earth has an average density of 5.5 grams per cubic centimeter.
They acknowledged, however, that, given its orbit, which resembles that of a short-period comet, Icarus might be a defunct comet nucleus. In that case, its density and mass would likely be considerably less. They also assumed that Icarus is a solid body; that is, that it is not made up of small pieces held together loosely by weak mutual gravitational attraction.
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) spacecraft had been postponed indefinitely following the Apollo 1 fire (27 January 1967) and the Saturn V Moon rocket had yet to fly. Apollo 4, the successful first Saturn V test flight, would not occur until 9 November 1967.
Nevertheless, the students wrote in their final report that "the awesome reality" of the giant structures NASA had built to launch men to the Moon had "completely erased" any doubts they might have had about using Apollo/Saturn technology in their project. The structures included the Vertical Assembly Building (VAB), in which Apollo spacecraft and three-stage Saturn V moon rockets were stacked together, and the twin Launch Complex 39 Saturn V launch pads (Pads 39A and 39B). One cannot help but wonder what their fall-back alternative might have been had they found the Apollo infrastructure wanting.
|Apollo 11 liftoff on 16 July 1969. If Project Icarus had been necessary, the Apollo 11 Saturn V would have launched the unmanned Saturn-Icarus 3 Interceptor, not the first manned Moon-landing mission. Image credit: NASA.|
Three of the nine Saturn V rockets would have been used for unmanned flight tests. The remainder would each have launched toward Icarus one heavily modified unmanned Apollo CSM bearing an 20,000-kilogram (44,000-pound) nuclear warhead with the destructive yield of 100 million tons of TNT.
Though the MIT students did not mention it, a 100-megaton warhead was never a component of the U.S. nuclear arsenal. Given the secrecy surrounding nuclear weapons during the Cold War, they probably would not have known that no 100-megaton warhead had ever been tested.
The most powerful nuclear bomb ever, the Soviet Union's 50-megaton "Tsar Bomba," exploded on 30 October 1961, triggering seismic sensors around the globe. Fifty megatons was about half its theoretical yield. The Soviet Union built only a single Tsar Bomba and the U.S. did not deign to match the Soviet feat.
Even had Tsar Bomba warheads been readily available, the Soviet super-bomb was likely so heavy that a Saturn V could not launch it to Icarus. It weighed as much as 27,000 kilograms (60,000 pounds).
|The Apollo 14 Saturn V rocket rolls out of the immense VAB at Kennedy Space Center. Had Project Icarus been necessary, the rocket would have launched the unmanned Saturn-Icarus 6 Interceptor on 14 June 1968. Image credit: NASA.|
The first Project Icarus Saturn V (Saturn-Icarus 1) would lift off from Cape Kennedy on 7 April 1968, 73 days before the asteroid was due to collide with Earth. Its payload, Interceptor 1, would reach Icarus 60 days later, when the asteroid was 13 days and 32.2 million kilometers (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 22 April 1968, 58 days before Icarus was due to strike. Interceptor 2 would reach its target 25 million kilometers (15.5 million miles) and 10 days out from Earth.
Saturn-Icarus 3 would lift off on 6 May 1968, 44 days before Icarus was due to arrive, and its Interceptor would reach Icarus one week and 17.7 million kilometers (11 million miles) from Earth. Saturn-Icarus 4 would lift off on 17 May 1968, 33 days before Icarus arrival, and Interceptor 4 would reach the asteroid 28 days later, when Earth and Icarus were 12.4 million kilometers (7.7 million miles) apart.
Saturn-Icarus 5 would leave Earth near dawn on the U.S. East Coast on 14 June 1968, and Interceptor 5 would reach Icarus 2.26 million kilometers (1.4 million miles) out from Earth, 22 hours before expected impact. By then, the asteroid would appear as a modest star in the predawn sky near the constellation Orion.
Saturn-Icarus 6 would lift off a few hours after Saturn-Icarus 5. When Interceptor 6 reached it, Icarus would be about 20 hours and 2 million kilometers (1.25 million miles) from impact.
As each Interceptor closed to within 400,000 kilometers (250,000 miles) of Icarus, an optical sensor in its nose would spot the asteroid. The modified AGC would then use the SPS and thrusters in the propulsion module to adjust the Interceptor's course to ensure a successful interception.
The MIT students acknowledged that Icarus might shatter; 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 required. Conversely, if fewer than six explosions were sufficient to deflect or pulverize the asteroid, then the remaining Saturn V rockets and Interceptors would stand down.
|The Project Icarus Intercept Monitoring Satellite (IMS) would have resembled NASA's Mariner 2 Venus flyby spacecraft. Image credit: NASA.|
The first IMS would leave Earth atop an Atlas-Agena rocket on 27 February 1968. It would pass between 115 and 220 kilometers (70 and 135 miles) from Icarus at the time of the first nuclear 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. IMS-1 would analyze the small fragments and hot gases to determine the asteroid's composition. A 23-kilogram (50-pound) foam-honeycomb "bumper" would shield IMS-1 during passage through the debris cloud.
No IMS would monitor the fifth interception (if it were judged necessary) unless the sixth interception had already been called off. The IMS for monitoring the sixth (or fifth) interception would lift off on 6 June 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 more damage to Earth than if it were permitted to impact intact. The probability that Project Icarus would reduce the damage Icarus would cause was, however, 86%, and the probability that it would succeed in preventing any part of the asteroid from reaching Earth was 71%.
During the June 1968 close approach, Icarus became the first asteroid to be detected using Earth-based radar. By analyzing data gathered during subsequent close approaches, scientists have found that Icarus is roughly spherical, rotates rapidly (about once every 2.25 hours), is probably a light-colored S-type asteroid made mostly of stony materials, and measures about 1400 meters (4600 feet) across. Its density is probably only about 2.5 grams per cubic centimeter.
The closest approach of Icarus to Earth since 19 June 1968 is taking place as I write this. On 16 June 2015, the asteroid will pass by Earth at a distance of about eight million kilometers (five million miles). It will zip through the northern-hemisphere constellations Ursa Major and Canes Venatici over the course of the day, though it will be too faint to view with unaided eyes. Closest approach to Earth will take place at 1539 UTC (11:39 AM U.S. Eastern Daylight Time). Icarus will not pass so close to Earth again until June 2090.
Project Icarus, MIT Report No. 13, Louis A. Kleiman, editor, The MIT Press, 1968.
Tsar Bomba: King of Bombs (http://www.tsarbomba.org/ — accessed 15 July 2015).
International Astronomical Union — Near Earth Asteroids: A Chronology of Milestones 1800-2200 (http://www.iau.org/public/themes/neo/nea/ — accessed 15 July 2015).
Earth Approaching Asteroids as Targets for Exploration (1978)
Multiple Asteroid Flyby Missions (1971)
To Mars By Way of Eros (1966)
Centaurs, Soviets, and Seltzer Seas: Mariner 2's Venusian Adventure (1962)