MIT Saves the World: Project Icarus (1967)

Image credit: NASA.
Walter Baade used the 48-inch reflecting telescope at Palomar Observatory in southern California to capture humankind's first image of asteroid 1566 Icarus on 26 June 1949. Icarus showed up as a nondescript streak set against the myriad stars on a glass photographic plate. Icarus, it was soon found, is unusual because its elliptical orbit takes it from the inner edge of the Main Asteroid Belt between the orbits of Mars and Jupiter to well within Mercury's orbit. Icarus needs 1.12 years to circle the Sun once. 

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 piloted 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 astronauts 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 automated Saturn-Icarus 3 Interceptor, not the first piloted Moon-landing mission. Image credit: NASA.
Professor Sandorff's students proposed to hijack Project Apollo, delaying NASA's first piloted lunar landing by about three years. They would have taken over the first nine Saturn V rockets earmarked for the Moon program, commenced construction in April 1967 of a third Launch Complex 39 launch pad (Pad 39C), and added a Saturn V assembly high bay to the VAB, bringing the total to four. NASA had planned to build Pad 39C, going so far as to build a road to the proposed pad site with appropriate signage (image at top of post), until funding cuts made an ambitious post-Apollo piloted space program increasingly unlikely.

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 automated 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 could 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 automated Saturn-Icarus 6 Interceptor on 14 June 1968. Image credit: NASA.
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's structural design but containing the 100-megaton nuclear device; and a stripped-down conical Command Module (CM) containing Icarus detection sensors and an MIT-designed Apollo Guidance Computer (AGC) modified for automated operation. Unlike the two-module Apollo CSM, the three modules of the Interceptor would have remained bolted together throughout its flight.

The first Project Icarus Saturn V (Saturn-Icarus 1) would have lifted off from Cape Kennedy on 7 April 1968, 73 days before the asteroid was due to collide with Earth. Its payload, Interceptor 1, would have reached 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 have detected Icarus for the first time.

Saturn-Icarus 2 would have launched on 22 April 1968, 58 days before Icarus was due to strike. Interceptor 2 would have reached its target 25 million kilometers (15.5 million miles) and 10 days out from Earth.

Saturn-Icarus 3 would have lifted off on 6 May 1968, 44 days before Icarus was due to arrive, and its Interceptor would have reached Icarus one week and 17.7 million kilometers (11 million miles) from Earth. Saturn-Icarus 4 would have lifted off on 17 May 1968, 33 days before Icarus arrival, and Interceptor 4 would have reached the asteroid 28 days later, when Earth and Icarus were 12.4 million kilometers (7.7 million miles) apart.

Saturn-Icarus 5 would have left Earth near dawn on the U.S. East Coast on 14 June 1968, and Interceptor 5 would have reached Icarus 2.26 million kilometers (1.4 million miles) out from Earth, 22 hours before expected impact. By then, the asteroid would have appeared as a modest star in the predawn sky near the bright stars of the constellation Orion.

Saturn-Icarus 6 would have lifted off a few hours after Saturn-Icarus 5. When Interceptor 6 reached it, Icarus would have been 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 have spotted the asteroid. The modified AGC would then have used the SPS and thrusters in the propulsion module to adjust the Interceptor's course to ensure a successful interception.

Apollo astronauts grew fond of the simple but capable MIT-developed Apollo Guidance Computer (AGC). For Project Icarus, MIT would have added an extra layer of automation so that the AGC could guide the unmanned Interceptor spacecraft to their target. Image credit: Wikipedia.
When an Interceptor closed to a distance of 170 meters (550 feet) from Icarus, a radar would have detected the asteroid and triggered the nuclear device, which would have exploded at a distance of from 15 to 30 meters (50 to 100 feet). If the students' assumptions about the asteroid's mass and density were correct, then each 100-megaton near-surface nuclear blast would have excavated on Icarus a bowl-shaped crater up to 300 meters (1000 feet) wide. The effect the explosions would have had on the asteroid's course was, of course, not known with precision; the students calculated that each blast would alter its velocity by between 8 and 290 meters (26 and 950 feet) per second.

The MIT students acknowledged that Icarus might shatter; in that event, subsequent Interceptors would have targeted the largest fragments. Data from each Interceptor as it approached Icarus and from Earth-based optical telescopes and radars would have been 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 have stood down.

The Project Icarus Intercept Monitoring Satellite (IMS) would have resembled NASA's Mariner 2 Venus flyby spacecraft. Image credit: NASA.
All but one of the Interceptors would be joined at Icarus by a separately launched 245-kilogram (540-pound) Intercept Monitoring Satellite (IMS) based on the Mariner 2 design. Mariner 2, the first successful interplanetary probe, had flown past Venus on 14 December 1962. In addition to data immediately useful for Project Icarus, the IMS would have provide pure science data.

The first IMS would have left Earth atop an Atlas-Agena rocket on 27 February 1968. It would have passed between 115 and 220 kilometers (70 and 135 miles) from Icarus at the time of the first nuclear explosion. This would have placed 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 have analyzed the small fragments and hot gases to determine the asteroid's composition. A 23-kilogram (50-pound) foam-honeycomb "bumper" would have shielded IMS-1 during passage through the debris cloud.

No IMS would have monitored 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 have lifted 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 over the decades 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 ( — accessed 15 July 2015).

International Astronomical Union — Near Earth Asteroids: A Chronology of Milestones 1800-2200 ( — accessed 15 July 2015).

More Information

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)


  1. The Tsar Bomba was capable of reaching 100MT, but would have released so much fallout, and also potentially destroyed the carrier aircraft, that the Soviets reconfigured it (by using a lead rather than U238 tamper) so that it only had an output of 50MT for it's only test.

  2. Thanks for this info. I find conflicting claims re: Tsar Bomba, including one that the design couldn't have reached 100 MT. But I confess that I do not know enough about nuclear weapon design to judge the various claims. I'll tweak the language a bit.

    One source I used said no one knows how much Tsar Bomba weighed. I came across the 60K lbs figure in a couple of places. Would you say that's a reasonable weight for such a device?


    1. Hi David,

      Tsar Bomba was delivered by a Tupolev 95; max weight for that aircraft is around 400,000 lbs give or take, with an aircraft weight around 200,000 lbs empty. Payload was normally 30,000 BUT the dropping aircraft was modified. So there was certainly some capacity for a 60,000 lb "device" to be carried; given the specs for the Tu-95 I imagine that 60,000 lbs is the upper weight limit, then once you factor in the USSR's penchant for slight exaggeration (cough!) I'd say it was probably closer to 50,000 lbs.

      Tsar Bomba was also a one-off device. 50 MT+ nukes weren't in the standard inventory of either nation, something that the Project Icarus kids might not have been aware of at the time.

  3. Thanks for the link! I wouldn't know for sure, but my sense is that the US opted for efficacy over sheer explosive power fairly early on. That would include device mass reduction to enable use of smaller missiles, MIRVs. I don't know when development would have been curtailed, though certainly by end of 1980s.



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