Icarus falling: Apollo nukes an asteroid

The proposal to deflect Icarus would have used several Saturn V rockets equipped with 100-megaton bombs encased in fairings like that used for the Skylab launch. (credit: NASA)

Apollo Revisited



In the late 1960s, as the Apollo program was in full-swing, a group of engineers in training at the Massachusetts Institute of Technology designed a defense against an asteroid heading toward Earth. Their plan would have involved a half-dozen Saturn V rockets carrying some really big bombs, aimed at an asteroid named Icarus.

Periodically, the large asteroid 1566 Icarus swings by planet Earth, often coming within 6.4 million kilometers of the planet—mere spitting distance in astronomical terms. Icarus last passed by Earth in 2015. It also crosses the orbits of Mars, Venus and even Mercury.

In early 1967, MIT professor Paul Sandorff gave his class of graduate students a task: suppose that instead of passing harmlessly by, Icarus was instead going to hit the Earth. The nearly 1.4-kilometer wide chunk of rock would hit the planet with the force of 500,000 megatons—far larger than any major earthquake or volcanic eruption, and over 33,000 times the size of the bomb that destroyed Hiroshima. At a minimum, it would kill millions, flattening buildings and trees for a radius of hundreds of kilometers. The dust it kicked into the atmosphere could even lead to a global winter lasting years. Sandorff posed a simple challenge: You have 15 months. How do you stop Icarus?

MIT was then deeply involved in the Apollo program. The guidance system for the Apollo spacecraft was developed there and the country’s foremost experts in aviation and space walked the school’s halls. Sandorff’s proposal was intended to teach his students how to improvise under pressure.

The class immediately split up into several working groups based upon their areas of expertise: orbits and trajectories, boosters and propulsion, spacecraft, guidance and control, communications, economics and management, and nuclear payloads. They began evaluating the different options for defeating the killer space rock.

Could they launch a big bomb to the asteroid and blow it to pieces? Quick calculations showed that pulverizing a rock the size of Icarus would require a 1,000-megaton bomb. No nuclear weapon even remotely that big had ever been theorized, let alone designed or built. There was no way it could be done in the short time available. Using a bunch of smaller bombs was also not possible because they would all have to be detonated at exactly the same time. Otherwise, one bomb would vaporize the others before they detonated.

The most desirable option would be to rendezvous with Icarus when it reached aphelion—the slowest point in its orbit—in November 1967. At that point it would be easiest to rendezvous with the asteroid and easiest to exert force to change its orbit. But such a mission would have had to be launched immediately, in spring 1967, and so it was out of the question. The group quickly determined that no rockets could conceivably be readied before 1968 and this greatly constrained their options. A slow rendezvous, or even a soft landing, was totally out of the question: Icarus would be moving too fast by 1968 for a spacecraft to reach it and then rendezvous.

The only option was a fast intercept—fly out to Icarus and detonate a bomb near the surface to change its course.

The best way to get the most payload to Icarus was to launch two modified Saturn V rockets into orbit. These would rendezvous with an Apollo “space tug” launched atop a Titan III rocket. The space tug would connect up the modified S-IVB third stages from the Saturns. They would then be used to push a relatively large spacecraft out to Icarus where it would detonate a large nuclear weapon.

But there were many problems with this proposal. The Saturn S-IVB third stages were not designed to carry fuel in orbit for more than six hours and would require extensive modification. A spacecraft would also have to be designed from scratch and built in under a year. Most importantly, the on-orbit operations required to link up the large craft were extensive and unproven. There would be no way to practice. The students rejected this plan.

What the group decided to do was to take six Saturn V rockets then in production, and with only minimal modifications to their payloads use them to carry smaller bombs to Icarus. The first launch would have to take place by April 1968, only a year away, and five more launches would have to follow at two-week increments.

The actual Icarus spacecraft would have consisted of an Apollo Service Module (SM) with a five-foot (1.5-meter) cylindrical extension known as the Payload Module (PM) at the top. Instead of a Command Module, the top of the stack would be a simple aluminum cone containing a few necessary systems. Although the Apollo Command Module and its associated guidance and control systems would have been useful, its weight was prohibitive and unnecessary. Weight had to be kept to a minimum in order to enable the rocket to carry the biggest possible bomb.

The Payload Module would have carried a 100-megaton bomb shaped as a cylinder roughly three feet (0.9 meters) in diameter and mounted horizontally along the diameter of the spacecraft. The bomb would weigh 18,150 kilograms. One side of the PM would sport a phased array radar antenna for tracking and rendezvous with the Icarus asteroid.

The plan would have used an essentially unmodified Saturn V rocket. At the time, the first Saturn V test was not scheduled until November 1967 and the planners did not know if it would work. The only real difference with the Icarus Saturn V was the modified adapter shroud at the top of the S-IVB third stage. On a standard Apollo mission to the Moon, these panels normally would have enclosed the Lunar Module, with the Service Module and Command Module mounted on top. But by modifying them and using them to enclose the entire Service Module and its attached Payload Module, the designers were able to improve the aerodynamics of the rocket, and more importantly, eliminate aerodynamic loads and heating on the radar antenna. In profile, the stack would have looked much like the Skylab launch vehicle lofted by the Saturn V in 1973, although slightly shorter.

The 100-megaton bomb would have been a challenge to design and build. At the time, the largest weapon ever developed for the American nuclear arsenal was a 25-megaton bomb. The Soviets had detonated a 58-megaton bomb earlier in the decade which could have easily been developed into a 100-megaton weapon. It is likely that their 100-megaton bomb would have weighed far more than the 18,150-kilogram weight limit for Icarus, so importing a Soviet warhead to save the world was a non-option.

The Icarus plan required a total of nine Saturn V rockets. Three were test flights and the remaining six were interceptors. At the time, NASA planned on having only six Saturn V rockets available by April 1968, so the production schedule would have to be dramatically increased. In addition, another launch pad would be needed at Cape Kennedy. Launch Complex 39C would have to be built in order to enable the high flight rate needed for the Saturn launches, all of which had to get off the ground in six weeks.

In addition to the nine Saturn Vs, the Icarus plan called for five Atlas Agena rockets carrying modified versions of the Mariner 2 deep space probe. Known as the Intercept Monitoring Satellite (IMS), these probes would be used to observe the detonation of the nuclear bombs when they reached the asteroid. Very little was known about how nuclear weapons would behave in space, let alone how the blast would affect an asteroid, and so the IMS was considered vital to the mission.

In late February 1968, the first IMS spacecraft would lift off atop its Atlas Agena booster. It would linger in Earth orbit only a short time before being sent on its way to rendezvous with Icarus. A little over a month later, Interceptor One would thunder aloft on 33 million newtons of thrust. After a coast of one orbit or less, the S-IVB stage would fire, boosting the Icarus spacecraft out of Earth orbit and toward the asteroid. Soon after, the adapter shroud panels would peel back like the petals of a flower and the Icarus spacecraft with its 100-megaton bomb would separate. Its Service Propulsion System engine would fire, adding more velocity to the spacecraft. Interceptor Two would follow a week later.

After a coast of approximately 60 days, with several course corrections along the way, an optical sensor aboard the spacecraft would acquire Icarus only three hours before rendezvous. The spacecraft would then enter the “terminal phase.” Four minutes before rendezvous the radar system would begin to supply range information for making final correction maneuvers. At five seconds before impact, a fusing radar would acquire the asteroid and arm the bomb. If all went as planned, detonation would occur within thirty meters of the surface of Icarus along the sunlit edge. The resulting explosion would either fragment or deflect the asteroid off its collision course with Earth.

The planners proposed six bombs for the mission. But they faced huge unknowns. The biggest problem was that nobody knew exactly what asteroids in general, and Icarus in particular, were made of. Was Icarus dense or light? Exactly how big was it? How was it shaped?

The Icarus project’s legacy was primarily to spawn a lousy 1970s movie called Meteor! (complete with exclamation mark) which was scientifically ridiculous, like every killer asteroid movie that followed it. But Sandorff’s class demonstrated that if you put enough smart people in a room and give them some tools, they can improvise in impressive ways. Maybe they can even save the Earth.


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