Perhaps 100 tons of interplanetary debris strike Earth's atmosphere every day. Some of this debris is seen in the night sky as shooting stars or, occasionally, as bright bolides that momentarily light the landscape. Many small asteroids burn up or explode high in the atmosphere. Very occasionally, an asteroid explodes low enough in the atmosphere that the blast wave reaches the ground, sometimes causing damage or even injuries. This is what happened in Russia, at Chelyabinsk, on 15 February 2013.
Even less frequently, one strikes Earth's surface more or less intact. A subset of these strike with enough energy to blast out an impact crater containing rare shocked minerals. On the scale of tens of millions of years, asteroids large enough to create craters readily visible from space strike Earth. Some of these - but by no means all - have been linked to mass extinctions.
People in the early 21st century have been encouraged to see asteroids as the interplanetary equivalent of sea monsters. We often hear talk of "killer asteroids," when in fact there exists no conclusive evidence that any asteroid has killed anyone in all of human history. The Tunguska airburst of 1908 might have caused deaths; this is, however, not known with certainty, for the explosion occurred over a remote area with a partially transient population. Compared with common daily dangers that people readily accept - for example, the risk of death by automobile accident - asteroids are positively benign.
In the 1970s, asteroids had yet to gain their present fearsome reputation. Some people had thought about the possibility that a large asteroid might strike Earth; in 1968, for example, students at Massachusetts Institute of Technology developed as a class assignment a plan for deflecting or destroying a menacing Near-Earth Asteroid (NEA) using Saturn V-launched nuclear bombs. However, most astronomers and planetary scientists who made a career of finding and studying asteroids rightly saw them as sources of fascination, not of worry.
In January 1978, NASA's Office of Space Science (OSS) sponsored a workshop at the University of Chicago to assess the state of asteroid studies and consider options for the future. The University of Chicago's Edward Anders and David Morrison of NASA OSS co-chaired the meeting. Fifteen invited participants presented papers - some written with collaborators - and talked about them. In June 1978, NASA published Asteroids: An Exploration Assessment, which included the papers and edited transcripts of the discussions.
Among the papers heard and discussed was one by Eugene Shoemaker and Eleanor Helin, both of the California Institute of Technology (Caltech). In it, Shoemaker and Helin called NEAs "targets for exploration" for both robotic and piloted spacecraft.
Shoemaker, an invited participant in the workshop, was a giant in the field of planetary geology in general and asteroids studies in particular. He had, among other accomplishments, been instrumental in the Ranger, Surveyor, and Lunar Orbiter robotic missions and in training Apollo astronauts to perform geologic investigations on the moon. Helin, Shoemaker's collaborator, was a pioneering female scientist in a field still dominated by men. In 1973, they had jointly established the Palomar Planet-Crossing Asteroid Survey to hunt for NEAs.
Their observing program had been inspired in part by the International Astronomical Union's March 1971 Tucson minor planets workshop. The Tucson workshop had not been kind to proponents of missions to asteroids; in fact, largely as the result of a persuasive paper by Anders, a consensus had emerged that launching spacecraft to asteroids would be "premature." Several of the Chicago workshop's invited participants, including Shoemaker and John Niehoff, a mission planner with Science Applications Incorporated in suburban Chicago, aimed to shape a new consensus.
In their paper, Shoemaker and Helin noted that most NEAs originated as fragments of larger asteroids in the Main Belt between Mars and Jupiter. The Main Belt contains about 95% of the Solar System's asteroids, including about 220 larger than 100 kilometers wide. The remaining NEAs are probably burnt-out comet nuclei. Shoemaker and Helin argued that the presence of NEAs meant that NASA could sample a variety of Main Belt asteroids without leaving Earth's vicinity. From the dead comets, NEA missions might extract "the most direct evidence obtainable concerning the early stages of accretion of solid matter in the solar system."
Of the NEAs discovered by mid-1977, Shoemaker and Helin estimated that spacecraft could rendezvous with and return from about one in 10 using less propulsive energy than is necessary to reach Mars. Because even the most massive NEA - 35-kilometer-wide 1036 Ganymed, discovered in 1924 - has a very low surface gravity, landing and takeoff would require very little energy. This meant that a single spacecraft could sample multiple sites on any given NEA.
|433 Eros was the first Near-Earth Asteroid discovered (13 August 1898) and, a century later, the first explored by an orbiting spacecraft. Image credit: NASA|
The Caltech scientists expected that NASA would find from seven to 10 Space Shuttle launches, each capable of placing about 13,640 kilograms of hardware and rocket propellants into Earth orbit, a reasonable number to dedicate to a piloted asteroid mission. This placed an upper limit on the quantity of propellants the spacecraft could expend to reach the asteroid. Acknowledging that the space environment might have adverse effects on human health and that a stepwise approach to extending biomedical experience would be wise, they imposed on their piloted asteroid mission a maximum round-trip duration of one year. This would include a 30-day stay at the destination asteroid.
Perhaps 1% of NEAs could provide opportunities for piloted missions that would meet Shoemaker and Helin's stringent criteria. This meant that, unless the NEA discovery rate immediately increased five-fold, no opportunity to launch "astronaut-scientists" to an NEA was likely to occur within a decade of the Chicago workshop.
Shoemaker and Helin cited a 1977 Niehoff study which showed that, if NASA were instead willing to dedicate from 28 to 31 Shuttle launches to a piloted NEA mission, then a round-trip voyage to 2062 Aten would last just six months. The 21 to 24 additional Shuttle flights would launch additional propellants for the piloted NEA spacecraft. A six-month round-trip voyage to 1943 Anteros would need 34 Shuttle launches. If, however, stay-time at Anteros were reduced to 10 days, it would need only 23. Additional propellants would, of course, also increase the number of opportunities available for astronauts to reach and return from an NEA in a year.
The Caltech scientists suggested that "more sophisticated strategies" be developed for piloted asteroid missions, such as pre-positioning Earth-return propellants at the target asteroid using robotic spacecraft that would follow fuel-efficient low-energy trajectories. They hastened to add, however, that the most economical near-term approach to preparing for a piloted NEA mission would be to increase funding support for Earth-based asteroid hunters so that they could use telescopes to add to the list of known accessible asteroids.
Shoemaker and Helin concluded their paper by stating that, while piloted asteroid missions were "highly appropriate," they did not believe that they should be carried out for scientific reasons alone. If science were the only objective, then many robotic asteroid missions could accomplish more than one or a few piloted missions. Rather, they saw launching a crew to an NEA as "the most achievable step [after the moon] in an orderly development of manned space exploration." They urged that piloted asteroid missions "be judged in the context of the larger goal of extending man's capabilities in space and in extending the frontier of exploration."
The response to Shoemaker and Helin's presentation at the 1978 Chicago workshop was divided, though in general the invited participants seemed more willing to accept the value of asteroid missions than had participants in the 1971 Tucson workshop (many of whom were, in fact, the same). Workshop co-chair and NASA official Morrison displayed a special interest in a concept Niehoff presented for a robotic multiple-asteroid rendezvous mission using electric (ion) propulsion. The choice of propulsion was in keeping with NASA support for electric propulsion research in the late 1970s.
NASA funded studies of multiple-asteroid rendezvous missions, but the agency's increased interest in asteroid exploration coincided with cuts in space science under President Ronald Reagan. No mission devoted wholly to asteroid exploration would leave Earth until the Near-Earth Asteroid Rendezvous (NEAR) Discovery mission to 433 Eros was launched on 17 February 1996.
The 487-kilogram spacecraft flew past 50-kilometer-wide Main Belt asteroid 253 Mathilde on 27 June 1997. Following Eugene Shoemaker's tragic death in an automobile accident while studying ancient impact craters in Australia (18 July 1997), NASA renamed the NEAR spacecraft NEAR Shoemaker. It orbited 34-kilometer-long, 17-kilometer-wide Eros 230 times between 14 February 2000 and 12 February 2001.
Though no multiple-asteroid mission has yet toured the NEAs, the Dawn spacecraft, which left Earth on 27 January 2007, has applied the principle to Main Belt exploration. Dawn used solar-electric propulsion to rendezvous with 4 Vesta, the third-largest asteroid, on 16 July 2011.
|1 Ceres is considered to be both an asteroid and a dwarf planet. In March 2015 it became the first (so far only) dwarf planet to be orbited by a spacecraft. Image credit: NASA|
"Reasons for Not Having an Early Asteroid Mission," E. Anders; in Physical Studies of the Minor Planets, Papers of the 12th Colloquium of the International Astronomical Union held in Tucson, Arizona, 6-10 March 1971, NASA SP-267, T. Gehrels, editor, 1972, pp. 479-485
"Asteroid Mission Alternatives," J. Niehoff; in Asteroids: An Exploration Assessment, Proceedings of a Workshop held at the University of Chicago, 19-21 January 1978, NASA CP-2053, D. Morrison and W. Well, editors, June 1978, pp. 225-244
"Earth-Approaching Asteroids as Targets for Exploration," E. Shoemaker and E. Helin; in Asteroids: An Exploration Assessment, Proceedings of a Workshop held at the University of Chicago, 19-21 January 1978, NASA CP-2053, D. Morrison and W. Well, editors, June 1978, pp. 245-266