|Pioneer 10/11. Image credit: NASA.|
Astronomers had spotted many thousands of other asteroids. Often they appeared as annoying streaks on photographic plates intended to capture images of objects deemed more worthy of an astronomer's attention: for example, distant galaxies. The vast majority of those asteroids were not knowingly observed again.
As the 1960s ended and the 1970s began, enthusiasm for asteroids as objects of study began to grow. In part this was because the future looked bright for robotic space exploration. No longer were automated spacecraft seen mainly as precursors for piloted moon missions.
Mars, with its two satellites Phobos and Deimos — believed to be captured asteroids — came in for special attention in NASA's 1970s robotic exploration program. Mariners 8 and 9 were scheduled for launch less than three months after the Tucson colloquium. The twin spacecraft, built by the Jet Propulsion Laboratory (JPL) in Pasadena, California, were planned to be the first Mars orbiters, though there was some concern that the Soviets might get there first.
The twin Viking spacecraft were scheduled to leave Earth for Mars in mid-1975. Each would include a Lander and an Orbiter. The Viking Landers were planned to be the first Mars soft-landers. NASA Langley Research Center (LaRC) in Hampton,Virginia, managed Viking; JPL served as contractor for the Mariner-based Viking Orbiters.
Pioneers 10 and 11 (image at top of post) were on track to become humankind's first emissaries to the Outer Solar System. To reach Jupiter (and, in the case of Pioneer 11, Saturn), they would become the first spacecraft to pass through the Asteroid Belt between Mars and Jupiter. Built by TRW and managed by NASA Ames Research Center (ARC) in Mountain View, California, they were due to launch in March 1972 and April 1973, respectively.
Most of the papers presented at the Tucson colloquium emphasized Earth-based telescope observations of asteroids, not spacecraft exploration, and in fact a consensus emerged by the end of the colloquium that spacecraft exploration of asteroids would be premature. Nevertheless, among those present in Tucson was a small cadre of asteroid mission proponents. LaRC engineers David Brooks and William Hampshire, for example, described missions to multiple Main Belt asteroids.
They looked first at the simplest multiple asteroid flyby mission: one which saw a spacecraft launched on a random date into an elliptical Sun-centered orbit with a perihelion of one Astronomical Unit (AU) and an aphelion within the Asteroid Belt at three AU. An AU is equal to the mean Earth-Sun distance. They assumed that, in order to obtain useful data, the spacecraft would need to pass no more than 15,000 kilometers from an asteroid. Based on these stipulations, they calculated that a randomly launched spacecraft stood virtually no chance of exploring even one asteroid.
They added, however, that a randomly launched spacecraft stood a good chance of passing within 0.1 AU of 10 asteroids on average. While 0.1 AU (about 15 million kilometers) was too great a distance for effective exploration, this finding meant that, if the spacecraft could change its velocity, then it could shape its orbital path to pass within 15,000 kilometers of multiple asteroids.
Large velocity changes would enable exploration flybys of many asteroids, but would require costly new spacecraft development. New asteroid discoveries would also increase the number of possible flyby candidates. Brooks and Hampshire were determined, however, to show what could be accomplished with small velocity changes, spacecraft already in the development pipeline, and the 1748 asteroids numbered as of March 1971.
The Viking Orbiter, they noted, would carry enough propellants to change its velocity by 1.5 kilometers per second. They revealed that NASA ARC had studied a Pioneer 10/11-class spacecraft modified to capture into a highly elliptical Jupiter orbit. A Pioneer 10/11-class spacecraft could change its velocity by only 0.2 kilometers per second, but the hypothetical Pioneer Jupiter orbiter would up this to about one kilometer per second.
The LaRC engineers then provided detailed multiple asteroid flyby sequences for three missions. The missions were: leave Earth in the 1980-1982 period and orbit the Sun in a one-AU-by-three-AU orbit; leave Earth in late 1975 and fly past 1 Ceres; and leave Earth in 1975 and travel through the Asteroid Belt to Jupiter.
In no case would a spacecraft perform maneuvers which together would change its velocity by more than one kilometer per second. In each case, Brooks and Hampshire assumed that the spacecraft would pass 15,000 kilometers from the first asteroid it encountered because its launch vehicle put it there; that is, the propellant cost of exploring the first asteroid in any multiple asteroid sequence would count against the rocket stage that boosted the spacecraft out of Earth orbit, not the spacecraft itself.
Brooks and Hampshire found that launch from Earth into a one-by-three-AU orbit on 14 July 1981 would cause the spacecraft to pass within 0.1 AU of 15 asteroids over the course of 659 days. Unfortunately, nudging the spacecraft's path so that it would pass within 15,000 kilometers of all 15 would require a total velocity change of 41.6 kilometers per second.
Large velocity changes were necessary in part because some flybys occurred close together. The spacecraft would, for example, pass about 0.1 AU from asteroid 149 Medusa on 10 January 1982, just nine days after a 15,000-kilometer flyby of asteroid 1515 Perrotin. A velocity change of several kilometers per second would be required to bend the spacecraft's path to enable it to pass just 15,000 kilometers from 149 Medusa so soon after leaving 1515 Perrotin.
Spacing out asteroid encounters meant that a small spacecraft velocity change immediately after a close asteroid flyby could yield a large spacecraft orbit change. If mission designers opted instead to follow the 1515 Perrotin close flyby with a close flyby of 1674 Groeneveld six months later (13 June 1982), then added a 12 July 1983 close flyby of 561 Ingewelde, the total spacecraft velocity change would amount to just 0.93 kilometers per second.
The LaRC engineers identified seven three-asteroid missions and one four-asteroid mission, all launched on 14 July 1981, that would need total velocity changes of less than one kilometer per second. A multiple flyby mission to 149 Medusa, 870 Manto, and 1720 Neils would require the smallest velocity change - just 0.58 kilometers per second. The four-asteroid mission, which would explore 1515 Perrotin, 1674 Groeneveld, 561 Ingewelde, and 1720 Neils, would need a total velocity change of 0.8 kilometers per second.
Brooks and Hampshire gave less attention to their 1975 Ceres and Jupiter multiple asteroid missions. They determined that a late 1975 launch would enable close flybys of 632 Pyrrha and either 946 Poësia or 947 Monterosa en route to 1 Ceres, at 950 kilometers across the largest asteroid. The 632 Pyrrha-947 Monterosa-1 Ceres flyby sequence would need the lowest total velocity change of any of mission they studied: just 0.24 kilometers per second.
Jupiter-bound spacecraft presented two new problems, Brooks and Hampshire explained. First, they would move fast. For example, a spacecraft bound for Jupiter in 1975 targeted to pass the large (124-by-75-kilometer) asteroid 27 Euterpe would zip past at 18 kilometers per second, making data collection difficult.
In addition, a Jupiter-bound spacecraft would follow a short path through the Asteroid Belt, so would pass few asteroids. Brooks and Hampshire were able to identify only one multiple asteroid flyby opportunity for the 1975 Jupiter mission. The spacecraft would fly first past 666 Desdemona on 19 November 1975, then past 396 Aeolia on 6 April 1976. It would change its velocity by 0.52 kilometers per second. The LaRC engineers did not indicate whether any part of the velocity change would be applied to correcting the spacecraft's course to Jupiter after the 396 Aeolia flyby.
NASA would need two decades to carry out its first multiple asteroid flyby mission, and when it did the mission would resemble none of Brooks and Hampshire's scenarios. The Galileo Jupiter Orbiter grew from the NASA ARC Pioneer Jupiter Orbiter they described at the Tucson colloquium. The mission received new-start funding in 1977. Launch aboard the Space Shuttle was scheduled for January 1982.
Space Shuttle delays, fierce political battles over the type of upper stage that would propel Galileo to Jupiter, costly redesigns to enable it to ride different upper stages (one redesign involved splitting Galileo into two spacecraft), and technical problems with the Centaur G' upper stage delayed Galileo's planned launch to May 1986. In January 1985, NASA Administrator James Beggs added to the Galileo mission the option of a flyby of the large (233-by-193-kilometer) asteroid 29 Amphitrite.
Destruction of the Space Shuttle Challenger on 28 January 1986 caused more delays and cancellation of the Centaur G' stage needed to boost Galileo directly to Jupiter. JPL re-planned the Galileo mission for an October 1989 launch with gravity-assist flybys of Venus and Earth (twice) and Jupiter arrival in December 1995. The new path put 29 Amphitrite far out of reach.
Following its first Earth gravity-assist, Galileo entered the Asteroid Belt and performed the first-ever asteroid flyby: a cruise past 18-by-nine-kilometer 951 Gaspra at a distance of 1604 kilometers on 29 October 1991. Galileo flew past Earth a second time to gain the final gravity-assist speed boost it needed to reach Jupiter; then, on 28 August 1993, it flew by 60-by-19-kilometer 243 Ida at a distance of 2410 kilometers, revealing that it has small moon. Dactyl, as the 1.6-by-1.4-by-1.2-kilometer satellite was named, brought to three the number of asteroids Galileo explored during its circuitous voyage to Jupiter.
|Main Belt asteroid 243 Ida and its moon Dactyl, 28 August 1993. Image credit: NASA.|
"Multiple Asteroid Flyby Missions," David Brooks and William Hampshire, NASA SP-267, Physical Studies of the Minor Planets, Proceedings of the 12th Colloquium of the International Astronomical Union held in Tucson, Arizona, 6-10 March 1971, Tom Gehrels, editor, 1972, pp. 527-537.
"Reasons for Not Having an Early Asteroid Mission," Edward Anders, NASA SP-267, Physical Studies of the Minor Planets, Proceedings of the 12th Colloquium of the International Astronomical Union held in Tucson, Arizona, 6-10 March 1971, Tom Gehrels, editor, 1972, pp. 479-485.
Memorandum, Clark Chapman to various, "Notes Concerning the 'Centaur Wars' and Possible Action by the Planetary Science Community," 14 September 1982.
Interoffice Memorandum GLL-JRC-84-189, Jet Propulsion Laboratory, J. R. Casani to W. E. Giberson, "Galileo Asteroid Flyby," 11 September 1984.
Memorandum, T. V. Johnson, W. J. O'Neil, and C. M. Yeates to PSG/IDS, "Galileo Asteroid Encounter," Jet Propulsion Laboratory, 1 October 1984.
Press Release, Public Information Office, Jet Propulsion Laboratory, "NASA Administrator James M. Beggs has approved the addition of an asteroid flyby option to the Galileo mission," 17 January 1985.
Interoffice Memorandum GLL/TCC-87.238, Jet Propulsion Laboratory, T. C. Clarke to Galileo Project Science Distribution list, "Minutes of Galileo PSG Meeting of 11/21/86," 20 April 1987.
Journey Into Space: The First Thirty Years of Space Exploration, Bruce Murray, W. W. Norton, 1989, pp. 180-237.
Earth-Approaching Asteroids as Targets for Exploration (1978)
The Challenge of the Planets, Part Three: Gravity