On 14 September 1951, for example, NGS-POSS astronomers Rudolph Minkowski and Albert George Wilson discovered asteroid 1951 RA. By noting its position on plates made as early as 31 August 1951, they were able to determine that it travels around the Sun in an inclined orbit that crosses the orbits of Earth and Mars. It completes an orbit every 508 days.
They determined that their new-found asteroid never comes very close to Mars — when it crosses the Red Planet's orbit about the Sun it is well above the orbital plane. When it crosses Earth's orbit, on the other hand, it is very near the orbital plane. As a result, it can pass very close to Earth. Soon after it was discovered, astronomers determined that 1951 RA would pass near Earth in 1969.
1951 RA was of sufficient interest to be made the 1620th named asteroid. In 1956, it was dubbed 1620 Geographos in honor of the National Geographic Society, which funded the NGS-POSS. The name Geographos means "geographer" in Greek.
1620 Geographos became a subject of special interest for University of California at Los Angeles astronomer Samuel Herrick. Following the 1969 close flyby, during which he refined knowledge of the parameters of its orbit, Herrick calculated that 1620 Geographos would pass close to Earth in 1994. He believed that its "ominous" orbit meant it stood a very good chance of striking Earth sometime during "the Third Millennium" (that is, in the interval between the years 2001 and 3001).
Herrick presented his results at the International Astronomical Union's Physical Studies of the Minor Planets colloquium held at Kitt Peak Observatory in March 1971. His contribution did not, however, appear in the colloquium proceedings NASA published later that year. Herrick subsequently passed away at age 62 on 24 March 1974.
According to Dutch-American astronomer Tom Gehrels, editor of the first Asteroids compilation volume published by the University of Arizona Press in 1979, Herrick turned his contribution into a proposal to use 1620 Geographos as a planetary engineering tool that might infuse Earth's crust with new mineral wealth. This led the editor and referees of the 1971 proceedings to declare Herrick's contribution "premature" and "outrageously innovative" and reject it. For his part, Gehrels was happy to publish Herrick's 1971 paper in the 1979 volume.
Herrick proposed a two-phase plan spanning about five years which would, he declared, generate much greater public enthusiasm than the Apollo lunar landings. In the first phase, engineers would used unspecified propulsive means to push 1620 Geographos into a safer orbit and "explosive cleaving" to separate a portion of the asteroid and push it toward Earth.
The second phase would see the separated portion guided toward an impact on the Isthmus of Panama. Herrick painted a target on the Atrato River in Colombia, which, he explained, had been proposed in the year 1540 as the site for a canal linking the Caribbean Sea and the Pacific Ocean.
On 25 March 1994, the impactor would streak through midnight skies over the cities of Quito, Bogota, and Medellin on its way to a "rendezvous" with the "jungle wasteland of northwestern Colombia." The resultant impact crater would form a "new canal from sea to sea." The "interocean Crater-Canal" would include no locks — it would be a sea-level passage through the isthmus that would permit the mixing of Caribbean and Pacific waters.
Herrick assumed that 1620 Geographos would be made of "nickel and the heavier elements that are mostly locked in the earth's core: rhenium, osmium, iridium, platinum, gold, etc." The impact would, he estimated, deposit on Earth extraterrestrial minerals worth $900 billion. They would be collected from the water-filled crater as Earth's supply of these valuable minerals became depleted.
Herrick acknowledged that the effects of the impact on the Earth would have to be studied and that 1620 Geographos might not be made of useful metals. He suggested that the techniques developed to deflect most of the asteroid away from Earth and excavate the canal in 1994 could, if necessary, be applied to another, more mineralogically suitable asteroid. Searching for a candidate impactor could also reveal future threats to Earth.
In 1994, 20 years after Herrick's passing, 1620 Geographos orbited nearer Earth than it had in two centuries. It was not, however, a threat — at its closest approach it passed about five million kilometers away (about 12 times the distance between the Earth and the Moon). The asteroid will not pass as close again until the 26th century. Earth is safe from 1620 Geographos on a time-scale of millions of years.
On 25 January 1994, the Clementine spacecraft lifted off from Vandenberg Air Force Base, California, bound for the Moon and 1620 Geographos. The spacecraft was intended to collect scientific data while demonstrating sensor and miniaturized spacecraft technologies that could be applied to ballistic missile defense systems. After a leisurely one-month transfer, Clementine orbited over the poles of the Moon for about two months (it was the first lunar polar orbiter).
On 3 May 1994, Clementine began a circuitous transfer to 1620 Geographos. Had it succeeded, it would have become the first spacecraft to fly by a near-Earth asteroid. Unfortunately, a computer malfunction on 7 May caused Clementine to fire one of its thrusters continuously, expending most of its remaining propellant supply and imparting a spin rate sufficient to render most of its instruments ineffective.
During the 1994 close approach Earth-based radar studies showed that 1620 Geographos is five kilometers long by about two kilometers wide. It is probably a "rubble pile" made up of many small asteroids loosely bound by mutual gravitational attraction. Earth-based studies have also revealed that it is a mainly stony asteroid not especially rich in metals.
"Voyage to the Planets," Kenneth F. Weaver, National Geographic, Volume 138, Number 2, August 1970, pp. 174-178.
"Exploration and 1994 Exploitation of Geographos," Samuel Herrick, Asteroids, Tom Gehrels, editor, The University of Arizona Press, 1979, pp. 222-226.
Dictionary of Minor Planet Names, Lutz D. Schmadel, Springer-Verlag, 1992, p. 211.