1620 Geographos

1620 Geographos (/dʒiːoʊˈɡræfɒs/; provisional designation 1951 RA) is a highly elongated, stony asteroid, near-Earth object and potentially hazardous asteroid of the Apollo group, with a mean diameter of approximately 2.5 km (1.6 mi). It was discovered on 14 September 1951, by astronomers Albert George Wilson and Rudolph Minkowski at the Palomar Observatory in California, United States. The asteroid was named in honor of the National Geographic Society.

Geographos orbits the Sun at a distance of 0.8–1.7 AU once every 1 years and 5 months (508 days). Its orbit has an eccentricity of 0.34 and an inclination of 13° with respect to the ecliptic. Its orbit is well-determined for the next several hundred years. Due to its high eccentricity, Geographos is also a Mars-crosser asteroid. The body's observation arc begins at Palomar, two weeks prior to its official discovery observation.

As a potentially hazardous asteroid, Geographos has a minimum orbital intersection distance (MOID) with Earth of less than 0.05 AU and a diameter of greater than 150 meters. The Earth-MOID is currently 0.0301 AU (4,500,000 km), which translates into 11.7 lunar distances. In 1994, Geographos made its closest approach to Earth in two centuries at 5.0 Gm – which will not be bettered until 2586.

Geographos was to be explored by the U.S.'s Clementine mission which was launched in January 1994. However, a malfunction in the spacecraft ended the mission before it could approach the asteroid.

In the Tholen and SMASS classification, Geographos is an S-type asteroid. This means that it is highly reflective and composed of nickel-iron mixed with iron- and magnesium-silicates.

Since the 1970s, several rotational lightcurve of Geographos have been obtained from photometric observations. Lightcurve analysis gave a rotation period (retrograde sense of rotation) between 5.222 and 5.224 hours with a very high brightness variation between 1.02 and 2.03 magnitude (U=3/3/3/2/3/3/3).

The Yarkovsky effect is causing a decrease in the orbital semimajor axis of 27.4±5.7 m yr⁻¹, while the YORP effect is increasing the asteroid's rotation at the rate of (1.14±0.03)×10⁻³ rad yr⁻².

Several lightcurve were also modeled from the abundant photometric observations. In 1994 and 1995, Polish astronomers obtained a concurring period 5.223328 hours and found a spin axis of (54.0°, −52.0°) in ecliptic coordinates (λ, β) (Q=3/3). Radiometric observations gave a period of 5.223327 hours and a pole of (55.0°, −46.0°). Two other international studies obtained a period of 5.223326 hours and a pole at (56.0°, −47.0°) and (55.0°, −45.0°), respectively (Q=3/3).