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.[3] The asteroid was named in honor of the National Geographic Society.[2]

1620 Geographos
Lightcurve-based 3D-model of Geographos
Discovery [1]
Discovered byA. G. Wilson
R. Minkowski
Discovery sitePalomar Observatory
Discovery date14 September 1951
Designations
(1620) Geographos
Pronunciation/dʒiːoʊˈɡræfɒs/
Named after
National Geographic Society [2]
1951 RA · 1983 CY3
Apollo · NEO · PHA [1][3]
Orbital characteristics [1]
Epoch 4 September 2017 (JD 2458000.5)
Uncertainty parameter 0
Observation arc65.37 yr (23,875 days)
Earliest precovery date31 August 1951
Aphelion1.6629 AU
Perihelion0.8276 AU
1.2453 AU
Eccentricity0.3354
1.39 yr (508 days)
246.00°
0° 42m 33.48s / day
Inclination13.337°
337.20°
276.90°
Earth MOID0.0301 AU · 11.7 LD
Physical characteristics
Dimensions(5.0 × 2.0 × 2.1) ± 0.15 km[4]
Mean diameter
1.77±0.1 km[5]
1.87±0.05 km[6]
1.96±0.06 km[7]
2.5 km[8][9]
2.56±0.15 km[4]
5.2220±0.0003 h[10]
5.22204±0.00007 h[lower-alpha 1]
5.22229±0.00004 h[11]
5.223±0.003 h[12]
5.223±0.005 h[13]
5.223 h[14]
5.223326 h[15][16]
5.223327 h[4]
5.223328 h[17][18]
5.224±0.007 h[19]
0.26[8][9]
0.290±0.038[6]
0.29±0.04[6]
0.32±0.04[7]
0.3258±0.051[5]
Tholen = S [1]
SMASS = S [1][8] · S [20]
B–V = 0.862[1]
B–V = 0.896±0.009[20]
U–B = 0.471[1]
V–R = 0.462±0.005[20]
V–I = 0.816±0.006[20]
15.09[8][9] · 15.41[7] · 15.55±0.37[21] · 15.6[1][5][6]

    Orbit and classification
    Animation of 1620 Geographos's orbit 2010-2020
       Sun ·    Venus   Earth ·   Mars ·    1620 Geographos

    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.[1] 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.[3]

    Close approaches

    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.[1] In 1994, Geographos made its closest approach to Earth in two centuries at 5.0 Gm – which will not be bettered until 2586.[22]

    Failed Clementine mission

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

    Physical characteristics

    Spectral type

    In the Tholen and SMASS classification, Geographos is an S-type asteroid.[1] This means that it is highly reflective and composed of nickel-iron mixed with iron- and magnesium-silicates.

    Rotation period

    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).[10][11][12][13][14][19][lower-alpha 1]

    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 (1.5±0.2)×10−3 rad yr−2.[23]

    Spin axis

    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).[18][17] Radiometric observations gave a period of 5.223327 hours and a pole of (55.0°, −46.0°).[4] 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).[15][16]

    Shape and structure

    The light curve shows a high amplitude, indicative of its elongated shape, measuring 5.0 × 2.0 × 2.1 kilometers, which corresponds to a mean-diameter of 2.5 km.[4]

    The interior of the asteroid probably has a rubble-pile structure. The asteroid's high thermal inertia indicates the surface is most likely a mix of fine grains and large rocks and boulders.[23] During the asteroid's close approach to Earth in 1994, a radar study of it was conducted by the Deep Space Network at the Goldstone Observatory, California. The resultant images show Geographos to be the most elongated object in the Solar System.[22]

    Diameter and albedo

    According to the observations with the Goldstone Observatory and the space-based surveys carried out by the Infrared Astronomical Satellite IRAS and the NEOWISE mission of the Wide-field Infrared Survey Explorer, Geographos measures between 1.77 and 2.56 kilometers in diameter and its surface has an albedo between 0.26 and 0.3258.[5][6][7][9][4] The Collaborative Asteroid Lightcurve Link adopts an albedo of 0.26 and a diameter of 2.5 kilometers based on an absolute magnitude of 15.09.[8]

    Naming

    This minor planet was named after the National Geographic Society, in recognition of its contribution to astronomy by supporting the National Geographic Society – Palomar Observatory Sky Survey (NGS-POSS), which produced a photographic atlas of the entire northern sky in the 1950s. NGS-POSS was headed by the second discoverer, Rudolph Minkowski. The Greek word geographos means geographer (from geo– 'Earth' + graphos 'drawer/writer').[2] The official naming citation was published by the Minor Planet Center in August 1956 (M.P.C. 1468).[24]

    Notes
    1. Hamanowa (2011): lightcurve plot of (1620) Geographos, by H. & H. Hamanowa at the Hamanowa Astronomical Observatory (D91). Obs. date: 2008-02-10; rotation period 5.22204 hours (0.217585 days) with a brightness amplitude of 1.39 mag. Quality Code of 3. Summary figures at the LCDB

    References
    1. "JPL Small-Body Database Browser: 1620 Geographos (1951 RA)" (2017-01-11 last obs.). Jet Propulsion Laboratory. Retrieved 5 September 2017.
    2. Schmadel, Lutz D. (2007). "(1620) Geographos". Dictionary of Minor Planet Names – (1620) Geographos. Springer Berlin Heidelberg. p. 128. doi:10.1007/978-3-540-29925-7_1621. ISBN 978-3-540-00238-3.
    3. "1620 Geographos (1951 RA)". Minor Planet Center. Retrieved 5 September 2017.
    4. Hudson, R. S.; Ostro, S. J. (August 1999). "Physical Model of Asteroid 1620 Geographos from Radar and Optical Data". Icarus. 140 (2): 369–378. Bibcode:1999Icar..140..369H. doi:10.1006/icar.1999.6142. Retrieved 5 September 2017.
    5. Tedesco, E. F.; Noah, P. V.; Noah, M.; Price, S. D. (October 2004). "IRAS Minor Planet Survey V6.0". NASA Planetary Data System – IRAS-A-FPA-3-RDR-IMPS-V6.0. Bibcode:2004PDSS...12.....T. Retrieved 17 October 2019.
    6. Nugent, C. R.; Mainzer, A.; Masiero, J.; Bauer, J.; Cutri, R. M.; Grav, T.; et al. (December 2015). "NEOWISE Reactivation Mission Year One: Preliminary Asteroid Diameters and Albedos". The Astrophysical Journal. 814 (2): 13. arXiv:1509.02522. Bibcode:2015ApJ...814..117N. doi:10.1088/0004-637X/814/2/117. Retrieved 5 September 2017.
    7. Nugent, C. R.; Mainzer, A.; Bauer, J.; Cutri, R. M.; Kramer, E. A.; Grav, T.; et al. (September 2016). "NEOWISE Reactivation Mission Year Two: Asteroid Diameters and Albedos". The Astronomical Journal. 152 (3): 12. arXiv:1606.08923. Bibcode:2016AJ....152...63N. doi:10.3847/0004-6256/152/3/63. Retrieved 5 September 2017.
    8. "LCDB Data for (1620) Geographos". Asteroid Lightcurve Database (LCDB). Retrieved 5 September 2017.
    9. Harris, Alan W. (February 1998). "A Thermal Model for Near-Earth Asteroids". Icarus. 131 (2): 291–301. Bibcode:1998Icar..131..291H. doi:10.1006/icar.1997.5865. Retrieved 5 September 2017.
    10. Higgins, David; Pravec, Petr; Kusnirak, Peter; Hornoch, Kamil; Brinsfield, James W.; Allen, Bill; et al. (September 2008). "Asteroid Lightcurve Analysis at Hunters Hill Observatory and Collaborating Stations: November 2007 - March 2008". The Minor Planet Bulletin. 35 (3): 123–126. Bibcode:2008MPBu...35..123H. ISSN 1052-8091. Retrieved 5 September 2017.
    11. Behrend, Raoul. "Asteroids and comets rotation curves – (1620) Geographos". Geneva Observatory. Retrieved 5 September 2017.
    12. Skiff, Brian A.; Bowell, Edward; Koehn, Bruce W.; Sanborn, Jason J.; McLelland, Kyle P.; Warner, Brian D. (July 2012). "Lowell Observatory Near-Earth Asteroid Photometric Survey (NEAPS) - 2008 May through 2008 December". The Minor Planet Bulletin. 39 (3): 111–130. Bibcode:2012MPBu...39..111S. ISSN 1052-8091. Retrieved 5 September 2017.
    13. Warner, Brian D. (April 2016). "Near-Earth Asteroid Lightcurve Analysis at CS3-Palmer Divide Station: 2015 October-December". The Minor Planet Bulletin. 43 (2): 143–154. Bibcode:2016MPBu...43..143W. ISSN 1052-8091. Retrieved 5 September 2017.
    14. Dunlap, J. L. (February 1974). "Minor planets and related objects. XV. Asteroid (1620) Geographos". Astronomical Journal. 79: 324. Bibcode:1974AJ.....79..324D. doi:10.1086/111546.
    15. Magnusson, P.; Dahlgren, M.; Barucci, M. A.; Jorda, L.; Binzel, R. P.; Slivan, S. M.; et al. (September 1996). "Photometric Observations and Modeling of Asteroid 1620 Geographos". Icarus. 123 (1): 227–244. Bibcode:1996Icar..123..227M. doi:10.1006/icar.1996.0151. Retrieved 5 September 2017.
    16. Kaasalainen, M.; Torppa, J.; Muinonen, K. (September 2001). "Optimization Methods for Asteroid Lightcurve Inversion. II. The Complete Inverse Problem". Icarus. 153 (1): 37–51. Bibcode:2001Icar..153...37K. doi:10.1006/icar.2001.6674. Retrieved 5 September 2017.
    17. Kwiatkowski, T. (February 1995). "Sidereal period, pole and shape of asteroid 1620 Geographos". Astronomy and Astrophysics. 294: 274–277. Bibcode:1995A&A...294..274K. Retrieved 5 September 2017.
    18. Michalowski, T.; Kwiatkowski, T.; Borczyk, W.; Pych, W. (April 1994). "CCD Photometry of the Asteroid 1620 Geographos". Acta Astronomica. 44: 223–226. Bibcode:1994AcA....44..223M. Retrieved 5 September 2017.
    19. Polishook, David (July 2009). "Lightcurves for Shape Modeling Obtained at the Wise Observatory". The Minor Planet Bulletin. 36 (3): 119–120. Bibcode:2009MPBu...36..119P. ISSN 1052-8091. Retrieved 5 September 2017.
    20. Dandy, C. L.; Fitzsimmons, A.; Collander-Brown, S. J. (June 2003). "Optical colors of 56 near-Earth objects: trends with size and orbit". Icarus. 163 (2): 363–373. Bibcode:2003Icar..163..363D. doi:10.1016/S0019-1035(03)00087-3. Retrieved 5 September 2017.
    21. Veres, Peter; Jedicke, Robert; Fitzsimmons, Alan; Denneau, Larry; Granvik, Mikael; Bolin, Bryce; et al. (November 2015). "Absolute magnitudes and slope parameters for 250,000 asteroids observed by Pan-STARRS PS1 - Preliminary results". Icarus. 261: 34–47. arXiv:1506.00762. Bibcode:2015Icar..261...34V. doi:10.1016/j.icarus.2015.08.007. Retrieved 5 September 2017.
    22. Ostro, Steven J.; Jurgens, Raymond F.; Rosema, Keith D.; Hudson, R. Scott; Giorgini, Jon D.; Winkler, Ron; et al. (May 1996). "Radar Observations of Asteroid 1620 Geographos". Icarus. 121 (1): 46–66. Bibcode:1996Icar..121...46O. doi:10.1006/icar.1996.0071. Retrieved 5 September 2017.
    23. Rozitis, B.; Green, S. F. (August 2014). "Physical characterisation of near-Earth asteroid (1620) Geographos. Reconciling radar and thermal-infrared observations". Astronomy and Astrophysics. 568: 11. arXiv:1407.2127. Bibcode:2014A&A...568A..43R. doi:10.1051/0004-6361/201323090.
    24. Schmadel, Lutz D. (2009). "Appendix – Publication Dates of the MPCs". Dictionary of Minor Planet Names – Addendum to Fifth Edition (2006–2008). Springer Berlin Heidelberg. p. 221. doi:10.1007/978-3-642-01965-4. ISBN 978-3-642-01964-7.
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