(436724) 2011 UW158

(436724) 2011 UW158, provisionally known as 2011 UW158, is a stony, walnut-shaped asteroid and fast rotator, classified as near-Earth object and potentially hazardous asteroid of the Apollo group, approximately 300 meters in diameter. It was discovered on 25 October 2011, by Pan-STARRS at Haleakala Observatory on the island of Maui, Hawaii, in the United States.[2]

(436724) 2011 UW158
Animation of Arecibo radar images of 2011 UW158
Discovery [1]
Discovered byPan-STARRS 1
Discovery siteHaleakala Obs.
Discovery date25 October 2011
Designations
(436724) 2011 UW158
2011 UW158
Apollo · NEO · PHA [1][2]
Orbital characteristics [1]
Epoch 4 September 2017 (JD 2458000.5)
Uncertainty parameter 0
Observation arc5.48 yr (2,003 days)
Aphelion2.2301 AU
Perihelion1.0109 AU
1.6205 AU
Eccentricity0.3762
2.06 yr (753 days)
12.072°
0° 28m 40.08s / day
Inclination4.5717°
286.00°
8.7537°
Earth MOID0.0020 AU · 0.8 LD
Physical characteristics
Dimensions0.22±0.04 km[3]
0.3 × 0.6 km[4]
0.311 km (calculated)[5]
0.61069±0.00002 h[6][lower-alpha 1]
0.61070±0.00003 h[3]
0.61073 h[5]
0.610752±0.000001 h[7]
0.20 (assumed)[5]
0.39±0.09[3]
S [5]
19.45±0.27[8] · 19.9[1][5] · 19.93±0.11[3]

    Orbital description

    2011 UW158 orbits the Sun at a distance of 1.0–2.2 AU once every 2 years and 1 month (753 days). Its orbit has an eccentricity of 0.38 and an inclination of 5° with respect to the ecliptic.[1] Its observation arc begins with its official discovery observation by Pan-STARRS, as no precoveries were taken, and no prior identifications were made.[2]

    Close approach

    The asteroid has an Earth minimum orbit intersection distance of 0.0020 AU (300,000 km), which corresponds to 0.8 lunar distances (LD). On 19 July 2015, it passed about 2.5 million kilometers from Earth (6.5 LD),[1] attracting the interest of astronomers.[1] The asteroid was listed as level 1 in the Torino Scale on 4 November 2011, 9 days after its discovery, but was removed two weeks later.[9][10]

    Left: Inner solar system orbits; right: Motion of 2011 UW158 across sky, 6 hour motion

    Physical characteristics

    Spectral type

    2011 UW158 is an assumed stony S-type asteroid, the most common type among the populations of near-Earth and inner main-belt asteroids.[5]

    Diameter and albedo

    Bruce Gary at Hereford Arizona Observatory (G95) estimated a mean-diameter of 220 meters with a high albedo of 0.39,[3] while the Collaborative Asteroid Lightcurve Link assumes a standard albedo for stony asteroid of 0.20 and calculates a mean-diameter of 311 meters based on an absolute magnitude of 19.9.[5]

    Fast rotator

    In July and August 2015, rotational lightcurves of this asteroid were obtained from photometric observations by Bruce Gary at Hereford Arizona Observatory and by Brian Warner at the CS3–Palmer Divide Station in California (U82). Lightcurve analysis gave a rotation period of 0.6107 hours (36.6 minutes) with a brightness variation between 0.52 and 2.05 magnitude (U=3/3/3).[3][6][lower-alpha 1] This makes 2011 UW158 one of the Top 200 fast rotator, suggesting it is a large boulder rather than a rubble pile.[6][11]

    Shape

    Radar observations by the Arecibo Observatory on 14 July 2015, revealed that the asteroid's shape looks like an unshelled walnut with a diameter of 300 by 600 metres. The radiometric observations also agreed with the previously obtained photometric ones and gave a period of 37 minutes.[4][11]

    Media attention

    It also attracted the media and even firms such as Planetary Resources[12] for its alleged content of platinum worth as high as 5 trillion U.S. dollars.[13][14][15] Commenters at StackExchange have denied these estimations as being orders of magnitude too high,[16] and radar observations have shown that the object contains no more metal than an average rocky asteroid.[17][18]

    Notes
    1. Lightcurve plot of 2011 UW158 by Brian Warner at the CS3–Palmer Divide Station in July 2015: rotation period of 0.61069±0.00002 hours with an amplitude of 0.65 magnitude

    References
    1. "JPL Small-Body Database Browser: 436724 (2011 UW158)" (2017-04-19 last obs.). Jet Propulsion Laboratory. Retrieved 2 June 2017.
    2. "436724 (2011 UW158)". Minor Planet Center. Retrieved 28 March 2017.
    3. Gary, Bruce L. (January 2016). "Unusual Properties for the NEA (436724) 2011 UW158". The Minor Planet Bulletin. 43 (1): 33–38. Bibcode:2016MPBu...43...33G. ISSN 1052-8091. Retrieved 28 March 2017.
    4. "Solar System Studies at Arecibo Observatory". Arecibo Observatory. Retrieved 2015-07-23.
    5. "LCDB Data for (436724)". Asteroid Lightcurve Database (LCDB). Retrieved 28 March 2017.
    6. Warner, Brian D. (January 2016). "Near-Earth Asteroid Lightcurve Analysis at CS3-Palmer Divide Station: 2015 June-September". The Minor Planet Bulletin. 43 (1): 66–79. Bibcode:2016MPBu...43...66W. ISSN 1052-8091. Retrieved 28 March 2017.
    7. Carbognani, Albino; Gary, Bruce L.; Oey, Julian; Baj, Giorgio; Bacci, Paolo (January 2016). "Pole and Shape for the NEA (436724) 2011 UW158". The Minor Planet Bulletin. 43 (1): 38–41. Bibcode:2016MPBu...43...38C. ISSN 1052-8091. Retrieved 28 March 2017.
    8. 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 28 March 2017.
    9. "2011 UW158 Impact Risk". Near Earth Object Program. NASA. Archived from the original on November 6, 2011.
    10. "NEOs Removed from Impact Risks Tables". Near Earth Object Program. NASA. Retrieved 28 March 2017.
    11. "First detailed images of rare asteroid to pass close by Earth on 19 July". Astronomy Now. 19 July 2015. Retrieved 28 March 2017.
    12. "Mining Firm Aiming For Platinum-Loaded Asteroid". Retrieved 2015-07-23.
    13. "UW-158: Watch live as asteroid worth £ 3 TRILLION passes close to Earth". Retrieved 2015-07-23.
    14. "What you can expect to see as a £3 trillion asteroid passes Earth tonight". Retrieved 2015-07-23.
    15. "$5 Trillion Dollar Asteroid Makes Close Approach to Earth". Retrieved 2015-07-23.
    16. "How is it known that asteroid 2011 UW158 has so much platinum?". Retrieved 2015-07-23.
    17. "5th IAA Planetary Defense Conference" (PDF). Retrieved 2017-06-03.
    18. "Ground-based radar observations of human space flight accessible targets". Retrieved 2017-06-03.
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