(415029) 2011 UL21

(415029) 2011 UL21, provisional designation 2011 UL21, is an Apollo class potentially hazardous asteroid discovered on October 17, 2011 by the Catalina Sky Survey project.[1] The asteroid is estimated to have a diameter of 2.5 kilometres (1.6 mi). It was rated at Torino Scale 1 on October 27, 2011 with an observation arc of 9.6 days.[4]

(415029) 2011 UL21
Discovery[1]
Discovered byCatalina Sky Srvy.
Discovery siteCatalina Stn.
Discovery date17 October 2011
Designations
(415029) 2011 UL21
2011 UL21
Apollo · NEO · PHA [2]
Orbital characteristics[2]
Epoch 13 January 2016 (JD 2457400.5)
Uncertainty parameter 0
Observation arc9379 days (25.68 yr)
Aphelion3.5091 AU (524.95 Gm)
Perihelion0.73606 AU (110.113 Gm)
2.1226 AU (317.54 Gm)
Eccentricity0.65323
3.09 yr (1129.5 d)
110.19°
0° 19m 7.392s / day
Inclination34.845°
275.60°
284.74°
Earth MOID0.0185982 AU (2.78225 Gm)
Physical characteristics
Mean diameter
2.5 km[3]
Mass2.1×1013 kg (assumed)[3]
15.8[2]

    Description

    2011 UL21 briefly had about a 1 in a million chance of impacting in 2029.[5] Its cumulative impact probability dropped to 1 in 71 million by 2 November 2011 when the observation arc reached 15 days. It was removed from the Sentry Risk Table on 4 November 2011 when all impact scenarios for the next 100 years or more were ruled out.[6] During 2029, the closest approach to Earth is 1.6 AU. Palomar Observatory precovery images from 1989 and 1990 have extended the observation arc to 22 years.[7] Its next notable close approach to the Earth will be on June 27, 2024 at a distance of 0.044 AU (6,600,000 km; 4,100,000 mi).[8]

    With an absolute magnitude of 15.8,[2] it is one the brightest and therefore largest potentially hazardous asteroids (PHA) detected since (242450) 2004 QY2.[9] The next largest PHA (based on absolute magnitude) discovered in 2011 is 2011 WO41 with an absolute magnitude of 16.8.[9]

    Chronology of close approaches of large near-Earth objects since 1981 (A)
    PHA Date Approach distance in lunar distances Abs. mag
    (H)    		 Diameter (C)
    (m)    		 Ref (D)
    Nominal (B) Minimum Maximum
    (143651) 2003 QO1041981-05-182.7612.7602.76116.01333–4306data
    2014 LJ211989-08-017.0346.8437.22416.01333–4306data
    4179 Toutatis1992-12-089.3999.3999.39915.302440–2450data
    4179 Toutatis2004-09-294.0314.0314.03115.302440–2450data
    (159857) 2004 LJ12038-11-167.7197.7197.71915.41746–4394data
    (4953) 1990 MU2058-06-058.9868.9848.98814.13199–10329data
    4179 Toutatis2069-11-057.7257.7247.72515.302440–2450data
    (52768) 1998 OR22079-04-164.6114.6114.61215.81462–4721data
    (415029) 2011 UL212089-06-256.9366.9356.93815.71531–4944data
    3200 Phaethon2093-12-147.7147.7097.71814.64900–5300data
    (52768) 1998 OR22127-04-166.5366.5106.56315.81462–4721data
    (A) This list includes near-Earth approaches of less than 10 lunar distances (LD) of objects with H brighter than 16.
    (B) Nominal geocentric distance from the center of Earth to the center of the object (Earth has a radius of approximately 6,400 km).
    (C) Diameter: estimated, theoretical mean-diameter based on H and albedo range between X and Y.
    (D) Reference: data source from the JPL SBDB, with AU converted into LD (1 AU≈390 LD)
    (E) Color codes:   unobserved at close approach   observed during close approach   upcoming approaches

    References
    1. "MPEC 2011-U39 : 2011 UL21". IAU Minor Planet Center. 28 October 2011. Retrieved 1 November 2011.
    2. "JPL Small-Body Database Browser: (2011 UL21)". Jet Propulsion Laboratory. Retrieved 8 April 2016. 2012-04-18 last obs (arc=22 years)
    3. "Earth Impact Risk Summary: 2011 UL21". NASA/JPL Near-Earth Object Program Office. Retrieved 1 November 2011. (Wayback Machine)
    4. "Observations of small Solar-System bodies". hohmanntransfer. 27 October 2011. Retrieved 4 September 2013. (1.5e-06 = 1 in 667,000 chance)
    5. David Morrison (26 October 2011). "Should we be concerned about 2011 UL21". NASA Ask An Astrobiologist. Retrieved 6 November 2011.
    6. "Date/Time Removed". NASA/JPL Near-Earth Object Program Office. Retrieved 6 November 2011.
    7. "2011 UL21 Orbit". Minor Planet Center. Retrieved 17 May 2012. 2012 04 18 (arc=22 years)
    8. "JPL Close-Approach Data: (2011 UL21)". Retrieved 17 May 2012. 2012-04-18 last obs (arc=22 years)
    9. "JPL Small-Body Database Search Engine: PHAs and H < 17 (mag)". JPL Solar System Dynamics. Retrieved 20 November 2011.
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