(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 by	Catalina Sky Srvy.
Discovery site	Catalina Stn.
Discovery date	17 October 2011
Designations
MPC designation	(415029) 2011 UL21
Alternative designations	2011 UL21
Minor planet category	Apollo · NEO · PHA [2]
Orbital characteristics[2]
Epoch 13 January 2016 (JD 2457400.5)
Uncertainty parameter 0
Observation arc	9379 days (25.68 yr)
Aphelion	3.5091 AU (524.95 Gm)
Perihelion	0.73606 AU (110.113 Gm)
Semi-major axis	2.1226 AU (317.54 Gm)
Eccentricity	0.65323
Orbital period	3.09 yr (1129.5 d)
Mean anomaly	110.19°
Mean motion	0° 19^m 7.392^s / day
Inclination	34.845°
Longitude of ascending node	275.60°
Argument of perihelion	284.74°
Earth MOID	0.0185982 AU (2.78225 Gm)
Physical characteristics
Mean diameter	2.5 km[3]
Mass	2.1×10^13 kg (assumed)[3]
Absolute magnitude (H)	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 QO104	1981-05-18	2.761	2.760	2.761	16.0	1333–4306	data
2014 LJ21	1989-08-01	7.034	6.843	7.224	16.0	1333–4306	data
4179 Toutatis	1992-12-08	9.399	9.399	9.399	15.30	2440–2450	data
4179 Toutatis	2004-09-29	4.031	4.031	4.031	15.30	2440–2450	data
(159857) 2004 LJ1	2038-11-16	7.719	7.719	7.719	15.4	1746–4394	data
(4953) 1990 MU	2058-06-05	8.986	8.984	8.988	14.1	3199–10329	data
4179 Toutatis	2069-11-05	7.725	7.724	7.725	15.30	2440–2450	data
(52768) 1998 OR2	2079-04-16	4.611	4.611	4.612	15.8	1462–4721	data
(415029) 2011 UL21	2089-06-25	6.936	6.935	6.938	15.7	1531–4944	data
3200 Phaethon	2093-12-14	7.714	7.709	7.718	14.6	4900–5300	data
(52768) 1998 OR2	2127-04-16	6.536	6.510	6.563	15.8	1462–4721	data
^(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.

External links

• (415029) 2011 UL21 at NeoDyS-2, Near Earth Objects—Dynamic Site
  • Ephemeris · Obs prediction · Orbital info · MOID · Proper elements · Obs info · Close · Physical info · NEOCC
• (415029) 2011 UL21 at the JPL Small-Body Database
  • Close approach · Discovery · Ephemeris · Orbit diagram · Orbital elements · Physical parameters