# (308635) 2005 YU55

(308635) 2005 YU$_{55}$, provisionally named 2005 YU$_{55}$, is a potentially hazardous asteroid 360±40 meters in diameter, as measured after its Earth flyby. Previously it was estimated to be 310 meters or about 400 m (1,300 feet) in diameter. It was discovered on 28 December 2005 by Robert S. McMillan at Steward Observatory, Kitt Peak. On 8 November 2011 it passed 0.85 lunar distances (324,900 kilometers; 201,900 miles) from Earth.

## 8 November 2011 flyby

In February 2010, (308635) 2005 YU$_{55}$ was rated 1 on the Torino Scale for a potential pass near Earth on 10 November 2103, that posed no unusual level of danger. On 19 April 2010, radar ranging by the Arecibo radio telescope reduced uncertainties about the orbit by 50%. This improvement eliminated any possibility of an impact with Earth within the next 100 years. It was removed from the Sentry Risk Table on 22 April 2010 and as such it now has a rating of 0 on the Torino Scale. It is now known that on 10 November 2103 (308635) 2005 YU$_{55}$ will be roughly 2 AU from Earth.
On 8 November 2011 at 23:28 UT the asteroid passed 0.85 LD (330,000 km; 200,000 mi) from Earth. On 9 November 2011 at 07:13 UT the asteroid passed 0.6231 LD (239,500 km; 148,800 mi) from the Moon. During the close approach the asteroid reached about apparent magnitude 11, and would have been visible to experienced observers using high-end binoculars with an objective lens of 80$^+$ mm if it were not for bright moonlight preventing a true dark sky. Since the gibbous moon did interfere with the viewing, observers trying to visually locate the asteroid required a telescope with an aperture of 6 inches (15 centimeters) or larger.
The next few times a known asteroid this large will come this close to Earth will be in 2028 when (153814) 2001 WN5 passes 0.65 LD from Earth, and in 2029 when the 325-meter 99942 Apophis comes even closer at just 0.10 LD.
According to Jay Melosh, if an asteroid the size of (308635) 2005 YU$_{55}$ (~400 m across) were to hit land, it would create a crater 6.3 km (3.9 mi) across, 518 m (1,699 ft) deep and generate a seven-magnitude-equivalent-earthquake. The chances of an actual collision with an asteroid like (308635) 2005 YU$_{55}$ is about 1 percent in the next thousand years.
| NEO | Date | Approach distance in lunar distances | Approach distance in lunar distances | Approach distance in lunar distances | Abs. mag (H) | Diameter $^{(C)}$ (m) | Ref $^{(D)}$ |
| NEO | Date | Nominal$^{(B)}$ | Minimum | Maximum | Abs. mag (H) | Diameter $^{(C)}$ (m) | Ref $^{(D)}$ |
| (152680) 1998 KJ9 | 1914-12-31 | 0.606 | 0.604 | 0.608 | 19.4 | 279–900 | data |
| (458732) 2011 MD5 | 1918-09-17 | 0.911 | 0.909 | 0.913 | 17.9 | 556–1795 | data |
| (163132) 2002 CU11 | 1925-08-30 | 0.903 | 0.901 | 0.905 | 18.5 | 443–477 | data |
| 2010 VB1 | 1936-01-06 | 0.553 | 0.553 | 0.553 | 23.2 | 48–156 | data |
| 2002 JE9 | 1971-04-11 | 0.616 | 0.587 | 0.651 | 21.2 | 122–393 | data |
| 2013 UG1 | 1976-10-17 | 0.854 | 0.853 | 0.855 | 22.3 | 73–237 | data |
| 2012 TY52 | 1982-11-04 | 0.818 | 0.813 | 0.822 | 21.4 | 111–358 | data |
| 2012 UE34 | 1991-04-08 | 0.847 | 0.676 | 1.027 | 23.3 | 46–149 | data |
| 2017 VW13 | 2001-11-08 | 0.373 | 0.316 | 3.236 | 20.7 | 153–494 | data |
| 2002 MN | 2002-06-14 | 0.312 | 0.312 | 0.312 | 23.6 | 40–130 | data |
| (308635) 2005 YU55 | 2011-11-08 | 0.845 | 0.845 | 0.845 | 21.9 | 320–400 | data |
| 2011 XC2 | 2011-12-03 | 0.904 | 0.901 | 0.907 | 23.2 | 48–156 | data |
| 2018 AH | 2018-01-02 | 0.773 | 0.772 | 0.773 | 22.5 | 67–216 | data |
| 2018 GE3 | 2018-04-15 | 0.502 | 0.501 | 0.503 | 23.7 | 35–135 | data |
| 2010 WC9 | 2018-05-15 | 0.528 | 0.528 | 0.528 | 23.5 | 42–136 | data |
| (153814) 2001 WN5 | 2028-06-26 | 0.647 | 0.647 | 0.647 | 18.2 | 921–943 | data |
| 99942 Apophis | 2029-04-13 | 0.0989 | 0.0989 | 0.0989 | 19.7 | 310–340 | data |
| 2012 UE$_{34}$ | 2041-04-08 | 0.283 | 0.274 | 0.354 | 23.3 | 46–149 | data |
| 2015 XJ351 | 2047-06-06 | 0.789 | 0.251 | 38.135 | 22.4 | 70–226 | data |
| 2007 TV18 | 2058-09-22 | 0.918 | 0.917 | 0.919 | 23.8 | 37–119 | data |
| 2005 WY55 | 2065-05-28 | 0.865 | 0.856 | 0.874 | 20.7 | 153–494 | data |
| (308635) 2005 YU55 | 2075-11-08 | 0.592 | 0.499 | 0.752 | 21.9 | 320–400 | data |
| (456938) 2007 YV56 | 2101-01-02 | 0.621 | 0.615 | 0.628 | 21.0 | 133–431 | data |
| 2007 UW1 | 2129-10-19 | 0.239 | 0.155 | 0.381 | 22.7 | 61–197 | data |
| 101955 Bennu | 2135-09-25 | 0.531 | 0.507 | 0.555 | 20.19 | 472–512 | data |
| (153201) 2000 WO107 | 2140-12-01 | 0.634 | 0.631 | 0.637 | 19.3 | 427–593 | data |
| 2009 DO111 | 2146-03-23 | 0.896 | 0.744 | 1.288 | 22.8 | 58–188 | data |
| (85640) 1998 OX4 | 2148-01-22 | 0.771 | 0.770 | 0.771 | 21.1 | 127–411 | data |
| 2011 LT17 | 2156-12-16 | 0.998 | 0.955 | 1.215 | 21.6 | 101–327 | data |
| $^{(A)}$ This list includes near-Earth approaches of less than 1 lunar distances (LD) of objects with H brighter than 24. $^{(B)}$ 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 0.05 and 0.25. $^{(D)}$ 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 Note: All close approaches between 1900 and 2200 are listed (with H<24 at less than 1 LD). Objects not observed during the approach, and simply estimated to have approached on this date, are colored grey. Generically estimated asteroid diameters are given in italics. ||||||||

## Study

During the 2011 passage (308635) 2005 YU$_{55}$ was studied with radar using Goldstone, Arecibo, the Very Long Baseline Array, and the Green Bank Telescope. The Herschel Space Observatory has made far-infrared measurements of (308635) 2005 YU$_{55}$ on 10 November, helping determine its temperature and composition.
Radar analysis has also helped to pin down the asteroid's albedo, or diffuse reflectivity. Although radar measurements do not detect visible light they can determine the distance and size of an object with a high degree of accuracy. This information, coupled with visible light measurements, provides a more accurate measure of an object's absolute magnitude, and therefore its albedo.
On 8 November 2011, NASA released a statement mentioning a number of structures on the surface of the asteroid, which were detected as it passed near Earth. On 11 November 2011, higher-resolution images showed concavities, a ridge near the asteroid's equator, and numerous features interpreted as decameter-scale boulders. Shape modeling based on the radar images shows that YU55's shape is close to spheroidal, with maximum dimensions of 360±40 m, and an equator-aligned ridge. A 150–200 meter-long, ~20 meter-high rise forms a portion of the ridge-line, and the number of boulders on the surface is comparable to that seen on the asteroid 25143 Itokawa by the Hayabusa spacecraft.
Optical lightcurve measurements during the flyby provided a more accurate estimate of the asteroid's spin period – about 19.3 hours. Because (308635) 2005 YU$_{55}$ is so nearly spheroidal, it was not significantly torqued by Earth's tides during the flyby, and there is no evidence of non-principal-axis rotation. Optical, near-infrared, and ultraviolet spectroscopy confirmed that 2005 YU$_{55}$ is a C-type asteroid.

## Future trajectory

On 19 January 2029, (308635) 2005 YU$_{55}$ will pass 0.0023 AU (340,000 km; 210,000 mi) from Venus. The close approach distance to Venus in 2029 will determine how close the asteroid will pass to Earth in 2041. Before the November 2011 observations, the uncertainties in the post-2029 trajectory showed that the asteroid would pass somewhere between 0.002 AU (300,000 km; 190,000 mi) and 0.3 AU (45,000,000 km; 28,000,000 mi) of Earth in 2041. Radar astrometry in November 2011 clarified the Earth passage in 2041 and beyond. As a result of the November 2011 radar observations, it is now known that (308635) 2005 YU$_{55}$ will pass between 0.1017 AU (15,210,000 km; 9,450,000 mi) and 0.1020 AU of Earth on 12 November 2041. Using the current uncertainty region integrated until the future, the 2075 approach will be between 0.0013 AU (190,000 km; 120,000 mi) and 0.0021 AU (310,000 km; 200,000 mi).