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Near Earth Asteroid (66391) 1999 KW4 was first discovered in 1999 by the Lincoln Laboratory’s Near Earth Asteroid Research survey (LINEAR) using telescopes at the Experimental Test Site (ETS) in White Sands Missile Range. The White Sands Missile Range is located near Socorro. New Mexico, close to the Very Large Array (VLA), which was featured in the movie Contact, starring Jodie Foster. White Sands Missile Range was the site of the Trinity test of the first nuclear bomb and also served as a alternate landing field for the Space Shuttle. The early shuttle mission STS-3 landed there in 1982, ironically due to flooding of the runway on the desert playa at Edwards Air Force Base in California.
With over three thousand measurements spanning twenty years taken by many observers since discovery, we have been able to find out a few interesting facts about 1999 KW4. 1999 KW4 orbits the sun every 0.51 years or about 186 days. Its orbit has an eccentricity of 0.69 meaning it is quite elliptical (the eccentricity of a circle is 0). Also its orbit is tilted at a high angle of 39 degrees compared to Earth's orbit.
The above video was produced by the Goldstone Radar Observing team
1999 KW4 has been classified as an Aten asteroid. Aten asteroids' orbits cross the Earth's orbit and 1999 KW4 also crosses the orbit of Mercury and Venus. It gets within 0.2 AU of the Sun. The closest 1999 KW4 will get is 0.0155 AU (1 AU is the average distance from the Earth to the Sun) on 25 May 2036. 1999 KW4 passed closest to the Earth on this visit on May 27th, 2018 when it came 0.078 AU to Earth or about 7.3 million miles. This is the fifth of a series of nine close approaches to the Earth each year, all around the end of May or start of June. Next year’s close approach on 25 May 2019 will be the closest of the series at 3.2 million miles from the Earth.
Radar images from the closest approach in the last set of close approaches in 2001 confirmed that 1999 KW4 is actually a binary asteroid. The main body (primary) of 1999 KW4 is about 1300 meters across, but is actually quite a complex shape. It is slightly squashed at the poles and with a mountain ridge around the equator, which runs all the way around the asteroid. This ridge gives the primary an appearance similar to a walnut or a spinning top. The secondary is about 500 meters across and the two asteroids orbit each other every 17.5 hours at a distance of about 1.6 miles. We have a good idea of what the asteroid is made up of from previous radar images. The asteroid is a rocky “rubble pile” but there must be considerable gaps between the “rubble” as the density is lower than that of typical Earth rocks.
1999 KW4 is a radar target for both of the planetary radars at Goldstone and the Planetary Radar Science group working at the Arecibo Observatory. The Arecibo Observatory is home to the world's largest radio telescope which is located in Puerto Rico. Their goals are to take observations that will improve the model of 1999 KW4's shape, surface properties and measure the rotation period of the binary and of the primary and secondary. They do this by sending radio waves at the asteroid and measuring the waves that bounce off. With these measurements, the radar groups will also be able to refine its orbit and determine the size precisely.
While the radar teams are planning on observing 1999 KW4 to find out more about the asteroid's physical properties and the separation between the two bodies, we will be trying to improve the knowledge of its rotational period from light curve analysis. The two previous measurements of the rotation period of the primary differ considerably with one value of about 2.76 hours and another of 9.6 hours. These are nearly 9 times or 2.5 times shorter than an a day on Earth!