An animation shows how the orbit looks.
Credit: Dr. Steven Ostro et al./NASA
A very large asteroid with its own little moon will pass next to Earth tonight (May 25), close enough so that, with a little preparation and a decent telescope, amateur astronomers can detect that it erases the stars.
This moon and asteroid system, called 1999 KW4, is made up of two rocks. The large one is about 0.8 miles (1.3 kilometers) wide, according to NASA, and has the shape of a spinning top. The smaller one is more elongated and extends to 0.35 miles (0.57 km) along its longest dimension. Aim along its much larger twin.
Together, the asteroid and its minimoon will pass to Earth at such a strange and pronounced angle that NASA called them "the least accessible … for a spacecraft mission of any known near-by binary asteroid on Earth." [Doomsday: 9 Real Ways Earth Could End]
But that does not mean they are not interesting to look at.
The two asteroids will pass closer to Earth at 7:05 p.m. EDT (1105 GMT), when they will be only 3,219,955 miles (5,182,015 km) from the planet's surface. That's more than a dozen times the distance between Earth and the Moon in its orbit around our planet, and too far for the space rocks to pose a threat. In fact, this is the fourth approach that binary asteroids have made to Earth since they were discovered in 1999, and not the closest one. This is not the first time, according to EarthSky, that astronomers plan to make radar images of these asteroids as they pass.
On May 25, 2001, according to NASA, asteroids passed about 6.7% closer to Earth than this time, at a distance of 3,005,447 miles (4,836,798 km). Within 17 years, on May 25, 2036, the rocks will pass 55.2% closer to Earth, at a distance of only 1,443,511 miles (2,323,106 km), once again, they do not pose a worthy threat to worry.
These large rocks have been frequent travelers in the neighborhood of our planet for a long time.
"The KW4 of 1999 approaches 0.05 AU on Earth several times every century," said the NASA report on the object. "This trend exists from at least [the year] 1600 [to] 2500. " [Black Marble Images: Earth at Night]
"AU" refers to "astronomical units", a unit equal to the distance between the Earth and the sun. Therefore, 0.05 AU is equal to one twentieth of the distance between Earth and the Sun, or approximately 4,650,000 miles (7,480,000 km). The two asteroids have passed even closer to Earth, without incident, several times per century since William Shakespeare was writing, and will continue to do so until this article is at least 500 years old.
EarthSky reported that during the closest approach of the space rocks, they will be more visible in the southern hemisphere, appearing as fast-moving shadows against the stars in the constellation of Puppis. However, the two asteroids will remain visible for several days, according to EarthSky. Asteroid hunters in North America can see objects near the Hydra constellation on the afternoon of May 27.
NASA said its Office of Planetary Defense Coordination will continue to closely monitor the asteroids.
Originally published in Living science.