Don’t get out your nuclear missiles. All dinosaurs are safe this time, as the closest approach of asteroid 1999 KW4 will be about 3 million miles—about 12 times the distance from Earth to Moon. So on the list of scary encounters, this is low on the “buy canned food” scale. In fact, this same asteroid has swung past four times before since its discovery, and on one of those occasions it came closer still.
However, it’s big on the interesting scale, because the asteroid sweeping by on Saturday isn’t one body, it’s two. The space rock first found by the Lincoln Near-Earth Asteroid Research (LINEAR) collaboration between NASA, the Air Force, and MIT back in 1994, was determined by JPL researchers in 2001 to actually be a “binary pair” where one asteroid acts as the moon of the other.
1999 KW4 Image from team led by Dr. Steven Ostro at NASA's Jet Propulsion Laboratory
Radar imaging during previous close approaches has made the nature of the system and the shape of the two big stones much more apparent. The larger of the pair is just under 0.8 miles wide, making it about 1/100th the mass of the asteroid or comet that carved out the Chicxulub crater and made Tyrannosaurs a non-factor in your daily commute. However, it’s still large enough that if it were to impact, it could be a civilization-ending event. Thankfully neither the big asteroid, or its 0.3 mile moonlet is going to to get all that near on Saturday night. However, they will be back in 2036 for an approach that’s more than twice as close.
Even then, they’re not any kind of threat; unlike the kilometer-wide asteroid 99942 Apophis which is set to pass closer to the Earth than many satellites in 2029. On that one, people will definitely be double-checking their math, and maybe hoping that the planned Asteroid Redirect Mission produces good results. Apophis was named for the “uncreator” who was an enemy of Ra in Egyptian myths so someone seems to have been a little pessimistic when working out those orbital results.
The 1999 KW4 pair will not be visible to the naked eye, but telescope operators may spot it in the night sky over the next few days near the constellation Hydra.
