A Near-Earth Object is an asteroid or comet that is in an orbit that sometimes brings it close to the Earth. The largest of these are known as Potentially Hazardous Objects as they could cause significant damage if they impacted our planet. There are sometimes news reports of a “close approach” where an object passes particularly near the Earth. An asteroid called 2013 TX68 will pass the Earth on March 8, 2016. It is estimated to be 100 feet in diameter.
Astronomers commonly use a unit of measurement called Lunar distance to describe how close to the Earth an object is. A Lunar distance (LD) of 1.0 is the average distance to our Moon. That is just under a quarter of a million miles.
Objects passing near the Earth happen more frequently than one might expect. A number of these approaches will happen this month. On March 1 an object called 2016 DL1 came within 4.7 LD of our planet and it is about 120 feet in diameter. On March 3 an object called 2016 DV1 passed within 1.0 LD and it is about 290 feet in diameter. Another called 2016 DN2 will pass within 1.8 LD on March 5 and it is about 140 feet across. The 2013 TX68 pass on March 8 will be a bit farther at 13.0 LD.
There have been at least 70 objects that have passed within 10 LD of our planet since early January 2016. While it is important for astronomers to track these object in case one is on a collision course with the Earth the majority of these objects safely fly past our home world and this happens somewhat frequently. It is, however, an opportunity for scientists to study the objects “up close” to learn more about them.
In November 2008 I observed the close approach of an asteroid called 1993 KH using a telescope and digital camera at the Brown University Department of Physics.
The movie shows a time-lapse of 40 minutes of motion. The image has a field of view of about 21 arcminutes across, or about 2/3 the diameter of the full moon. There are seven more close approaches by 1993 KH in the next century, but none of them are much closer than this one. The exact size is not known but it is likely between 2,000 and 4,000 feet across. At close approach it was about 39 LD. It missed us by about 9 million miles. The next close approach is in November of 2019. Click on the image below to see the video.
There is also a second asteroid visible in the frame. Can you spot it? It is a Trojan out near Jupiter. Because of the much greater distance it barely appears to move. Menestheus is much larger, about 40 miles in diameter.