What You Missed v. What Missed You

Wow, that was close!

Did you see that?!?

There was not one, but two, near misses in the inner solar system last week. Asteroid TU24 buzzed by Earth last Tuesday while Asteroid WD5 just missed Mars on Wednesday. Who knew there was so much traffic amongst the planets?!

The Minor Planet Center (MPC) knew. They know a lot. They operate from the Smithsonian Astrophysical Observatory (SAO) under the auspices of the International Astronomical Union (IAU). They are responsible for naming, compiling, checking, and making available all data related to minor bodies in the solar system. These include all natural satellites (moons), asteroids, centaurs, Trans-Neptunian/Kuiper Belt objects, and comets. Traditionally asteroids are small rocky bodies that collectively occupy and define the asteroid belt, the region between the orbits of Mars and Jupiter. It is thought that there are close to two million asteroids in the solar system above 1 km in diameter. (For reference the smallest moon in the solar system is Mars' second moon, Deimos, which is only seven miles in diameter). This number is only an estimate because due to their small size and composition, asteroids are inherently hard to detect. 

To be visible through a telescope, even a very powerful one, an object must reflect a fairly large amount of sunlight. The larger the object, the more light it reflects. And the more light it reflects, the farther away you can see it. The same is true when you drive on a dark road at night; the larger and more reflective an object on the road is, the better and the earlier you will see it in your headlights. Asteroids typically have an albedo, or reflectivity, of less than 5%, meaning they reflect less than 5% of the sunlight that hits them, much like fresh asphalt. This is what makes them so hard to see.

The term asteroid is Greek for "star-like" and the first asteroid, Ceres, was discovered in 1801. The discovery of their numbers was slow at first until computerized methods were developed late last century. At first, they weren't of much interest to astronomers - some even termed them "vermin of the skies" - but when asteroids with orbits very near to Earth began to be discovered, the potential threat of an Earth impact caused astronomers to take notice. With the 1994 and crash of Comet Shoemaker-Levy into Jupiter and the increasing acceptance of the proposal that an asteroid impact may have caused the Cretaceous-Tertiary extinction, the threat became real. Now everyone is paying attention and each new discovery of a near-Earth object (NEO) brings with it both the hype and the fear that this may be the big one. One day that may be the case (that's a topic for another post), but today NASA's Near-Earth Object Program Office has it all under control.

Asteroids TU24 and WD5 were discovered by the Catalina Sky Survey on October 11th and November 20th, 2007, respectively, but merely seeing an asteroid once means nothing. For astronomers to properly calculate its physical properties such as size, mass, composition, rotation, and orbit, an asteroid must be observed multiple - the more, the better. Of these, size and orbit matter most when a potential collision with Earth is at stake. Multiple observations over consecutive nights enable astronomers to see how the light reflected by the asteroid changes, thereby helping them to estimate its overall shape and size. Tracking its path with respect to background stars gives a measure of its distance from Earth and its orbital path. Taken together, these parameters help astronomers to determine whether or not the asteroid is likely to pose a danger to us or not. Potentially hazardous asteroids whose orbits bring them close to Earth or across our orbital path are put on a watch list and are continually monitored by telescopes and observers all over the world. As a result of detailed follow-up observations of Asteroids TU24 and WD5 we now know much more about them.

Meet Asteroid TU24:


This sequence of images was taken by the Goldstone Solar System Radar Telescope out in the Mojave Desert, over the course of a few hours. Though the resolution isn't great, about 20-meters per pixel, the shift of the brightest spot in each image clearly shows how the asteroid is rotating. These images also allowed astronomers to refine their estimate of the asteroid's size to approximately 800 feet (250 meters) in diameter, or about the length of one and a half cross-town blocks in Manhattan. In the end, TU24 passed us at the safe distance of 0.003 astronomical units - 1.4 times this distance to the moon - roughly 334,000 miles. Twelve hours before this closest approach, the Arecibo and Green Bank radio telescopes were able to get this snazzy image.


And now for Asteroid WD5:

This one is a bit camera shy, and so with fewer observations, less is known about it. But the most important detail - that it would not hit Earth - was thankfully confirmed. Mars was a different story though. It was given 1 in 75 odds to brace for impact, an exceptionally high probability when astronomers primarily deal with one in a million and one in a billion statistics. Still, that's almost a 99% chance of WD5 missing Mars completely. The asteroid's orbit skims close to Earth's orbital path (long after we had been there) and continues on to intersect Mars', as illustrated in this diagram released by NASA in mid-December.


To determine if Mars and WD5 would both reach the same point in space at the same time, astronomers additional observations were required to accurately measure the asteroid's speed. Upper limits estimated WD5 to be traveling 13-and-a-half kilometers per second which still left some uncertainty as to whether or not a collision with Mars was inevitable. This animation (click to play) shows how that uncertainty factors in to where the asteroid is likely to pass across Mars' orbit.


While both Earth and Mars escaped the events of last week unscathed, near Earth objects are an actual concern that might one day pose a true danger. As of today, the inner solar system looks like this...


The Minor Planet Center updates this plot daily and it's hard not to look at it and realize that the statistics may not be in our favor in the long run.

(Photo credits: Credit: Arecibo/Green Bank, NASA/JPL-Caltech, Gareth Williams/Minor Planet Center)