On October 31, a 1,300-foot space rock being called the "Halloween Asteroid" (its actual name is "2015 TB145") will hurtle by at about 1.3 times the distance to the Moon. The timing of 2015 TB145 is spooky, but so is the fact that it managed to sneak up on us. We didn't even know it existed until October 10--just 21 days before its arrival.
  1. 1.
    Why Didn't We See It Coming?
    Asteroid 2015 TB145 was discovered by Pan-STARRS, a large telescope in Hawaii that searches the skies for moving objects. According to principal investigator Richard Wainscoat, it was difficult to spot due to its strange orbit. The Sun & the planets are arranged in a flat disk shape ("the ecliptic plane"). Most of the asteroids in the belt between Mars and Jupiter can be found along the ecliptic plane, but the Halloween Asteroid cuts a path at a 40-degree angle to the line of the ecliptic plane.
  2. 2.
    Not finding it until it was so close to the Earth could have been dangerous.
    A 1,300-foot (400-meter) asteroid like the Halloween one, could smack Earth like 2800 metric tons of dynamite, leaving behind a crater 3.7 miles wide and producing continental scale devastation.
  3. 3.
    However, 2015 TB145 will remain at a safe distance from Earth.
    If it were on a crash course with Earth, it would take more than a handful of candy to keep it from defacing our home. Paul Chodas, who heads up NASA's Near Earth Object program, told Popular Science, "If it were headed directly for Earth, that would have been too late to do anything about it. An asteroid of this size is really difficult to deflect with only 20 days warning."
  4. 4.
    It's not near Earth very often.
    It orbits the Sun every three years, but like other asteroids with oval-shaped orbits, it doesn't move at a constant velocity. When it is furthest from the inner solar system (and Earth) it moves slowly. At that point it is far away, faint and hard to detect. And when it swings back near Earth, the Sun's gravity speeds it up and it zips past us. The asteroid spends less than 5 percent of its time in the inner solar system, says Chodas.
  5. 5.
    But it has passed by the Earth before.
    "The last time it made a close approach like this was in 1975, when we were not really searching the sky," he says. At that time, the asteroid came near Earth's track around the Sun, but at a time when Earth was in a different spot along the track.
  6. 6.
    So the "Halloween asteroid" won't hit Earth. But what about others like it?
    Although our telescopes have spotted about 90% of the kilometer-sized (3,300-foot) asteroids that would endanger life on Earth, the medium-sized rocks like 2015 TB145 are harder to track. "I wouldn't expect a large number of asteroids to be in this kind of orbit," says Chodas. "It's quite peculiar… The majority are in orbits that are much easier to detect." Rocks the size of the "Halloween asteroid" only hit the Earth about once every 100,000 years or so. So the odds of an impact are very small.
  7. 7.
    You should be able to see the "Halloween asteroid" as it passes us.
    Asteroid 2015 TB145 is expected to reach its closest approach to Earth at about 1:18 pm Eastern. It'll blast past us at a speed of about 78,300 miles an hour, and should be bright enough to see with a decent telescope and a star chart.
  8. 8.
    What can we learn from the asteroid?
    There's an important reason to observe the asteroid. "The more asteroids we can see up close," says Chodas, "the more we'll learn about them, and the better prepared we'll be if we ever need to deflect one." The asteroid's proximity will give scientists an excellent chance to study it. Scientists hope to image it with a resolution as high as 6.5 feet (2 meters) per pixel, and radar measurements will help shed light on the asteroid's precise size and composition.
  9. 9.
    Happy Halloween, Halloween Asteroid!
    Read more about the asteroid at!