Asteroid 2014 JO25 was imaged by radar from NASA's Goldstone Deep Space Communications Complex in California one day before its closest approach to Earth. A grid composed of 30 images shows the two-lobed asteroid in different rotations. The space rock passed Earth on April 19, 2017, at a distance of 1.1 million miles (1.8 million kilometers).

A graphic shows asteroid 2014 JO25 as it is projected to fly safely past Earth on April 19, 2017, at a distance of about 1.1 million miles or about 4.6 times the distance from Earth to the moon.

This graphic illustrates asteroid 2016 HO3 orbiting Earth as the pair go around the sun together. The asteroid was first spotted on April 27, 2016, by the Pan-STARRS 1 asteroid survey telescope on Haleakala, Hawaii.

All about asteroids This graphic shows the track for asteroid 2004 BL86, which flew about 745,000 miles from Earth on January 26, 2015. That's about three times as far away as the moon.

This graphic shows the path Asteroid 2014 RC took as it passed Earth on September 7, 2015. The space rock came within one-tenth the distance from Earth to the moon.

A diagram shows the orbit of an asteroid named 2013 TV135 (in blue), which made headlines in September 2013 when it passed close by Earth. The probability of it striking Earth one day stands at 1 in 63,000, and even those odds are fading fast as scientists find out more about the asteroid. It will most likely swing past our planet again in 2032, according to NASA.

Asteroid 2012 DA14 made a record-close pass -- 17,100 miles -- by Earth on February 15, 2013. Most asteroids are made of rocks, but some are metal. They orbit mostly between Jupiter and Mars in the main asteroid belt. Scientists estimate there are tens of thousands of asteroids and when they get close to our planet, they are called near-Earth objects.

Another asteroid, Apophis, got a lot of attention from space scientists and the media when initial calculations indicated a small chance it could hit Earth in 2029 or 2036. NASA scientists have since ruled out an impact, but on April 13, 2029, Apophis, which is about the size of 3 football fields, will make a close visit -- flying about 19,400 miles (31,300 kilometers) above Earth's surface. The images above were taken by the European Space Agency's Herschel Space Observatory in January 2013.

If you really want to know about asteroids, you need to see one up close. NASA did just that. A spacecraft called NEAR-Shoemaker, named in honor of planetary scientist Gene Shoemaker, was the first probe to touch down on an asteroid, landing on the asteroid Eros on February 12, 2001. This image was taken on February 14, 2000, just after the probe began orbiting Eros.

The first asteroid to be identified, 1 Ceres, was discovered January 1, 1801, by Giuseppe Piazzi in Palermo, Sicily. But is Ceres just another asteroid? Observations by NASA's Hubble Space Telescope show that Ceres has a lot in common with planets like Earth. It's almost round and it may have a lot of pure water ice beneath its surface. Ceres is about 606 by 565 miles (975 by 909 kilometers) in size and scientists say it may be more accurate to call it a mini-planet. NASA's Dawn spacecraft is on its way to Ceres to investigate. The spacecraft is 35 million miles (57 million kilometers) from Ceres and 179 million miles (288 million kilometers) from Earth. The photo on the left was taken by Keck Observatory, Mauna Kea, Hawaii. The image on the right was taken by the Hubble Space Telescope.

One big space rock got upgraded recently. This image of Vesta was taken by the Dawn spacecraft, which is on its way to Ceres. In 2012, scientists said data from the spacecraft show Vesta is more like a planet than an asteroid and so Vesta is now considered a protoplanet.

Asteroids have hit Earth many times. It's hard to get an exact count because erosion has wiped away much of the evidence. The mile-wide Meteor Crater in Arizona, seen above, was created by a small asteroid that hit about 50,000 years ago, NASA says. Other famous impact craters on Earth include Manicouagan in Quebec, Canada; Sudbury in Ontario, Canada; Ries Crater in Germany, and Chicxulub on the Yucatan coast in Mexico.

NASA scientists say the impact of an asteroid or comet several hundred million years ago created the Aorounga crater in the Sahara Desert of northern Chad. The crater has a diameter of about 10.5 miles (17 kilometers). This image was taken by the Space Shuttle Endeavour in 1994.

In 1908 in Tunguska, Siberia, scientists theorize an asteroid flattened about 750 square miles (1,200 square kilometers) of forest in and around the Podkamennaya Tunguska River in what is now Krasnoyarsk Krai, Russia.

One of the top asteroid-tracking scientists is Don Yeomans at the Jet Propulsion Laboratory, which is managed by the California Institute of Technology. Yeomans says every day, "Earth is pummeled by more than 100 tons of material that spewed off asteroids and comets." Fortunately, most of the asteroid trash is tiny and it burns up when it hits the atmosphere, creating meteors, or shooting stars. Yeomans says it's very rare for big chunks of space litter to hit Earth's surface. Those chunks are called meteorites.

Asteroids and comets are popular fodder for Earth-ending science fiction movies. Two of the biggest blockbusters came out in 1998: "Deep Impact" and "Armageddon." (Walt Disney Studios) Others include "Meteorites!" (1998), "Doomsday Rock" (1997), "Asteroid" (1997), "Meteor" (1979), and "A Fire in the Sky" (1978). Can you name others?

Asteroid 1998 QE2 is about 3.75 million miles from Earth. The white dot is the moon, or satellite, orbiting the asteroid.

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NASA's planetary defense system will be put to the test in October - CNN