In Depth

Asteroid 99942 Apophis is a near-Earth asteroid more than 1000 feet (over 300 meters) in size that will harmlessly pass close to Earth on April 13, 2029. When it was discovered in 2004, the asteroid caused a stir because initial calculations indicated a small possibility it would impact Earth in 2029.

After searching through some older astronomical images, scientists ruled out the possibility of a 2029 impact. It’s now predicted the asteroid will safely pass about 19,800 miles (31,900 kilometers) from our planet’s surface. While that’s a safe distance, it’s close enough that the asteroid will come between Earth and our Moon, which is about 238,855 miles (384,400 kilometers) away. It’s also within the distance that some spacecraft orbit Earth.

It’s rare for an asteroid of this size to pass so close to Earth, although smaller asteroids, in the range of 16 to 33 feet (5 to 10 meters), in size have been observed passing by at similar distances.

“The Apophis close approach in 2029 will be an incredible opportunity for science,” said Marina Brozović, a radar scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California, who works on radar observations of near-Earth objects (NEOs). “We’ll observe the asteroid with both optical and radar telescopes. With radar observations, we might be able to see surface details that are only a few meters in size.”

During its 2029 flyby, Apophis will first become visible to the naked eye in the night sky over the southern hemisphere and will look like a speck of light moving from east to west over Australia. It will be mid-morning on the U.S. East Coast when Apophis is above Australia.

Apophis will then cross above the Indian Ocean, and continuing west, it will cross the equator over Africa.

At its closest approach to Earth, just before 6 p.m. EDT, April 13, 2029, Apophis will be over the Atlantic Ocean. It will move so fast that it will cross the Atlantic in just an hour. By 7 p.m. EDT, the asteroid will have crossed over the United States.

As it passes by Earth, it will get brighter and faster. At one point it will appear to travel more than the width of the full Moon within a minute and it will get as bright as the stars in the Little Dipper.

Apophis is named for the demon serpent who personified evil and chaos in ancient Egyptian mythology.

Exploration

Apophis was discovered on June 19, 2004, by astronomers Roy Tucker, David Tholen and Fabrizio Bernardi at the Kitt Peak National Observatory in Tucson, Arizona. They were only able to observe the asteroid for two days because of technical and weather problems. Fortunately, a team at the Siding Spring Observatory in Australia spotted the asteroid again later in the same year.

Since its discovery, optical and radar telescopes track Apophis as it orbits the Sun and scientists are confident they know its future trajectory. Current calculations show that Apophis still has a very small chance of impacting Earth — less than 1 in 100,000 many decades from now.

The most important observations of Apophis will come during its close Earth flyby in 2029. Scientists around the world will study the asteroid’s size, shape, composition and possibly even its interior.

Size and Distance

Apophis is a 1,120-foot-wide (340-meter-wide) asteroid. That’s about the size of three-and-a-half football fields.

At its farthest, Apophis can reach a distance of about 2 astronomical units (One astronomical unit, abbreviated as AU, is the distance from the Sun to Earth.) away from Earth. It’s expected to safely pass close to Earth — within 19,794 miles (31,860 kilometers) from our planet’s surface — on April 13, 2029. This is the closest approach by an asteroid of this size that scientists have known about in advance.

Orbit and Rotation

The orbit of Apophis crosses the orbit of Earth. It completes an orbit around the Sun in a bit less than one Earth year (about 0.9 years). This places it in the group of Earth-crossing asteroids known as "Atens," whose orbits are smaller in width than the width of Earth's orbit, or 1 AU. As a result of its close encounter with Earth in 2029, the asteroid's orbit will be widened to become slightly larger than the width of Earth's orbit. At this point it will be reclassified from the Aten group to the "Apollo" group (the group of Earth-crossing asteroids with orbits wider than 1 AU).

The asteroid “wobbles” as it spins about its short axis, typically rotating about once every 30 hours. Sometimes, there is also a “rocking” motion back and forth about its long axis, as well, which occurs over a longer period than the short axis wobble. (The technical term for this rocking motion is “non-principal axis rotation.”)

Structure

Apophis is classified as an S-type, or stony-type asteroid, made up of silicate (or rocky) materials and a mixture of metallic nickel and iron. Radar images suggest it is elongated and possibly has two lobes, making it look something like a peanut. Much more will be learned about this asteroid's structure following its close flyby of Earth in 2029.

Formation

Like all asteroids, Apophis is a remnant from the early formation of our solar system about 4.6 billion years ago. It originated in the main asteroid belt between Mars and Jupiter. Over millions of years, its orbit was changed primarily by the gravitational influence of large planets like Jupiter so that it now orbits the Sun closer to Earth. As a result, Apophis is classified as a near-Earth asteroid, as opposed to a main-belt asteroid.

Surface

There are no high-resolution images of the surface of asteroid Apophis, but it is likely similar to surfaces of other stony-type asteroids like Itokawa, the first asteroid from which samples were captured and brought to Earth for analysis.

Atmosphere

None

Magnetosphere

None

Rings

None

Moons

None

Potential for Life

None

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