Asteroid Apophis to sweep close 7 years from now

Animation of the orbits of Earth and Apophis showing how often they cross each other.
Orbit of asteroid Apophis (pink) in contrast to the orbit of Earth (blue). The yellow dot represents the sun. See the day and year in the upper left corner? And see how close Apophis and Earth will come in 2029? Image via Phoenix7777/ Wikimedia Commons.

Seven years from today – on Friday, April 13, 2029 – a relatively large and extremely infamous asteroid named 99942 Apophis will zoom past Earth. It’ll be easily visible to the eye. Many astronomers will study it. But Apophis will not strike us in 2029. For a time, initial observations of this asteroid suggested that – if, at the 2029 pass, Apophis passed through a region of space only half a mile wide (about 800 meters wide), dubbed a “keyhole” by astronomers – then it might strike us exactly seven years later on April 13, 2036. But, by 2006, that idea had also been disproven.

Apophis is exciting! But it’s not frightening. Here’s the updated story on this amazing asteroid.

And, by the way, Apophis is pronounced uh-pah’-fs.

Location, location, location

Apophis is a space rock about 1,000 feet (340 meters) across. Calculations in recent years have proven the asteroid will safely glide past Earth in both 2029 and 2036. In 2029, Apophis should pass at a nominal distance of 19,662 miles (31,643 km) from the Earth’s surface. That’s in contrast to the moon’s average distance of about 250,000 miles (380,000 km). And it’s closer than many Earth-orbiting satellites. As the asteroid encounters Earth’s gravitational field in 2029, one result could be asteroid-quakes on Apophis. This passage will also change the orbit of Apophis slightly.

Not everyone will be able to see Apophis in 2029. If you are in Australia, southern Asia, southern Europe, or Africa, you will have a front-row seat to see this asteroid when it is at its brightest. As the asteroid moves farther from the Earth and dims, it becomes visible in eastern South America. As evening falls along the east coast of North America, the asteroid will be a telescopic object located in a part of the sky about 15 degrees north of the Pleiades. An ephemeris for the asteroid is here.

Black and white shaded image of a large irregular body with small pebbles stuck to it.
Artist’s concept of asteroid Apophis. Radar measurements estimate Apophis is roughly 1,500 feet (450 meters) wide and 550 feet (170 meters) tall. Image via The Planetary Society.

Discovery of Apophis

Astronomers at Kitt Peak National Observatory near Tuscon, Arizona, discovered Apophis on the evening of June 19, 2004. The team of Dave Tholen, Fabrizio Bernardi, and the late Roy Tucker were searching for asteroids low in the western sky. They were specifically looking for objects in the direction of the sun. The asteroid they found was originally designated 2004 MN4. It was 57 degrees from the sun, unusually close for an asteroid.

But astronomers quickly recognized this asteroid was different from most. It orbits the sun in less than one Earth-year (Apophis takes 323.6 days to orbit the sun. Earth takes 365.3 days). And Apophis gets nearly as close to the sun as the planet Venus, then heads out to just beyond Earth’s orbit. Its orbit defines Apophis as what astronomers call an Aten-class asteroid.

In the course of its orbit, Apophis can pass very close to the Earth. This fact quickly caught the attention of astronomers worldwide. By December 2004, they had enough data to make a rough calculation of the future orbit of the asteroid. And they found it had a 2.7% chance of hitting the Earth in April 2029, on Friday the 13th. That same month, Apophis was moved to the top of the list of potentially hazardous asteroids.

You can imagine the media frenzy that resulted.

Probability of collision reduced by precision

It took several more years of studying this asteroid to learn it would not strike Earth in 2029. The fact is, an asteroid’s orbital path can be changed slightly, every time it passes near another astronomical object. And it can be changed by what’s called the Yarkovsky effect, a minor push on the asteroid, caused by sunlight.

Both are known effects. But astronomers can determine the extent of these effects only after careful measurement of an asteroid’s positions over the course of years.

And observing this asteroid year after year isn’t as straightforward as you might think. Some years, the asteroid isn’t observable because it appears too close to the sun, as seen from the Earth. So astronomers imaged Apophis extensively whenever it was visible. And, by 2006, they were able to determine that Apophis won’t hit the Earth in 2029.

Whew, we dodged that one! But what about the next close approach in 2036? That possibility was eliminated in 2013.

In early March of 2013, all eyes turned toward Apophis as the asteroid made a relatively close sweep (though not nearly as close as in 2029) to our planet on March 6. The Goldstone Deep Space Communications Complex tracked the asteroid for about two weeks around the closest approach. Researchers at the Green Bank Telescope took observations, coordinating with Goldstone because the use of these two telescopes together allows the data to be sharper. The coordination between the two telescopes meant that Goldstone was transmitting data while Green Bank was receiving, performing what is known as a bistatic experiment that doubled the strength of the received signal. Marina Brozovic of NASA’s Jet Propulsion Laboratory explained:

Apophis made a [close approach in 2013] with Earth, it was still nearly 10.6 million miles (17 million kilometers) away. Even so, we were able to acquire incredibly precise information about its distance to an accuracy of about 490 feet (150 meters).

Later calculations let NASA scientists announce on March 26, 2021, that Earth is safe from an impact with the relatively large asteroid for at least the next 100 years. Radar observations taken at NASA’s Goldstone Deep Space Communications Complex in California and the Green Bank Observatory in West Virginia have officially ruled out an impact in 2068, the only year out of the next 100 that previously showed a slight risk. Earlier observations had ruled out impacts during the upcoming 2029 and 2036 flybys.

Davide Farnocchia of NASA’s Center for Near-Earth Object Studies said:

A 2068 impact is not in the realm of possibility anymore, and our calculations don’t show any impact risk for at least the next 100 years.

This new analysis means that Apophis is no longer on the Sentry Impact Risk Table, which is a list of objects that pass so close by Earth that astronomers have not yet been able to rule out a possible strike.

This campaign not only helped us rule out any impact risk, but it also set us up for a wonderful science opportunity in 2029.

Field of stars with a brighter smudge in three places, each circled in white.
These images show asteroid Apophis during 3 days of its flyby on March 8, 9 and 10, 2021. Radio antennas at the Deep Space Network’s Goldstone Complex in California and the Green Bank Telescope in West Virginia, worked together to acquire these images. The asteroid was 10.6 million miles (17 million km) away, and each pixel has a resolution of 127 feet (39 meters). Image via NASA/ JPL-Caltech/ NSF/ AUI/ GBO.

The images seen at top are the product of the collaboration. Brozovic went on to describe the excellent quality achieved through the collaboration, which she called:

… a remarkable resolution, considering the asteroid was 10.6 million miles (17 million kilometers) away, or about 44 times the Earth-moon distance. If we had binoculars as powerful as this radar, we would be able to sit in Los Angeles and read a dinner menu at a restaurant in New York.

Many tiny white parallel streaks on a dark gray background; an arrow pointing to a dot in the middle.
The Virtual Telescope Project, based in Rome, Italy, captured asteroid (99942) Apophis on March 2, 2021. The asteroid shows as a dot – while the stars around it show as streaks – because the telescope was tracking the asteroid’s motion. It is moving through space with respect to Earth at 2.894 miles/sec (4.658 km/sec). Image via Virtual Telescope.

Astronomers also studied asteroid Apophis using NASA’s NEOWISE infrared space telescope in April 2021. This is the same telescope that discovered 2020’s favorite comet, Comet NEOWISE. They found the asteroid is about 1181 feet (360 meters) across and reflects about 30% to 50% of the light that strikes it. They also suspect the asteroid is “significantly elongated”. The NEOWISE report is here.

A gentle effect that pushes a rock

Astronomers in Hawaii studied how Yarkovsky acceleration, or pushes due to sunlight, would change Apophis’ orbit. In some instances, acceleration – a change in an object’s speed and direction through space – can help avoid a collision. Studies of Yarkovsky acceleration as related to asteroid Apophis suggest this is the case for this asteroid.

Astronomer Dave Tholen and colleagues suggest that Apophis is drifting more than 500 feet (about 170 meters) per year from its expected position in its orbit. These observations aren’t easy to obtain and analyze. Factors such as the asteroid’s distance at the time of observation, its composition, its shape, and its surface features all affect the outcome.

Read more about the Yarkovsky effect: Pushing asteroids around with sunlight

Earth inside ring of very many dots, and the path of the asteroid as a yellow line passing close to the dots.
This animation shows the distance between the Apophis asteroid and Earth at the time of the asteroid’s closest approach in 2029. The blue dots are manmade satellites orbiting our planet, and the pink represents the International Space Station. Image via NASA/ JPL-Caltech.
Diagram of Earth with moon's orbit and a line coming very close to Earth, slightly bent as it passes.
As a result of the extremely close approach of April 2029, it is expected that perturbations caused by Earth’s gravity will change Apophis’ orbit from the Aten to the Apollo class. Image via Marco Polo/ Wikimedia Commons.

Between now and then

Apophis is now in a part of the sky that is not observable from Earth. It will remain so until we see it again in 2029.

Bottom line: Seven years from today Apophis will zoom safely past the Earth. This much-anticipated event is a “must-see” for all.



Read more about the Yarkovsky effect: Pushing asteroids around with sunlight

April 13, 2022

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