In some instances, acceleration – a change in an object’s speed and direction through space – can help avoid a collision. This seems to be the case for infamous asteroid 99942 Apophis, whose close approaches to Earth in this century have caused astronomers to watch this object carefully. Apophis is a large near-Earth asteroid, expected to pass close to Earth in 2029, 2036 and again in 2068. Impacts in 2029 and 2036 had already been ruled out. As of February 2021, the chances of impact during the 2068 flyby of Apophis are now 1 in 380,000. That’s a 99.99974% chance the asteroid will miss the Earth.
Previous calculations (made in 2016) had all but ruled out the probability of an impact in 2068. The chance of an impact was seen in 2016 as vanishingly small, at just 1 in 150,000 odds of impact, or a 99.99933% chance the asteroid would miss the Earth. The more recent observations, first discussed in October 2020 and updated again in early 2021, show a decreasing risk.
It’s a Yarkovsky acceleration of asteroid Apophis – detected by astronomers at the University of Hawaii – that has reduced the impact probability for the 2068 flyby. Dave Tholen and collaborators used the 323-inch (8.2-meter) Subaru Telescope at Maunakea, Hawaii, to make the most recent observations. These astronomers were then able to update the Earth-impact risk from Apophis, including the latest measurements of the Yarkovski effect, which arises from a minuscule push imparted by sunlight.
The new work by Tholen and colleagues suggests that Apophis – whose estimated diameter is between 1,115 and 1,214 feet (340 to 370 meters) – is drifting more than 500 feet (about 170 meters) per year from its expected position in its orbit.
Tholen – who has been tracking the motion of Apophis in the sky since he and his colleagues discovered it from Kitt Peak National Observatory near Tucson, Arizona, in on June 19, 2004 – commented in the statement:
We have known for some time that an impact with Earth is not possible during the 2029 close approach.
The new observations we obtained with the Subaru telescope earlier this year were good enough to reveal the Yarkovsky acceleration of Apophis, and they show that the asteroid is drifting away from a purely gravitational orbit by about 170 meters [about 500 feet] per year, which is enough to keep the 2068 impact scenario in play.
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.
But astronomers are pushing to understand the orbit of asteroid Apophis because of its close sweeps past our planet in this century and beyond.
In recent years, astronomers have been able to find and track tiny asteroids sweeping very near the Earth. For example, on September 24, 2020, asteroid 2020 SW swept even closer to us than our meteorological and television satellites as well as other geostationary satellites, which orbit our planet at some 22,300 miles (35,900 km) from Earth’s surface. Asteroid 2020 SW came within about 7% of the Earth-moon distance. But asteroid 2020 SW is estimated to be only about 14 to 32 feet (about 4.5 to 10 meters) in diameter. That’s very small in contrast to asteroid Apophis.
Asteroid Apophis will have an extremely close encounter with Earth on April 13, 2029. At its closest in 2029, Apophis will sweep just 23,441 miles (37,725 km) from our planet, or about 10% of the Earth-moon distance. That’s very close for a space rock over 1,115 ft (340 meters) across! Lance Benner of NASA/JPL commented:
This will be the closest approach by something this large currently known. (In 2029) Apophis will be visible to the unaided eye for several hours, and Earth tides will probably change its spin state.
Astronomers will know well before 2068 if there is any chance of an impact from Apophis. And opportunities for observing asteroid Apophis are coming up again soon.
Astronomers are planning to study 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, which has now faded from view.
Due to its short orbital period around the sun of only 323.6 days (less than an Earth year), asteroid Apophis comes by this region of the solar system quite often and will have a distant encounter with Earth in just a few months. On the night of March 5, 2021, the huge space rock will pass at 10,471,577 miles (16,852,369 km) from our planet, i.e. about 44 times the distance to the moon. Apophis is then expected to reach a visual magnitude of 15 to 16, too faint to be seen in small telescopes, but within the reach of 12-inch-diameter and larger telescopes, and possibly even smaller telescopes using sensitive cameras.
Astronomers will also study Apophis during the 2021 flyby with radar observations, using the Goldstone observatory in California. These observations of Apophis should improve our knowledge of its shape and spin state, and they’ll help reduce uncertainties in the space rock’s orbit caused by Yarkovsky acceleration.
This 2021 flyby will be Apophis’ closest approach until the great encounter in 2029.
Friday, April 13, 2029, will be more of a showtime for the general public and astronomers alike. Apophis will come so close that it’ll be visible to the unaided eye alone; something that almost never happens with asteroids! According to NASA, Apophis will first become visible in the Southern Hemisphere and will look like a speck of light moving across Australia during this close encounter. It will be over the Atlantic Ocean at its closest approach to Earth. It will move so fast that it crosses the Atlantic in just an hour, and will have crossed the U.S. in the late afternoon/early evening within the next hour. Calculations indicate that Apophis will reach a visual magnitude of 3.1 during this approach, comparable to the stars in the Little Dipper. It is expected to be visible to the unaided eye from some areas of Australia, western Asia, Africa, and Europe.
An asteroid is quite the traveler: While the semi-major axis (the largest diameter of its elongated orbit) of Apophis’ orbit now is shrinking by 170 meters (500 feet) per year, Dave Tholen told EarthSky that this will change:
The 2029 close approach to the Earth will increase the semi-major axis significantly, changing Apophis from an asteroid of the Aten variety to one of the Apollo variety. Average orbital speed will slow down as a result.
So currently, Apophis has a semi-major axis smaller than that of the Earth and occasionally crosses paths with our orbit. This makes it an Aten category asteroid in the Near Earth Objects (NEO) classifications, and means that the asteroid spends the majority of its time inside the Earth’s orbit around the sun. After 2029, it will join the ranks of Apollo objects, which still cross Earth’s orbit but have a semi-major axis larger than the Earth’s. That is, Apophis will then live its majority of time outside Earth’s orbit.
According to some estimates, an asteroid the size of Apophis can be expected to strike Earth about every 80,000 years.
Bottom line: Asteroid Apophis is a relatively large body, noteworthy for its extremely close approaches to Earth in 2029, 2036 and 2068. In February 2021, astronomers at the University of Hawaii updated their measurements for the Yarkovsky effect for this asteroid – a change in its orbital motion, due to a tiny push imparted by sunlight as the asteroid spins – reducing the impact probability for the 2068 flyby further.
Eddie Irizarry of the Sociedad de Astronomía del Caribe (Astronomical Society of the Caribbean) has been a NASA Solar System Ambassador since 2004. He loves public outreach and has published multiple astronomy articles for EarthSky, as well as for newspapers in Puerto Rico. He has also offered dozens of conferences related to asteroids and comets at the Arecibo Observatory.
Asteroid 33012EddieIrizarry, a 7.8 km space rock, has been named in his honor.