In 2004, the asteroid 99942 Apophis was identified as one of the most hazardous asteroids that could impact Earth. Nonetheless, as astronomers tracked Apophis and its orbit, the impact assessment changed.
According to the results of a new radar observation campaign and a precise orbit analysis, there is no likelihood that Apophis will impact our planet for at least the next century. Radar observations have determined that the near-Earth object poses no risk of impacting the Earth in 2068.
Estimated to be about 340 metres across, Apophis quickly gained notoriety as an asteroid that could pose a serious threat to Earth when astronomers predicted that it would come uncomfortably close in 2029. Thanks to additional observations of the near-Earth object (NEO), the risk of an impact in 2029 was later ruled out, as was the potential impact risk posed by another close approach in 2036. Until this month, however, a small chance of impact in 2068 still remained.
During Apophis' near flyby of Earth on March 5th, astronomers took advantage of the opportunity to refine their understanding of its orbit around the Sun with extreme precision, enabling them confidently to rule out the possibility of any impact in 2068 and beyond.
“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,” said Davide Farnocchia of NASA’s Center for Near-Earth Object Studies (CNEOS), which is managed by NASA’s Jet Propulsion Laboratory in Southern California. “With the support of recent optical observations and additional radar observations, the uncertainty in Apophis’ orbit has collapsed from hundreds of kilometres to just a handful of kilometres when projected to 2029. This greatly improved knowledge of its position in 2029 provides more certainty of its future motion, so we can now remove Apophis from the risk list.”
The Sentry Impact Risk Table is what Farnocchia was referring to that keeps tabs on the few asteroids whose orbits take them so close to Earth that an impact can’t be ruled out. As a result of the recent findings, Apophis is no longer included in the Risk Table.
In support of NASA's Planetary Defense Coordination Office, CNEOS calculates high-precision orbits based on optical telescopes and ground-based radar to help characterize every known near-Earth object's orbit.
The Opportunity for Science Type Stuff?
The Deep Space Network's Goldstone Deep Space Communications Complex near Barstow, California, boasts a 70-meter radio antenna that astronomers used to precisely track Apophis' motion in the latest calculations. “Although Apophis made a recent close approach with Earth, it was still nearly 17 million kilometres away. Even so, we were able to acquire incredibly precise information about its distance to an accuracy of about 150 metres,” said JPL scientist Marina Brozovic, who led the radar campaign. “This campaign not only helped us rule out any impact risk, but it also set us up for a wonderful science opportunity.”
Furthermore, Goldstone collaborated with the 100-metre Green Bank Telescope in West Virginia in order to facilitate the imaging of Apophis; Green Bank was transmitting while Goldstone was receiving - a "bistatic" experiment that doubled the received signal's strength.
Although the radar imagery of Apophis appears pixelated, the images have a resolution of 38.75 metres per pixel, “which is a remarkable resolution, considering the asteroid was 17 million kilometres away, or about 44 times the Earth-Moon distance,” added Brozovic. “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.”
These images of asteroid Apophis were recorded by radio antennas at the Deep Space Network’s Goldstone complex in California and the Green Bank Telescope in West Virginia. The asteroid was 17 million kilometres away, and each pixel has a resolution of 38.75 metres.
Credits: NASA/JPL-Caltech and NSF/AUI/GBO
During the further analysis of the radar data, the radar team hopes to uncover more information about the asteroid's shape. Radar observations have suggested that Apophis has the appearance of a "bilobed" or peanut-like object. One in six near-Earth asteroids larger than 200 metres in diameter exhibit this shape.
Astrophysicists are also attempting to better understand the asteroid's rotation rate and the axis around which it spins (known as the asteroid's spin state). In conjunction with this knowledge, they can predict the orientation the asteroid will have with Earth as it encounters our planet's gravitational field in 2029, which may result in a change to its spin state and even an asteroid quake.
It is predicted that the asteroid Apophis will pass less than 32,000 kilometres from the surface of our planet on April 13th, 2029 - closer than the distance between geosynchronous satellites. During that 2029 close approach, Apophis will be visible to observers on the ground in the Eastern Hemisphere without the aid of a telescope or binoculars. It’s also an unprecedented opportunity for astronomers to get a close-up view of a solar system relic that is now just scientific curiosity and not an immediate hazard to our planet.
“When I started working with asteroids after college, Apophis was the poster child for hazardous asteroids,” said Farnocchia. “There’s a certain sense of satisfaction to see it removed from the risk list, and we’re looking forward to the science we might uncover during its close approach in 2029.”