On and around our planet there are thousands of comets and asteroids known as Near-Earth Objects (NEOs). Several space agencies and government affiliates are responsible for tracking them, especially those known as Potentially Hazardous Asteroids (PHAs). These objects are so named because they will pass through Earth’s orbit and may even collide with it one day. Considering how impacts in the past have caused mass extinctions (like the Chicxulub impact event that killed the dinosaurs), future impacts are something we would like to avoid!
Surveillance of PHAs is a huge responsibility that requires a global effort, including monitoring, alerts and disaster preparedness. Last year, more than 100 participants from 18 countries (including scientists from NASA and the NEOWISE mission) conducted an international exercise simulating an encounter with an asteroid that flew past Earth. As NASA revealed in a recently published study, the exercise was a complete success. Lessons learned could help avoid real impacts in the near future or significantly limit the devastation this could cause.
The study, published in the May 31 issue of The Journal of Planetary Science (entitled “Apophis Planetary Defense Campaign”), was conducted by the Planetary Defense Exercise Task Force and led by Vishnu Reddy – Associate Professor at the Lunar and Planetary Laboratory (LPI) at the University of Arizona . The working group is made up of more than 100 participants from 18 countries and includes facilities such as NASA’s Planetary Defense Coordination Office (PDCO), ESA’s NEO Coordination Center, the Russian Academy of sciences, the Korea Institute of Astronomy and Space Science (KASI), and many universities and research institutes around the world.
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Simulate a threat
As Reddy and his colleagues describe in the article, the planetary defense exercise was the culmination of work that began in 2017, which aimed to test the operational readiness of our global planetary defense capabilities. . The exercise was carried out with the support of NASA’s PDCO, the Minor Planet Center (MPC) – the internationally recognized authority for monitoring the position and movement of small celestial bodies – and the International Asteroid Warning Network (IAWN). The exercise was named “Apophis Campaign” because it coincided with the close approach of NEO (99942) Apophis, which flew by Earth from December 2020 to March 2021.
Apophis is one of several PHAs regularly monitored by the Planetary Defense Monitoring Database. Shortly after its discovery in 2004, it was determined that Apophis had a significant chance of impacting Earth in 2029 or later. But after years of tracking and several close approaches, astronomers have refined Apophis’ orbit and concluded that it poses no risk of impact on Earth for a century or more. Apophis was specifically selected for this campaign because planetary defense experts knew it would come close to Earth early on December 4, 2020.
To make the exercise more realistic, the MPC removed Apophis from the planetary defense monitoring database to see if it could be properly detected again, tracked, and characterized by the planetary defense system. With no prior registration in the database, astronomers had nothing to refer to, making it seem like astronomers were seeing it for the first time. Other goals included the system’s ability to perform observations, hypothetical risk assessment, risk prediction, and hazard communication.
Find Apophis (again)
On December 4, the asteroid began to brighten in the night sky and the NASA-funded Catalina Sky Survey in Arizona was able to detect it and report its position to MPC. The NASA-funded Asteroid Terrestrial-impact Last at Alert System (ATLAS) and Panoramic Telescope and Rapid Response System (Pan-STARRS) in Hawaii made subsequent detections. This was followed by observations from NASA’s NEOWISE (Near-Earth Object Wide-field Infrared Survey Explorer) mission, an asteroid hunting campaign that relies on the Wide-field Infrared Survey Explorer (WISE) space telescope .
The NEOWISE space probe provided infrared observations of Apophis that were not possible with ground-based telescopes due to atmospheric interference – that is, water vapor in our atmosphere absorbs light at these wavelengths. On December 23, the MPC announced the discovery of this “new” asteroid and added it to the list of known PHAs. During Apophis’ next close approach (in March 2021), JPL astronomers used NASA’s Goldstone Solar System Radar (GSSR) in California to take photos (pictured above) and precisely measure the speed and distance of the asteroid from Earth.
NASA engineer Davide Farnocchia, who led the orbit determination calculations for JPL’s Center for Near-Earth Object Studies (CNEOS), explained in a recent NASA press release:
“Even though we knew that in reality Apophis had no impact on Earth in 2029, from square one – with only a few days of astrometric data from survey telescopes – there were great uncertainties. in the object’s orbit that theoretically allowed for an impact that year.
Combined with measurements from other observatories, the exercise not only “discovered” the object but succeeded in continually reassessing its chances of hitting Earth. As Reddy indicated, they were able to rule out the possibility of an impact in 2029 and any possibility of an impact for 100 years or more. “This real-world science contribution stress-tested the entire planetary defense response chain, from initial detection and orbit determination to measuring the physical characteristics of the asteroid and even determining if and where it might hit Earth,” Reddy said.
A second article describing the results obtained by the NEOWISE mission during the exercise (entitled “NEOWISE Observations of the Potentially Hazardous Asteroid (99942) Apophis”) was also recently published in The Journal of Planetary Science. This study was led by undergraduate researcher Akash Satpathy and NEOWISE principal investigator Amy Mainzer, both from the University of Arizona. They were joined by researchers from Caltech, UCLA, the Astronomical Research Institute and NASA’s Jet Propulsion Laboratory. As Satpathy explained:
“The independent infrared data collected from space greatly benefited the results of this exercise. NEOWISE was able to confirm the rediscovery of Apophis while quickly gathering valuable information that could be used in planetary defense assessments, such as its size, shape, and even clues to its composition and surface properties.
The NEOWISE measurements also allowed for updated assessments of the asteroid’s size. As they stated in the study, Apophis is an elongated object with an “effective spherical diameter” of 270 to 410 meters (~885 to 1345 feet). These improved measurements allowed the research team and participating scientists from NASA’s Ames Research Center to place new estimates on the impact energy this asteroid would deliver. In a series of simulations, which looked at impacts in different geographic locations, the team found that an impact from an Apophis-like object “would likely cause damage regionally, not globally.”
This collaborative effort could help disaster management agencies plan for future impacts and develop possible mitigation and evacuation strategies. Michael Kelley, a program scientist with OFSP who guided exercise participants, said:
“Watching the planetary defense community come together during Apophis’ final close approach was impressive. Even during a pandemic, when many exercise participants were forced to work remotely, we were able to detect, track and learn about a potential danger with great efficiency.The exercise was a resounding success.
Last year, NASA approved the next-generation NEO Surveyor spacecraft – an asteroid-hunting space telescope and the direct successor to NEOWISE – for further development. This mission will launch no earlier than 2026 and will provide updated data that will greatly expand our knowledge of the near-Earth asteroids that populate our solar system (and those at risk of collision with Earth). On September 26, 2022, NASA’s Double Asteroid Redirection Test (DART) mission will encounter the asteroid Didymos and collide with its moon to test a key asteroid defense technique (the kinetic impact method).
As noted, a major impact (and extinction-level event) is something humanity would like to prevent. A considerable amount of resources are currently directed towards the development of all means to avoid this. But the best tool we have in our arsenal is regular monitoring, which ensures that we are prepared for any threats that may arise one day.
Further reading: NASA, The Planetary Science Journal