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Most Dangerous Asteroid Apophis “Rediscovered” In Successful Test Of Detection System

Prior to the asteroid Apophis's most recent close approach records of it were deleted in a test to see if sky scanners would pick it up. Not only did it get detected by three institutions, but much was learned about its size, shape, and orbit, showing how fast these things could be compiled in the absence of prior information.


Stephen Luntz

Stephen has a science degree with a major in physics, an arts degree with majors in English Literature and History and Philosophy of Science and a Graduate Diploma in Science Communication.

Freelance Writer

The asteroid Apophis is the exemplar of space rocks that could eventually threaten the Earth. Image Credit: Philipp Nedomlel/

In a readiness test, scientists hid knowledge of infamous asteroid Apophis from their systems to see if it would be picked up during the 2020-21 close approach. The good news is that it was, increasing confidence the same processes will detect any similar objects lurking in the darkness ready to land on us.

The asteroid (99942) Apophis is considered the most threatening to Earth of any we know, even though it won't be hitting any time soon. Initial fears that close approaches in 2029 or 2068 could turn into disasters have been resolved – but Apophis passes close to the Earth so often that, left to its own devices, there is a high likelihood of eventual impact.


Efforts to work out how to deal with this particular threat are used as a template to consider how to deal with other threatening space rocks and dirty snowballs.

We know how many potentially hazardous asteroids we have found, but knowing how many we are missing is much harder. One way to test this is to remove knowledge of existing objects and see if they get picked up again. The results of such a test, conducted on a truly grand scale, are reported in the Planetary Science Journal.

Asteroid trackers have conducted two tests of planetary defense systems – at least as they apply to threat identification – in 2017 and 2019. The first studied an object small enough to do only local damage, while the second investigated the relative threats of the two components of binary asteroid 66391. This time, the International Asteroid Warning Network (IAWN) and NASA's Planetary Defense Coordination Office (PDCO, made famous in Don't Look Up) operated on a bigger scale.

If one of the many facilities that keep a look out for space threats conducted such a test, while others didn't, the testers might be alerted by observations from collaborators.


Consequently, the operation involved over 100 scientists from 48 institutions in 18 countries. “This real-world scientific input stress-tested the entire planetary defense response chain, from initial detection to orbit determination to measuring the asteroid’s physical characteristics and even determining if, and where, it might hit Earth,” said project leader Dr Vishnu Reddy of the University of Arizona in a statement.

For the operation, telescopes scanned the skies as usual, rather than focusing on where they knew Apophis would be, and the Minor Planet Center (MPC) blocked connecting new data on Apophis' movements to anything taken during previous close approaches.

Images of Apophis recorded by radio antennas at NASA’s Goldstone Deep Space Communications Complex in California and the Green Bank Telescope in West Virginia in 2021, during the asteroid’s close approach, when it was about 10.6 million miles (17 million kilometers) away. Image Credit: NASA/JPL-Caltech and NSF/AUI/GBO

Arizona's Catalina Sky Survey won kudos for being the first institution to spot the visitor, and its report was logged at the MPC as if it was a new discovery. Several other institutions also detected the movement of something unknown across the sky, increasing confidence in the robustness of the detection aspect of the Earth's defenses.

After the MPC announced the “new” discovery, others joined in the observations to provide data to calculate the orbit, measure the asteroid's size and calculate the damage if it hit Earth. Data from the NEOWISE satellite proved particularly valuable on these questions.


“Even though we knew that, in reality, Apophis was not impacting Earth in 2029, starting from square one – with only a few days of astrometric data from survey telescopes – there were large uncertainties in the object’s orbit that theoretically allowed an impact that year,” said JPL's Dr Davide Farnocchia.

The more data that was added, the smaller these uncertainties became until a 2029 collision could be ruled out.


spaceSpace and PhysicsspaceAstronomy
  • tag
  • asteroids,

  • Astronomy,

  • apophis,

  • planetary defense