Enjoying the show? Support our mission and help keep the content coming by buying us a coffee: https://buymeacoffee.com/deepdivepodcastThe Unprecedented Threat of Apophis

The saga of asteroid 99942 Apophis is a monumental case study in planetary defense. Named after the Egyptian God of chaos, Apophis briefly became the highest-rated cosmic threat known to humanity.

• Immense Scale: Initial size estimates were revised after 2013 observations, increasing its diameter by 20% (from 270m to 325m). Due to cubic scaling, this translated to a 75% increase in mass and kinetic energy. Objects of this magnitude are statistically rare, expected to approach Earth only once every 800 years.

• The Highest Hazard Rating: In late 2004, refined orbital estimates focused on a potential impact on Friday, April 13, 2029. The calculated probability temporarily peaked at an unprecedented 2.7% (a 1-in-37 chance). This combination of probability and catastrophic consequence caused Apophis to hit Level 4 on the 10-point Torino Scale (a yellow warning signifying a close encounter with a 1% or greater threat of regional devastation)—the highest rating ever assigned to an object. Simultaneously, its risk on the specialist-used Palermo Technical Impact Hazard Scale peaked at 1.10, meaning the hazard was over 12 times the average background risk.

The crisis was averted and the risk ruled out through rapid, high-precision science and orbital mechanics:

• Precovery Images: The turning point occurred in late December 2004 when researchers successfully located precovery images—archival photographs of the night sky taken months earlier—that extended the asteroid's observational arc back to March 2004. This new, longer data set was precise enough to instantly rule out the 2029 impact chance entirely.

• Decisive Tracking: Years of sustained effort and Goldstone radar observations ruled out the 2036 chance. The final definitive success came in March 2021 following refined radar observations. The Jet Propulsion Laboratory formally announced that Apophis has absolutely no chance of impacting Earth in the next 100 years.

• Negligible Risk: After holding the highest hazard rating ever, Apophis was removed from the risk table with a cumulative Palermo scale rating of -3.22, meaning its risk is less than one thousandth the average background hazard level.

• The Yarkovsky Effect: Scientists successfully detected and measured the Yarkovsky acceleration on Apophis, a non-gravitational force caused by uneven thermal radiation from the sun acting as a tiny, continuous thruster. It was found to be decreasing the asteroid's average distance from the sun by about 170 meters per year. Measuring and modeling this small nudge was critical to confirm that Apophis would miss all keyholes entirely, definitively ruling out any future collision setup (like 2068).

• Stellar Occultation: To achieve the astonishing precision of $\pm$3km for the 2029 approach, scientists successfully used the technique of stellar occultation—observing the exact moment the asteroid blocked the light of a distant star. Apophis, at 325m, was the smallest object ever successfully tracked using this method, a major technical record.

The near-miss has transformed Apophis into an unparalleled scientific target:

• Close Approach: On Friday, April 13, 2029, Apophis will pass Earth at approximately 31,860 kilometers from the surface—closer than our ring of geosynchronous communication satellites (which orbit at 35,800km). It will literally be crossing the boundary of human space infrastructure.

• Beyond Gravity: Former astronaut Rusty Schweickart and the B612 Foundation strongly advocated for placing a dedicated radio transponder on Apophis to precisely monitor how the subtle Yarkovsky effect influences its orbit over the decades.