Asteroid Day: 8 Key Dates That Shaped Our Understanding of Asteroids

Asteroids, once referred to as the “vermin of the sky” by 19th-century astronomers due to their tendency to streak across long-exposure images, are remnants from the formation of the solar system. Most asteroids reside in the asteroid belt between Mars and Jupiter, yet some are near-Earth asteroids that pose potential threats to our planet.

Asteroid Day commemorates the largest asteroid explosion ever witnessed by modern humans. On the morning of June 30, 1908, an unknown object detonated approximately 10 kilometers above the remote Tunguska region of Siberia, unleashing an explosion equivalent to three to five megatons of TNT. This “Tunguska event” flatlined roughly 60 million trees over an area of 2,200 square kilometers.

Had the explosion occurred over a densely populated area, the consequences would have been devastating. Interestingly, Europe felt the shockwaves, as atmospheric pressure waves and seismic tremors were registered, and a strange glow lit up the night sky.

In the age of modern space exploration, the first asteroid was visited by a spacecraft. NASA’s Galileo mission flew past the asteroid Gaspra on its way to Jupiter, coming within 1,600 kilometers of its surface. Contrary to the belief that asteroids were static rock masses, photographs of Gaspra revealed its complex geology, showcasing unique shapes and geological characteristics unlike any previously known.

This date marks the first successful landing of a spacecraft on an asteroid, with the NEAR Shoemaker landing on the peanut-shaped asteroid 433 Eros. Conducting an extensive one-year study, NEAR revealed a surprisingly varied surface marked by craters and large boulders. During its final descent, NEAR transmitted 69 close-up images before landing, continuing to operate for an additional 16 days.

The first close encounter with an asteroid in Europe occurred when the ESA’s Rosetta spacecraft encountered Šteins during its journey to comet 67P/Churyumov–Gerasimenko. This diamond-shaped body showed extensive impact craters, helping scientists understand the historical significance of collisions in the formation of our solar system.

Japan’s Hayabusa mission launched in 2003 with the ambitious goal of returning material from the 330-meter-diameter asteroid Itokawa. Despite facing multiple challenges, including equipment failures, Hayabusa successfully collected particles and returned a capsule containing approximately 1,500 microscopic grains of asteroid material, a scientifically invaluable treasure.

No one saw the Chelyabinsk meteor approaching. At over 18 km/s, a 20-meter asteroid entered the atmosphere and exploded above the Ural Mountains, injuring around 1,500 people. It was the largest asteroid to strike Earth since Tunguska, highlighting the need to monitor near-Earth objects.

The Double Asteroid Redirect Test (DART) by NASA involved a spacecraft crashing into the 151-meter asteroid Dimorphos, testing the kinetic impact method for planetary defense. Successfully shortening Dimorphos’s orbital period by 33 minutes, this marked a significant leap in asteroid deflection strategies.

Following DART’s impact, Hera, an ESA mission set to launch in October 2024, will investigate Dimorphos’s structure and changes post-impact. The data gathered will enhance our understanding of kinetic impact as a viable planetary defense method.

On this date, the asteroid Apophis will make a close approach to Earth, observable by over two billion people. Scheduled missions aim to study its interaction with Earth’s gravity, providing insights into its structure and planetary defense strategies.

In recognition of the significance of Apophis’s close approach, the United Nations has declared 2029 as the International Year of Asteroid Awareness and Planetary Defense. For more information, visit Asteroids2029.