Near Miss Day

What the day is

Near Miss Day is observed on March 23. On that date in 1989 an asteroid several hundred meters across passed Earth at a distance of a few hundred thousand miles, and the alarming part was not the distance but the timing: it was not detected until days after it had already gone by.

Origin and background

Finding asteroids is difficult for an unintuitive reason. They are dark, they are small, and they are visible only by reflected sunlight, so a survey telescope is looking for a faint moving dot against a field of fixed stars. The modern approach is to photograph the same patch of sky repeatedly and let software pick out whatever has moved.

The serious blind spot is the sun. An object approaching from the sunward direction is lost in daylight and effectively invisible from the ground, which is exactly what happened in 2013, when an object roughly twenty meters across arrived unseen over Chelyabinsk in Russia and exploded in the atmosphere, injuring around 1,500 people mostly through broken window glass. Nobody saw it coming, because nobody could.

Surveys have since found the large majority of the near-Earth objects big enough to cause global effects, and none of them is on a collision course in the foreseeable future. The remaining risk sits in the middle range, objects large enough to destroy a city but small enough to be easy to miss, and those are still being catalogued.

The response is no longer only observation. In 2022 a spacecraft was deliberately flown into a small asteroid moon and measurably changed its orbital period, demonstrating that deflection works if you have enough warning. Warning is the whole problem, and warning is what detection buys.

How to take part

• Look up the current near-Earth object tally, which is updated continuously.
• Read about the 2013 Chelyabinsk event and why it was undetectable.
• Follow the deflection mission results, which are genuinely encouraging.
• Note that early detection, not interception, is the hard part.