Aarti Holla-Maini, director of the UN’s Office for Outer Space Affairs (Unoosa), is the designated person to alert the UN secretary general if a large asteroid is on a collision course with Earth. Based in Vienna, she had trained for this scenario through drills and table-top exercises, but a year ago she faced the real thing.
On 27 December 2024, a robotic telescope in Chile spotted asteroid 2024 YR4, initially estimated to be the size of a small building. Its Earth impact probability rose from less than 0.05% to over 1% over three weeks, meeting Unoosa’s criteria for a global notification. Although the chance of impact was minuscule, the asteroid’s size and speed meant it could release energy comparable to hundreds of times the Hiroshima bomb, potentially wiping out a city or region.
Romana Kofler, a programme officer and Unoosa’s point of contact for planetary defence, had been in discussions with the International Asteroid Warning Network, Nasa, and Esa. After Kofler informed Holla-Maini, they drafted a letter to UN Secretary General António Guterres, who then disseminated the message to 193 member states. Holla-Maini described it as the first real-time test of an international response.
The threat from space objects is not theoretical. In 2013, a 20-metre meteor exploded over Chelyabinsk, Russia, releasing energy equivalent to 500 kilotonnes of TNT, injuring over 1,200 people. For a period last year, 2024 YR4 was the most significant near-term threat since Apophis in 2004, which was later downgraded. The asteroid has since been monitored closely, but no impact is expected.
