How will we defend Earth now?

As well as the discovery and study of thousands of NEOs, NEOWISE established the foundation of knowledge that has informed a new, dedicated planetary defence mission. NASA’s NEO Surveyor will be launched in 2027.

NEO Surveyor’s goal is to discover approximately two thirds of all NEOs larger than 140 metres in diameter, over a five year baseline survey. This is a big step toward fulfilling the mandate United States Congress has provided to NASA: to discover 90% of all NEOs in this size range.

If they hit Earth, asteroids of this size could cause mass casualties if the impact were over a large metropolitan region.

You might think this poses a bit of a risk – shutting down NEOWISE three years before launching NEO Surveyor. What happens if one of these big asteroids comes our way in the next few years?

The risks are very small, as estimates show asteroids 140 metres in diameter impact Earth only approximately every 20,000 years. So, we would have to be extremely unlucky to have one in any given three-year period, especially impacting a place that would cause a large amount of damage. Only around 3% of Earth’s surface is occupied by urban areas.

NASA doesn’t really have much of a choice with the end of NEOWISE. The Sun’s 11-year activity cycle is picking up and causing Earth’s upper atmosphere, the ionosphere, to thicken. NEOWISE is flying through this ionosphere and can’t raise its orbit, so the ionosphere is inevitably dragging NEOWISE back to Earth.