For most of history, an asteroid impact was something humans could only watch happen. Today, we have started building a real defense system. Apophis in 2029 is a perfect chance to test our knowledge.
You cannot deflect what you cannot see. Surveys such as Catalina, Pan-STARRS and ATLAS scan the sky every night. NASA has found the large majority of near-Earth asteroids over one kilometer wide, and none of them poses a threat for the next century. The harder task is finding smaller ones, a few hundred meters or less, which are far more numerous. New facilities, including the Vera C. Rubin Observatory and the planned NEO Surveyor space telescope, are built to find many more.
When an asteroid is found, astronomers measure its orbit repeatedly. Each measurement narrows the uncertainty. Risk is rated on tools such as the Torino Scale, from 0 (no hazard) to 10 (certain global catastrophe). Apophis was briefly rated 4 in 2004, the highest ever recorded, and was downgraded to 0 once more data arrived.
In September 2022, NASA's DART spacecraft deliberately crashed into Dimorphos, a small moonlet of the asteroid Didymos. The impact shortened its orbit around Didymos by about 32 minutes, much more than the 73-second minimum NASA had set as success. It was the first time humans changed the motion of a celestial body. The test showed that a "kinetic impactor" works, and ESA's Hera mission is following up to study the results in detail.
The key is time. If an asteroid is found decades ahead, a tiny change in speed adds up to a huge change in position by the time it reaches Earth. That is why early detection matters more than raw power.
Which method is best depends on the size of the asteroid and how much time is left. With decades of warning, a small push is enough.
Asteroids ignore borders, so planetary defense is an international task. After the United Nations endorsed the idea in 2013, two groups were established: the International Asteroid Warning Network (IAWN), which shares observations and confirms threats, and the Space Mission Planning Advisory Group (SMPAG), which prepares possible deflection missions. Space agencies including NASA, ESA and JAXA take part.
Astronomers have found only part of the population of asteroids around 140 meters across and larger, which is the size class that surveys are trying to complete. NASA's NEO Surveyor, an infrared space telescope planned for launch in the late 2020s, is designed to find many of the missing ones. Better detection means more warning time, and more warning time makes every other method easier.
Apophis will not hit us, but its 2029 flyby is a free natural experiment. Earth's gravity will tug on it, and scientists will see how a real asteroid responds. Spacecraft such as OSIRIS-APEX are planned to observe it, which will improve the models we would need if a dangerous asteroid were ever discovered. See the full story in our Apophis guide.
Is there an asteroid headed for Earth right now? No known object poses a significant risk in the coming decades. Astronomers keep watching, and new discoveries are checked and rated within days.
Who would warn the public? Observatories report discoveries to the Minor Planet Center. Space agencies and international groups then analyze the risk and, if needed, inform governments.
Could we really stop an impact? For an object found with enough warning, yes. The challenge is detection time. The earlier an asteroid is found, the smaller the push we need. This is why surveys are the most important part of the defense.
For more detail about one famous example, read Is Apophis dangerous? and Apophis vs. the dinosaur asteroid.
Asteroid impacts are one of the few natural disasters we can actually prevent. Humanity has the tools and is building more. Real space news in 2029 will be more exciting than any rumor.
Facts in this article are drawn from public scientific sources. For primary information, see: