Meteor

The object is an enormous asteroid, measured at roughly two hundred meters across, that blazes with light as it tears into the planet's atmosphere.

Stargazers first spot the incoming rock while it is still bound for Earth, but it abruptly alters course and locks onto Z-City. Every S-Class hero is summoned to intercept it, yet only Bang, Metal Knight, and Genos manage to reach the site in time, most of the others left too far away by how little warning there was. The asteroid brushes off Metal Knight's barrage of missiles and endures Genos's fully charged incineration blast without a scratch. Saitama then arrives and breaks it apart with one punch. Its forward momentum is halted, but the shattered pieces still rain across Z-City and leave it in ruins, though remarkably no one is killed. The event stands as the first disaster the Hero Association ever ranked that involved no living being at all.

Its Dragon classification stems from the staggering kinetic energy it carried, which Genos judged capable of wiping out Z-City and threatening the towns nearby. Even after Saitama pulverized the main mass, the leftover shards held enough force to devastate the city, and had he not blunted the impact and its shockwave, the surrounding region would have suffered regardless. The rock was also tremendously heavy and tough, unbothered by Metal Knight's missiles or Genos's full-power flames. In the anime it was placed somewhere between six and nine on the Torino scale, a real-world system used to gauge the danger of incoming space objects.