Asteroid
The dwarf planet Ceres is by far the largest asteroid, with a diameter of . The next largest are 4 Vesta and 2 Pallas, both with diameters of just over . Vesta is the only main-belt asteroid that can, on occasion, be visible to the naked eye. On some rare occasions, a near-Earth asteroid may briefly become visible without technical aid; see 99942 Apophis.
Asteroid
In April 2018, the B612 Foundation reported "It's 100 percent certain we'll be hit [by a devastating asteroid], but we're not 100 percent sure when." Also in 2018, physicist Stephen Hawking,
in his final book "Brief Answers to the Big Questions", considered an asteroid collision to be the biggest threat to the planet. In June 2018, the US National Science and Technology Council warned that America is unprepared for an asteroid impact event, and has developed and released the ""National Near-Earth Object Preparedness Strategy Action Plan"" to better prepare. According to expert testimony in the United States Congress in 2013, NASA would require at least five years of preparation before a mission to intercept an asteroid could be launched.