# Internet of Spacecraft for Multi-Planetary Defense and Prosperity
## 3 Space Threats and Planetary Defense History
### Serious Threats from Outer Space
#### 3.1.2 Other Significant Events  
In addition to the serious K-Pg extinction event, some other significant events in the more recent timeline are also believed to be related to a comet or asteroid. The Younger Dryas that happened around 12,900 to 11,700 years before the present (BP) [68] was a return to glacial conditions after the Late Glacial Interstadial (LGI), which temporarily reversed the gradual climatic warming after the Last Glacial Maximum (LGM) started fading around 20,000 BP. For example, within decades, a sudden decline of temperatures in Greenland by 4 to 10 \({}^{\circ}\)C took place [68]. It is hypothesized that an impact event occurred in North America around 12,900 BP and initiated the Younger Dryas (YD) cooling [69] that lasted for 1200 years and became one of the possible major causes of the Quaternary extinction. Nanodiamonds which are usually the high-temperature products during extraterrestrial collision [70] have been found at the Younger Dryas Boundary (YDB) in the northern hemisphere [71, 72]. The catastrophic consequences of this impact, including the abrupt YD cooling, contributed to the late Pleistocene megafaunal extinction (e.g., woolly mammoth, saber-toothed predator), promoted human cultural changes, and resulted in immediate decline in some post-Clovis human populations [69]. When the timeline moves closer to the present, on the morning of 30 June 1908, an enormous explosion of around 12 megatons (of TNT) [73] (approximately 800 Hiroshima A-bombs) occurred near the Podkamennaya Tunguska River in what is now Krasnoyarsk Krai, Russia. An estimated 80 million trees over a forest area of 2150 km\({}^{2}\) were flattened [74] with three people possibly dead in the event as reported by witnesses [73]. The atmospheric explosion of a stony meteoroid with an estimated dimension of 50-60 m [75] happened at an altitude of around 5 to 10 km after entering Earth's atmosphere with a high speed of about 27 km/s [73, 76]. The meteoroid is believed to have disintegrated and exploded at a similar altitude. Therefore, there is no impact crater. The sounds were accompanied by a shock wave that knocked people down and broke windows hundreds of kilometers away [73]. The explosion was registered at seismic stations across Eurasia, and air waves were detected in Germany, Denmark, Croatia, the United Kingdom, and Washington, D.C. [77]. Over the next few days, night skies in Asia and Europe were aglow [78]. Moreover, a Smithsonian Astrophysical Observatory program at the Mount Wilson Observatory in California observed a months-long decrease in atmospheric transparency and an increase in suspended dust particles [79]. The Tunguska event is the largest impact event on Earth in recorded history. Within the last decade, the most significant meteor event happened on 15 February 2013 at about 03:20 UTC over the southern Urals region of Russia. An approximately 20 m-sized asteroid entered the atmosphere at a shallow 18.3 \(\pm\) 0.4 degree angle with a speed relative to Earth of 19.16 \(\pm\) 0.15 km per second [80]. According to the record, the meteor generated brighter light than the Sun and was visible as far as 100 km, and some eyewitnesses also felt intense heat from the fireball. Its peak radiation occurred at 29.7 \(\pm\) 0.7-km. Then the fragmentation left a thermally emitting debris cloud in this period, the final burst of which occurred at 27.0-km altitude over Chelyabinsk Oblast. The Chelyabinsk meteor shone 30 times brighter than the Sun and had an energy equivalent to more than 500 kilotons of TNT [81, 82] which is around 33.3 times the energy released from the Hiroshima A-bomb. Furthermore, the object did not release all of its kinetic energy in the form of a blast wave as some 90 kilotons of TNT of the total energy was emitted as visible light according to NASA's Jet Propulsion Laboratory (JPL) [83]. The explosion resulted in injuries to about 1500 people (a large majority due to shattered glass) and damaged 7200 buildings in six cities across the region.