# Internet of Spacecraft for Multi-Planetary Defense and Prosperity
## 4 Planetary Defense: State-of-the-Art
### Origins of Asteroids and Comets
#### 4.1.2 The Kuiper Belt  
Comets, which are cosmic snowballs of frozen gases, rock, and dust, orbit the sun and have highly eccentric elliptical orbits with a wide range of orbital periods from several years to several million years. When passing close to the Sun, they are warmed up due to solar radiation and the solar wind and begin to release gases, which produce a visible atmosphere or coma, and sometimes also a tail. The tail can stretch away from the Sun for millions of miles [123]. As estimated, there are likely billions of comets orbiting the Sun in the Kuiper belt and in the even more distant Oort cloud. There are two major categories of comets in terms of their orbital periods. Short-period comets are believed to originate in the Kuiper belt, which lies beyond the orbit of Neptune. As illustrated in Figure 12, the Kuiper belt is a donut-shaped region of icy bodies in the outer solar system [124], extending from the orbit of Neptune at approximately 30 AU to 55 AU from the Sun [124]. The Kuiper belt is similar to the MAB, but is much larger, around 20 times as wide and 20-200 times as massive [125]. It takes less than 200 years for short-period comets to orbit the Sun, usually the appearance of short-period comets is predictable since they have passed by before. The representative short-period comet is Halley's Comet, officially designated 1P/Halley and visible from Earth every 75-76 years. Furthermore, long-period comets are believed to originate in the Oort cloud, which is a distant region of our solar system [126]. The Oort cloud is believed to be a giant spherical  
Figure 12: Illustration of the solar system with the main asteroid belt and the Kuiper belt [119]. shell surrounding the rest of the solar system. The inner edge is between 2000 and 5000 AU and the outer edge is 10,000 or even 100,000 AU ( 1.58 light years), which is one-quarter to halfway between the Sun and the nearest neighboring star system, Alpha Centauri (a triple star system 4.35 light years from Earth) [126]. The Oort Cloud might contain billions of objects, or even trillions of objects larger than 1 km [127]. Long-period comets move toward the Sun away from the Oort cloud due to gravitational perturbations caused by passing stars and the galactic tide. A typical example of long-period comets is Comet C/2013 A1 Siding Spring. It made a very close pass by Mars in 2014 and will not return to the inner Solar System for about 740,000 years [126]. On the one hand, comets bombarding the young Earth about four billion years ago brought vast quantities of water. Organic molecules, such as polycyclic aromatic hydrocarbons [128], have been found in comets, which has led to speculation that comets may have brought the critical precursors of life or even life itself to Earth. On the other hand, comets may have caused many significant events on Earth. There have been debates on whether an asteroid or a comet was responsible for the Chixulub impact [129]. Similar discussions or uncertainties on the impactor have also been seen in the investigation of the Tunguska event [130] and the Younger Dryas event [70]. It is important to mention humanity's first direct observation of an extraterrestrial collision of solar system objects, Comet Shoemaker-Levy 9's collision with Jupiter in July 1994. The comet, formally designated D/1993 F2, was discovered by astronomers [PERSON] and [PERSON] and [PERSON] in 1993. By calculation, it may had been captured by Jupiter many years earlier and went through tidal breakup [131] in July 1992, eventually fragmenting and colliding with Jupiter in July 1994. This impact created a giant dark spot over 12,000 km (nearly one Earth diameter) across [132] and was estimated to have released an energy equivalent to 6,000,000 megatons of TNT (600 times the world's nuclear arsenal) [133]. In addition, the asteroid/comet may rarely come from neither the Kuiper belt nor the Oort cloud. 'Oumuamua is the first known interstellar object detected passing through the solar system.