The Space Shuttle was a partially reusable low Earth orbital spacecraft system that was operated from 1981 to 2011 by the U. S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. Its official program name was Space Transportation System (STS), taken from a 1969 plan for a system of reusable spacecraft of which it was the only item funded for development. The first of four orbital test flights occurred in 1981, leading to operational flights beginning in 1982. In addition to the prototype whose completion was cancelled, five complete Shuttle systems were built and used on a total of 135 missions from 1981 to 2011, launched from the Kennedy Space Center (KSC) in Florida. Operational missions launched numerous satellites, interplanetary probes, and the Hubble Space Telescope (HST); conducted science experiments in orbit; and participated in construction and servicing of the International Space Station. The Shuttle fleet’s total mission time was 1322 days, 19 hours, 21 minutes and 23 seconds.
On 21 March 2016, after a lifespan extended by two years, the China Manned Space Engineering Office announced that Tiangong-1 had officially ended its service. They went on to state that the telemetry link with Tiangong-1 had been lost. A couple of months later, amateur satellite trackers watching Tiangong-1 found that China’s space agency had lost control of the station. In September, after conceding they had lost control over the station, officials speculated that the station would re-enter and burn up in the atmosphere late in 2017. According to the China Manned Space Engineering Office, Tiangong-1 started reentry over the southern Pacific Ocean, northwest of Tahiti, on 2 April 2018 at 00:15 UTC.
Falcon 9 is a partially reusable two-stage-to-orbit medium lift launch vehicle designed and manufactured by SpaceX in the United States. It is powered by Merlin engines, also developed by SpaceX, burning cryogenic liquid oxygen and rocket-grade kerosene (RP-1) as propellants. Its name is derived from the Millennium Falcon and the nine engines of the rocket’s first stage. The rocket evolved with versions v1. 0 (2010–2013), v1. 1 (2013–2016), v1. 2 “Full Thrust” (2015–present), including the Block 5 Full Thrust variant, flying since May 2018. Unlike most rockets, which are expendable launch systems, since the introduction of the Full Thrust version, Falcon 9 is partially reusable, with the first stage capable of re-entering the atmosphere and landing back vertically after separating from the second stage. This feat was achieved for the first time on flight 20 with the v1. 2 version in December 2015.
The Oort cloud is thought to occupy a vast space from somewhere between 2,000 and 5,000 au (0. 03 and 0. 08 ly) to as far as 50,000 au (0. 79 ly) from the Sun. Some estimates place the outer edge at between 100,000 and 200,000 au (1. 58 and 3. 16 ly). The region can be subdivided into a spherical outer Oort cloud of 20,000–50,000 au (0. 32–0. 79 ly), and a torus-shaped inner Oort cloud of 2,000–20,000 au (0. 0–0. 3 ly). The outer cloud is only weakly bound to the Sun and supplies the long-period (and possibly Halley-type) comets to inside the orbit of Neptune. The inner Oort cloud is also known as the Hills cloud, named after Jack G. Hills, who proposed its existence in 1981. Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo; it is seen as a possible source of new comets to resupply the tenuous outer cloud as the latter’s numbers are gradually depleted. The Hills cloud explains the continued existence of the Oort cloud after billions of years.
The Solar System formed 4. 6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud. The vast majority of the system’s mass is in the Sun, with the majority of the remaining mass contained in Jupiter. The four smaller inner planets, Mercury, Venus, Earth and Mars, are terrestrial planets, being primarily composed of rock and metal. The four outer planets are giant planets, being substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are gas giants, being composed mainly of hydrogen and helium; the two outermost planets, Uranus and Neptune, are ice giants, being composed mostly of substances with relatively high melting points compared with hydrogen and helium, called volatiles, such as water, ammonia and methane. All eight planets have almost circular orbits that lie within a nearly flat disc called the ecliptic.