a. A galaxy is a collection of stars held together by their mutual galaxy. In other words, all the stars in a galaxy are kept together by the gravity of all the other stars (as well as the invisible, mysterious dark matter).
b.
Mercury, Diameter: 4879.4km 57.9 million km away from sun. 0 moons
Venus, Diameter: 12104km 108.2 million km away from sun. 0 moons
Earth, Diameter: 12756km 149.6 million km away from sun. 1 moon
Mars, Diameter: 6779km 227.9 million km away from sun. 2 moons
Jupiter, Diameter: 142800km 778.3 million km away from sun. 67 moons
Saturn, Diameter: 120660km 1,427.0 million km away from sun. 62 moons
Uranus, Diameter: 51118km 2,871 million km away from sun. 27 moons
Neptune, Diameter: 49528km 4,497.1 million km away from sun. 14 moons
c. Many comets have an elliptical orbit around the Sun that brings them closer at some times and farther away at others. The adjective elliptical refers to the shape of an ellipse, which is an elongated circle, stretched into an oval.
2. Day and night are due to the rotation of a planet. (How long it takes for the planet to orbit the sun.)
3. A lunar eclipse occurs when the Earth passes between the Moon and the Sun, and the Earth's shadow obscures the moon or a portion of it. A solar eclipse occurs when the Moon passes between the Earth and the Sun, blocking all or a portion of the Sun. An eclipse can be total, partial, or annular.
4. The International Space Station (ISS) is a space station, or a habitable artificial satellite, in low Earth orbit. Its first component launched into orbit in 1998, and the last pressurised module was fitted in 2011. ... The ISS consists of pressurised modules, external trusses, solar arrays, and other components.
5.
- Maintain and grow U.S. leadership at Mars with a rover in 2020, as a first step of a sample-return strategy, searching for past life and demonstrating oxygen production. Use this mission as a building block for a subsequent round-trip robotic mission with the historic first launch off another planet and sample return through the lunar gateway and the broader exploration architecture.
- Prioritize and guide investments and partnerships in long-pole technology areas and resource characterization needed for the exploration of Mars and other deep space destinations.
- Develop standards for human long-duration deep space transportation vehicles.
