10 most difficult as­tron­o­my riddles

And cosmic velocity - also referred to as the circular velocity, it is the speed that should be given to the body to enter into orbit around Earth. If a rocket launches from the surface of the earth, then it has to get at least 7.9 kilometers per second to get into orbit.

A Cepheid variable is a type of star that pulsates radially, varying in both diameter and temperature and producing changes in brightness with a well-defined stable period and amplitude. A strong direct relationship between a Cepheid variable's luminosity and pulsation period established Cepheids as important indicators of cosmic benchmarks for scaling galactic and extragalactic distances.

The 'avenue of heroes' at Baikonur in Kazakhstan, where the shuttle will launch, provides a poignant memorial to each mission conducted over the past 50 years. The tradition started with Yuri Gagarin, the first human to enter space, when he planted a single tree before blasting off further from Earth than anyone had ever been. Every Russian cosmonaut, and then later astronauts from other countries, that has left the bounds of Earth from the Cosmodrome has first planted a tree on the path to the launchpad.

The ridge is 13 km high, 20 km wide and 1300 km long. Its origin is not known, but considering the amount of craters, it's safe to assume that it is a very old geological feature.

A black dwarf is a theoretical stellar remnant, specifically a white dwarf that has cooled sufficiently that it no longer emits significant heat or light

Auroras are produced when the magnetosphere is sufficiently disturbed by the solar wind that the trajectories of charged particles in both solar wind and magnetospheric plasma, mainly in the form of electrons and protons, precipitate them into the upper atmosphere (thermosphere/exosphere) due to Earth's magnetic field, where their energy is lost.

Sojourner was the first Mars rover. It was much smaller than Curiosity, whose wheel is about the size of the whole Sojourner. It landed on July 4, 1997 as part of the Mars Pathfinder mission on Mars's Ares Vallis and remained in contact with Earth till September 27, 1997.

Canopus is the brightest star in the southern constellation of Carina, and the second brightest star in the night time sky, after Sirius. Its name is generally considered to originate from the mythological Canopus, who was a navigator for Menelaus, king of Sparta.

Iron is most abundant in the Earth's outer and inner core, and it is the fourth most common element in the Earth's crust.

It was named by in honour of Father Lucian Kemble (1922–1999), a Franciscan friar and amateur astronomer who wrote a letter to Houston about the asterism, describing it as "a beautiful cascade of faint stars tumbling from the northwest down to the open cluster NGC 1502" that he had discovered while sweeping the sky with a pair of 7×35 binoculars