611 articles
Proper elements
Proper elements are quasi-constant orbital parameters of an asteroid, computed from its osculating elements by removing periodic planetary perturbations; stable over millions of years, they reveal clustered asteroid families.
Proton aurora
A proton aurora is an auroral emission from energetic protons striking a planetary atmosphere, first observed beyond Earth when MAVEN detected one at Mars in 2018.
Puck
Puck (S/1985 U1, Uranus XV) is the largest small inner moon of Uranus, about 150 km across, discovered December 30, 1985 by Voyager 2, orbiting between the epsilon ring and Miranda, named for Shakespeare's sprite.
Quadrantids
The Quadrantids are an annual meteor shower peaking early January, one of the strongest of the year, with an asteroid parent body and a name from a constellation that no longer exists.
Quasi-satellite
A quasi-satellite, or quasi-moon, is a small body that orbits the Sun in step with a planet, appearing to circle the planet without being bound to it.
Radiant (meteor shower)
The radiant of a meteor shower is the point in the sky the meteors appear to come from, a perspective effect on nearly parallel paths.
Radiative zone
The radiative zone is the layer of the Sun's interior between the core and the convection zone, where energy travels outward as photons over more than 170,000 years.
Radio occultation
Radio occultation is a remote sensing technique in which a radio signal between two spacecraft is bent by a planetary atmosphere, yielding temperature, pressure, and electron density profiles.
Rahu
Rahu (Sanskrit राहु) is one of the nine navagraha of Hindu tradition, a shadow planet identified with the Moon's ascending node and with the severed head of the asura Svarbhanu.
Resonance capture
Resonance capture is the trapping of a celestial body into a mean-motion resonance when slow orbital migration sweeps the resonance past its orbit, explaining Pluto's 3:2 lock with Neptune.
Retrograde and prograde motion
Retrograde and prograde motion, also called direct motion, describe the two directions of orbital or apparent motion: prograde matches the central body's rotation, retrograde opposes it.
Retrograde rotation
Retrograde rotation is the rotation of a body opposite to its orbital direction, seen in Venus, which spins backward every 243.690 days, and Uranus, tilted 97.77 degrees.
Rhea (moon)
Rhea, designated Saturn V, is the second largest moon of Saturn, an airless, heavily cratered body of ice and rock discovered by Giovanni Cassini on December 23, 1672.
Richard C. Hoagland
Richard C. Hoagland, born 1945 in Morristown, New Jersey, is an American science writer best known for promoting the 'Face on Mars' as an artificial monument from a lost civilization.
Ring rain
Ring rain is the flow of charged water particles from Saturn's rings along magnetic field lines into its upper atmosphere, first confirmed by Keck observations in 2013.
Ring seismology
Ring seismology, also known as kronoseismology, is a method of planetary seismology that reads a planet's internal oscillations from waves they raise in its rings.
Ring system
A ring system is a disk of small particles orbiting a planet in a common plane. All four giant planets of the solar system have known rings.
Rings of Chariklo and Haumea
The rings of Chariklo and Haumea are the first ring systems found around Solar System bodies other than the giant planets, discovered in 2013 and 2017.
Rings of Earth
Rings of Earth, or Earth's rings, are a hypothetical ring system around Earth; none exists today, but a debris ring may have existed 466 million years ago.
Rings of Jupiter
The Rings of Jupiter, or Jupiter's rings, are faint dusty rings discovered by Voyager 1 in 1979, fed by dust from the planet's small inner moons.
Rings of Neptune
The rings of Neptune are five faint, narrow rings and four dusty ring arcs encircling Neptune, confirmed by Voyager 2 images in 1989 and named Galle, Le Verrier, Lassell, Arago, and Adams.
Rings of Saturn
Saturn's rings, also called the Saturnian rings, are the largest and brightest planetary ring system in the Solar System, made of water ice and spanning about 70,000 km.
Rings of Uranus
The rings of Uranus, also called the Uranian rings, are a system of 13 dark, narrow rings around the ice giant Uranus, discovered during a stellar occultation on 10 March 1977.
Roche limit
The Roche limit, also called the Roche radius, is the distance from a planet within which tidal forces tear apart a satellite held together only by gravity.
Rosalind (moon)
Rosalind is a small inner moon of Uranus, about 36 km in radius, discovered by the Voyager 2 team in January 1986 and named for a Shakespeare character.
Rubble pile (asteroid)
A rubble pile asteroid is not a solid rock but a collection of boulders held together by gravity, typically 200 m to 10 km in diameter.
Ryugu samples
The Ryugu samples are about 5.4 grams of pristine material from the carbonaceous asteroid Ryugu, returned by JAXA's Hayabusa2 spacecraft in December 2020, the first from a carbonaceous asteroid.
S-type asteroid
An S-type asteroid is a stony asteroid of silicate minerals and nickel-iron, making up about 17 percent of asteroids and commonest in the inner main belt.
S/2015 (136472) 1
S/2015 (136472) 1, nicknamed MK 2, is the only known moon of the dwarf planet Makemake, spotted in Hubble Space Telescope images and announced on April 26, 2016.
Sade Sati (साढ़े साती)
Sade Sati (साढ़े साती) is a Vedic astrology concept covering the roughly seven-and-a-half-year transit of Saturn through the signs before, on, and after the natal Moon.