- How did Edmund Halley figure out that comets seen in 1531, 1607, and 1682 were the same object?
- Why did the public fear Earth passing through the comet's tail in May 1910?
- Why did astronomers say the comet's tail posed no real danger despite containing a poisonous compound?
- What connects Mark Twain's birth and death to the 1835 and 1910 passages of Halley's Comet?
Planetary Science
The study of planets, moons, and other solar system bodies.
- What is the Zeeman effect, and how did it let George Ellery Hale detect a magnetic field on the Sun?
- Why were sunspots reinterpreted as magnetic flux tubes rather than simple dark patches after Hale's observation?
- What is the eleven-year solar magnetic cycle that Hale went on to map?
- How does the Sun's magnetic activity connect to modern space weather and satellite disruption?
- How did introducing elliptical electron orbits explain the doublet splitting in hydrogen's spectral lines?
- Why did Eddington need a total solar eclipse to measure the bending of starlight by the Sun's gravity?
- How did George Ellery Hale and his Mount Wilson colleagues determine that sunspots are regions of intense magnetic field?
- What is the Hale cycle, and why does it take twenty-two years rather than eleven for the Sun's full magnetic polarity to repeat?
- Why does the sunspot cycle matter for forecasting geomagnetic storms that can disrupt satellites and power grids?
- What evidence led Percival Lowell to predict a ninth planet before Pluto was ever seen?
- Who proposed the name Pluto and why was it chosen?
- Why was Pluto later reclassified in 2006, after seventy-six years as a planet?
- Why did the hiss that repeated every 23 hours and 56 minutes point to a source among the stars rather than on Earth?
- Why were Britain's wartime radar stations picking up mysterious interference in February 1942?
- What was happening on the Sun that caused it to emit strong radio-frequency noise?
- How did a wartime search for enemy bombers end up founding the field of radio astronomy?
- What orbital patterns in long-period comets led Jan Oort to propose a distant cometary reservoir?
- What strategic fact about satellites did Western newspapers explain to the public after Sputnik's launch?
- Why did Explorer 1's Geiger counter report almost no radiation at high altitude, and what did that actually mean?
- How did Explorer 3 confirm the existence and shape of the two radiation belts around Earth?
- Roughly how far from Earth do the inner and outer Van Allen belts extend?
- Why do the Van Allen belts matter for spacecraft shielding and crewed missions beyond low Earth orbit?
- Why did Luna 1 miss the Moon, and by how much?
- What did Luna 1's instruments detect that became known as the solar wind?
- What does it mean for an object to become 'heliocentric,' and why was Luna 1 the first human-made object to do so?
- What measurements did Luna 2 confirm about the solar wind and the Moon's magnetic field before impact?
- How long did Yuri Gagarin's Vostok 1 flight last, and what altitude and orbital path did it follow?
- Why did Venera 1 need to be launched toward a point in space where Venus would be months later rather than where it currently was?
- How many orbits of Earth did John Glenn complete, and how fast was Friendship 7 traveling?
- What caused Telstar 1 to go silent just seven months after launch?
- What surface temperature did Mariner 2 measure on Venus, and what did that rule out about habitability?
- How did Mariner 2 confirm the existence of the solar wind?
- Why did the first Mariner probe to Venus fail before Mariner 2 succeeded?
- How did Mariner 2's findings overturn the science-fiction image of a habitable Venus?
- Why did gravitational contraction fail as an explanation for how long the Sun could keep shining?
- Why were Venera 5 and 6 considered successes even though the probes were destroyed?
- What is a free-return trajectory, and how did it help bring Apollo 13's crew home safely?
- How did Mariner 9's photographs of sinuous channels change scientific thinking about water on Mars?
- Why was Mariner 9 the first spacecraft to orbit another planet rather than simply flying past it?
- How did Pioneer 11 use Jupiter's gravity to redirect itself toward Saturn?
- What did Pioneer 11 discover during its 1979 flyby of Saturn?
- What made the 1977 planetary alignment of Jupiter, Saturn, Uranus, and Neptune so rare, and how did it enable the Voyager missions?
- What does it mean for Pluto and Charon to be tidally locked in a mutual gravitational stare?
- How did the discovery of Charon contribute to Pluto's 2006 reclassification as a dwarf planet?
- Why couldn't NASA boost Skylab into a higher, safer orbit before it fell?
- How did higher-than-expected solar activity speed up Skylab's reentry?
- How does Axion Esti fuse Byzantine liturgy with surrealist imagery?
- Why did the changing plumes on Io matter scientifically beyond just confirming volcanism?
- How did Voyager 2 use Saturn's gravity to redirect itself toward Uranus, and why was the timing so rare?
- What did the first colour photographs of the Venusian surface actually show?
- Why had no one ever seen the nucleus of Halley's Comet before the 1986 spacecraft encounters?
- Why did the United States decline to send a spacecraft to meet Halley's Comet in 1986?
- Why is Uranus's magnetic field, tilted 60 degrees from its rotation axis and offset from its center, so hard to explain?
- What did the near-total absence of ancient impact craters on Venus reveal about its geological history?
- How did Voyager 2 use gravitational slingshots from Jupiter, Saturn, and Uranus to reach Neptune in 1989?
- Where is Voyager 2 now, and what is it still transmitting from interstellar space?
- What did the near-total absence of impact craters suggest about the age and history of Venus's surface?
- What happens during a performance of John Cage's 4'33"?
- Why does a shrinking orbital period between two neutron stars count as evidence for gravitational waves?
- What longstanding question in planetary science did the 1994 impacts help settle?
- What roles did Carl Sagan hold in the Voyager missions and the Planetary Society during his career?
- How did Alan Hale and Thomas Bopp independently discover the same comet on the same night?
- What made Comet Hale-Bopp visible to the naked eye for a record eighteen months?
- What did astronomers detect in Hale-Bopp's coma for the first time in any comet?
- What tragedy became linked to Comet Hale-Bopp's approach in March 1997?
- How does Chandra's elliptical orbit help it avoid interference from Earth's radiation belts?
- Why is Titan considered the most Earth-like body in the solar system despite its alien chemistry?
- What new definition of 'planet' did the International Astronomical Union adopt in 2006?
- What three criteria did the IAU's 2006 definition require for something to count as a planet?
- Which specific criterion did Pluto fail to meet, and why?
- How did Kepler's transit method detect planets by watching for tiny periodic dimming in starlight?
- How many exoplanets did Kepler confirm over its mission, and what did that reveal about how common planets are?
- How long did Bill Borucki work to get the Kepler mission funded before it launched?
- How does the Kepler space telescope detect exoplanets using the transit method?
- How did MESSENGER use flybys of Earth, Venus, and Mercury to slow down enough to enter orbit?
- Why are Mercury's polar craters permanently shadowed even though the planet orbits so close to the Sun?
- How did NASA use plasma wave data to confirm Voyager 1 had crossed the heliopause?
- How did researchers use a planetarium to discover that dung beetles navigate by the Milky Way?
- What did fitting beetles with tiny cardboard visors prove about how they orient themselves?
- What about ʻOumuamua's trajectory proved it originated outside our solar system?
- Why did ʻOumuamua's unexplained acceleration lead to competing theories, including Avi Loeb's alien-technology hypothesis?
- Why did Cassini's controllers choose to dive it into Saturn rather than leave it in orbit?
- Why does an efficient launch window to Mars only open once every 26 months?
- What is the Alfvén critical surface and why does it mark the boundary of the solar corona?
- How did Venus flybys let the Parker Solar Probe shrink its orbit close enough to reach the Sun?
- What are magnetic switchbacks, and why had they only been inferred before 2021?
- Why was JWST's deployment sequence so risky compared to a telescope in low Earth orbit?
- What is the L2 Lagrange point, and why does it put JWST beyond the reach of repair missions?
- Why was the L2 Lagrange point chosen as Gaia's observing position?
- How had some of these objects' orbits been inferred before Vera Rubin's direct observations confirmed them?