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I keep reading about Mars meteorites found on Earth. How do scientists know they came from Mars without samples of martian rocks to compare them?

Where does Enceladus get its water?

When the Sun produces a coronal mass ejection, is the blast of sufficient force to alter our star's path through the cosmos? Do these eruptions significantly perturb our solar system?

Besides computer simulations, what evidence do astronomers have to support the popular hypothesis that elliptical galaxies form from merging spirals?

If all photons have the same speed (the speed of light) and the same mass (zero), why does an individual gamma-ray photon have more energy than a radio-wave photon?

How do mission designers know at what angle to land on an astronomical body such as the Moon, Mars, or Earth?

How much color should I be able to see in sky objects through a 10-inch telescope?

Is it possible for a quark star to exist?

How do scientists know that the particles revealed after a proton or lead ion collision are different from each other, such as quarks and muons, and not just smaller pieces of protons or lead ions? (If I were to smash two lead bullets together, they would break into smaller pieces, but they would still be lead.)

How can six Trojan asteroids reside in Neptune's L4 region without colliding?

I have always heard that everything we see is "star stuff" from previous stars that died after converting their hydrogen into heavier elements. So how is there any hydrogen left for the Sun and planets?

Earth spins as it orbits the Sun, and the solar system is orbiting the galactic center. So, if I go outside and look up, in what direction are we heading?

How massive are globular clusters’ stars? They’re old, so they can't be too big, right?

How old is the Milky Way, and has it merged with other galaxies along the way?

Have astronomers seen any spectral lines they can’t identify?

Why is the Sun’s corona so hot?

Why does the zodiacal dust cloud still exist after billions of years? Does something replenish it?

As I understand, a star will “blow away” any remaining gas and dust in the immediate area once it begins fusion. So why can some stars collect billions of times more mass before blowing away the remainder of their embryonic material?

What is the hexagon at Saturn’s north pole, and what causes it?

Has dark matter ever been detected in or around the Milky Way Galaxy?

How many transiting exoplanets might NASA’s Kepler spacecraft find?

What is the best and cheapest way to get into solar observing?

What percent of star systems have orbits in the right orientation for scientists to find exoplanets through the transit method?

Would it be possible to find a moon around an extrasolar planet? If so, how?

What are the coldest places in the solar system?

Do globular clusters form in a galaxy's halo or migrate in from outside? If the latter, how?

How massive must a colliding body be to disturb Earth's orbit?

What is the heaviest element that astronomers can detect?

Why does Mercury have such a large core?

What is the average time for dust discharged from a supernova to be reconsolidated in part or whole to form a new start?

I’ve heard that most of Earth’s water came from comets, but where did all the water in those comets come from?

If you could get close enough to the Sun and hang out a microphone that wouldn't melt, would it pick up any sound from our star?

What is responsible for the horizontal bands in Jupiter's atmosphere?

How difficult is it to accurately implement planetary flybys? What factors determine the desired closest approach of a space probe during a flyby?

Could we detect a comet or asteroid hitting the Sun?

How close to each other do two stars have to be for one to steal material from the other?

How can scientists be sure that the space probes they send to the planets and other celestial objects aren't contaminating these worlds?

Why do the outer planets have more moons than the inner planets?

How is it possible for the universe to be infinite if it had a beginning, 13.7 billion years ago by most estimates?

While watching a total lunar eclipse, I still can faintly discern the Moon's disk, even during the umbral portion of the eclipse. How is this possible, given that no significant source of light reflects from the Moon at that time?

Why do we see only one side of the Moon from Earth? Have humans explored the other side, and, if so, is it different from the nearside?

How do scientists know there is a central black hole in almost every galaxy?

Can snow form "snowbows" similar to rainbows?

Is there evidence of Oort-Cloud-like structures around other stars?

Planets, orbits, and comets are somewhat round. Is there an underlying mechanism or physical law?

What is the circle that Earth’s axis traces out in its 26,000-year precession cycle? What bright stars would be close to it?

How deep are the craters on the Moon?

Would it be possible to send a probe vertically above the solar system's plane to observe the entire solar system and capture a mosaic photo? How far would it need to go to get a complete photo of everything?

When will Comet Halley next pass near Earth? Are there any other comets that will come close enough to our planet that people in the United States can see it at night?

When a really big rock (or a huge chunk of iron) hits Earth, why does it explode instead of just making a lot of rubble?

I read recently about the possibility of two white dwarfs colliding to create a type la supernova. Why do astronomers think it has to be two white dwarfs instead of a white dwarf colliding with a regular star?

The Hubble Space Telescope found a galaxy that was completely formed within 480 million years of the Big Bang. How could gas and dust clouds condense into stars and organize themselves via gravity into an entire galaxy in less time than it takes the Milky Way to make two rotations?

Is it possible that the gravitational effects attributed to dark matter could be caused by Jupiter-sized planets discovered orbiting other stars and also floating freely outside planetary systems?

What determines the length of totality during a solar eclipse?

If the Sun suddenly disappeared, it would take about eight minutes for Earth to become dark (due to the speed of light). How long would it take to feel the absence of the Sun’s gravity?

How do light pollution filters work? Do they actually help?

Are there plans to further the Hubble Space Telescope’s capacity to look even deeper?

Why do some stars appear to flash a variety of different colors when you look at them through a telescope?

If a brown dwarf orbited the Sun at the same distance as Jupiter, would life have evolved on Earth?

How can astronomers tell the difference between a group or cluster of stars and an actual galaxy?

I’ve read about background radiation fields that fill the universe. Is the infrared radiation as ancient as the microwave, and are there other cosmic fields of radiation besides these two?

Could the huge gamma-ray bubbles that extend above and below the center of the Milky Way be a boundary of matter and antimatter, whose collision emits gamma rays?

What would cause Earth’s orbit to become more circular instead of its present elliptical orbit, and has this ever happened before?

What is the Higgs particle?

Is the incredibly slow rotation of Venus a result of a geologic cataclysm?

If the Andromeda and Milky Way galaxies collide head-on when the two are predicted to merge five billion years from now, what is likely to happen to the Large and Small Magellanic Clouds?

What are typical star-to-star distances in globular clusters compared to stellar distances in our region of the galaxy?

If space is only a few degrees above absolute zero, how can hot nebulae gases from exploded stars stay warm for a long time?

Earth’s axis is tilted 23½° with respect to the Earth-Sun plane. What factors contribute to its “wobble” over extended periods?

What causes a planet to be tidally locked as opposed to rotating freely?

I've seen Astronomy magazine label M33 as the Pinwheel Galaxy, but I thought M101 was the Pinwheel Galaxy and M33 was the Triangulum Galaxy. Which is correct?

Is the universe infinite or finite? Or is it so close to infinite that for all practical purposes it is?

What is the baseline for determining the magnitude scale of celestial objects? Why do brighter objects have negative numbers?

Do the paths of totality during solar eclipses follow a pattern that repeats after a certain period?

What factors determine if a massive star (say, 100 solar masses) forms instead of a Sun-like star?

What types of energy are produced when matter and antimatter annihilate each other?

How do astronomers determine what causes starlight to shift toward the red end of the electromagnetic spectrum?

Why do the gas giant planets have so much gravity when they do not have solid cores like the terrestrial planets? Or do they have solid cores?

I understand that new stars form out of mostly hydrogen with smaller amounts of heavier elements. If helium, carbon, oxygen, and other heavier elements were abundant enough to form stars, could they?

Why are most impact craters nearly perfectly round? I'd expect the impacts to result from an obtuse or glancing blow that would leave an oblong or ellliptical shape.

Has New Horizons taken any pictures of Saturn, Uranus, or Neptune?

How can astronomers predict that meteor showers will return year after year?

I’ve read recently about new revelations regarding Cepheid variable stars. Given that these stars are used to measure cosmic distances, will this new data change measured distances to various objects?

To avoid dew, do professional observatories heat the mirrors and lenses like we amateurs do?

How is it possible that Mars can have dust devils and global dust storms with such a thin atmosphere?

How could the universe experience a period of inflation that would have resulted in faster-than-light travel?

The Sun is a G2V star and Rigel is a B8Iab. How do astronomers get the classifications this precise?

What do the terms arcminutes (') and arcseconds (") mean?

Where does the magnetism in neutron stars come from, as neutrons are neutral particles?

Does Eta Carinae have enough mass to eventually become a neutron star, or even a black hole?

I am perplexed by sidereal time; would you go over this concept?

On Earth, scientists measure the heights of mountains with respect to sea level. What baseline do they use on other planets or moons?

How does the Sun's magnetic field affect Earth?

Could gas giants be considered stars that failed to ignite?

Do galaxies rotate clockwise or counterclockwise? Do all galaxies rotate in the same direction?

Photos of recent comet flybys show their nuclei to be barbell-shaped rather than spherical. What are the forces or processes that produce this shape?

How do astronomers know that the universe is 13.7 billion years old?

What is the benefit of a 2" eyepiece compared to a 1¼" eyepiece?

Gravity decreases with distance. Could the repulsive force of dark energy increase with distance, causing the universe’s expansion to accelerate?

On Earth, methane is a flammable fuel. If you introduced a flame to the methane on Saturn’s moon Titan, would it burn?

What causes the central bar in some spiral galaxies?

How dense are giant gas clouds like the "Pillars of Creation"?

Have astronomers detected elements in the universe that aren’t found in our solar system?

I see the term dust in many Astronomy articles. What is it, and where does it come from?

What is a blue straggler?

How do scientists identify planetary compositions and temperatures (both internal and external)?

Magnetic fields, subatomic particles, etc., traveling in different directions fill the universe. Do they deflect or otherwise affect one another? If so, isn’t the information we get from them distorted?

What causes the highly differentiated color banding on Jupiter?

How does one differentiate between an exoplanet orbiting in front of a distant sun and a large starspot traveling across that star’s disk?

Why are all planetary systems and most galaxies disk-shaped?

How often does a large meteroid hit the Moon? Can I see such an impact?

Does the time it takes different methods to create all the elements coincide with the universe’s accepted age?

Scientists identified galaxies at 600 million light-years after the Big Bang in the Hubble Ultra Deep Field. Shouldn’t this light have passed us long ago? Why is it just getting to us now?

What causes spikes around stars in pictures, and how can I prevent them?

Is it possible to see a star’s disk? I’ve heard that Betelgeuse has been imaged, but what about the Alpha Centauri system or other stars?

Hubble is in space and the mirror is exposed, so how has the mirror stayed clean? Is it possible that space debris and micro particles could hit it?

What exactly are blazars and quasars? How are they different?

Why isn’t dark energy considered the “fifth force” in the way gravity, electromagnetism, weak nuclear, and strong nuclear are forces?

How can we tell if a star belongs to a star cluster or is in the space between the cluster and the observer?

Why doesn’t our Moon have a name?

What is a white hole?

When using my astronomy software or looking at various books, I notice that not everyone draws the constellations the same way. Why?

Do astronomers know of or suspect any stars that were original members of the same star cluster as our Sun?

Would I be able to see Saturn from the foggy surface of Titan?

What are carbon stars? How do they form?

What is the largest star known, and how far out would its habitable zone be?

How is it possible that scientists estimate that the universe is 13.7 billion years old, but more than 90 billion light-years across?

I read that ultraviolet light is the cause of HII regions, but this light is invisible. So why are these objects the color red?

When scientists talk about the rotation of a black hole, what are they referring to? If we can’t see a black hole, how can we tell if it rotates?

What happens to dark matter in the gravity field of a black hole?

What causes the variety of totality length from one solar eclipse to the next? Why does the length of totality vary along the path of an eclipse?

Why do impacting bodies on Jupiter leave black splotches even though the gas giant does not have a solid surface?

At the end of the Sun's life, when it expands into a red giant, what will become of the planets in the solar system?

Have scientists thought of what type of universe would evolve if physical constants had different values?

Does the Hubble Space Telescope actually take a number of sub-exposures and then stack them? If so, how long are typical sub-frames?

How do you distinguish between absorption lines that originate in a distant astronomical object and those from Earth's atmosphere?

Why doesn't our solar system contain any moons orbiting a moon?

What does "maximum usable" magnification mean?

Billions of years from now, what will be the fate of our Sun? Would it be a hard surface we could walk on?

How do astronomers make generalizations about the ages of stars, galaxies, and other celestial objects?

What are gravitational waves, and what do they have to do with the Big Bang? Can scientists detect them?

When we send spacecraft to the outer gaseous planets, how do they survive the trip through the asteroid belt between Mars and Jupiter?

In the December 2009 issue of Astronomy, you explained why we couldn't bring the Hubble Space Telescope back to Earth. But can we attach it to the International Space Station?

I heard that the light from nuclear fusion reactions in the Sun's core takes about 200,000 years to reach its surface. I thought the speed of light is constant. Why does it take so long?

Is there a difference between visual and imaging filters?

With many new planet discoveries being hot Jupiters, is it possible that Mercury is the leftover core of a gas giant?

I've heard that wormholes could make time travel in the future possible. How realistic is this phenomenon?

How difficult is it to launch an artificial satellite into a circular orbit versus an elliptical one? Is the final altitude of the orbit a key factor?

Could the elusive neutrino particle be a component of dark energy?

I saw a shooting star tonight; as it burnt up in the atmosphere, it became a brilliant green. I have seen only yellow and white ones before. What causes the green color? Or could I have seen space junk burn up?

The universe has billions of galaxies and billions of stars, so why is space dark?

If all the known matter in the universe were compressed into one colossal black hole, how big would it be (both size and mass)?

Is every star in a designated galaxy? Are there any "rogue" stars?

Even though we won't be around when M31 approaches, what will be the earliest effects of the collision on Earth, the Sun, and the solar system's planets?

Is there any evidence that Earth's magnetic poles (or any other planet's) have reversed? What would cause such a reversal?

Why are all moon photos in black and white, and yet other planets show color?

The current theory of lunar genesis says a Mars-sized object collided with Earth and ejected material that later coalesced into the Moon. Is it just by chance that the newly formed Moon adopted an orbit that coincides with the ecliptic plane?

If Earth had more than one moon, how would it have affected life's evolution on the planet?

Can extrasolar planets be detected with radio telescopes and then studied via radio astronomy?

It looks like the Moon stays higher in the sky on some days than on others. Why is that?

Why don't globular clusters ultimately collapse, rather than getting old and stable?

What is the maximum number of spatial dimensions any universe could have?

How are we sure that images classified as gravitational lensing are a distortion of light, not existing objects?

My astronomy book reads, "Heavy-element fusion in massive stars ends with iron." How are elements heavier than iron in the periodic table created?

What is the number one piece of advice a beginning astronomer needs to receive?

Why didn't any of the other gas giants (Jupiter, Uranus, or Neptune) develop a large ring system like Saturn?

How do scientists measure dark matter, dark energy, and normal matter using the CMB?

Could a near-Earth asteroid enter into orbit around Earth?

If our galaxy's central supermassive black hole devoured a star, what effects or phenomena would be visible to observers on Earth? How likely is this sort of event to happen within a few human lifetimes?

When did observers determine that "wandering stars" were not stars at all, but rather planets?

Is it normal for telescope optics to freeze up when the temperature is below freezing with high humidity? If so, how can I prevent this?

Do Io's volcanic eruptions create the electromagnetic torus between the moon and Jupiter? Could Io deorbit into Jupiter in the next several million years because of energy lost in these interactions?

Could our solar system's habitable zone hold two earthlike planets?

Do stars experience the same phenomena as our Sun, such as sunspots, prominences, solar wind, and flares?

If we live in a multiverse, why do other universes not collide with ours?

How is it that galaxies merge and collide but the universe is expanding and everything is moving away from everything else?

I have an 8-inch telescope but can see only Mars as orange instead of red. All other planets appear colorless. Why?

Have astronomers seen massive young clusters like the Milky Way's Arches Cluster in other spiral galaxies?

Is it true that there are more stars in the universe than there are grains of sand on Earth? How could one calculate such figures?

Why can't the recently launched Herschel Space Telescope produce sharper images than the Spitzer Space Telescope?

What is the largest rocky extrasolar planet ever found and its size comparison to Earth?

How do globular clusters form? Can such massive concentrations of stars arise from gas clouds alone, or is a central black hole required?

Dark energy is consistently described as a property of space itself. Does this mean that, in the total absence of matter, the universe would still exist?

I was taught that the gravity of a black hole is so great that nothing, not even light, can escape. But I frequently see pictures of dual energy jets emanating from a black hole. How is this possible?

Why hasn't the asteroid belt formed a planet?

Why do comets have such eccentric orbits?

How did Olympus Mons get to be so tall? Is it because the gravity on Mars is weaker than that on Earth?

Does Titan have the requirements to support life?

How do supermassive black holes form at the centers of galaxies?

What is the smallest star in the universe, and where is it?

Is there a plan to retrieve the Hubble Space Telescope from space to preserve it in the Smithsonian Institution or elsewhere for future generations to appreciate?

I have a 70-millimeter refracting telescope. Would a scope with a larger diameter make objects look bigger or closer?

Does flying in a plane underneath the aurora expose a person to radiation? Would it be dangerous?

Is it possible for a large star to collapse itself out of existence rather than just forming a black hole?

Will the Messenger probe to Mercury be able to get a photograph of the asteroid Icarus?

Will the galaxy M33 be involved in the merger between our Milky Way Galaxy

I read somewhere that mercury is being used to make liquid-mirror telescopes. What are the advantages of collecting light this way?

Do black holes and their accretion disks always spin in the same direction? Does a supermassive black hole always spin in the same direction as its host galaxy?

Saturn's rings are said to contain chunks of icy debris as big as cars. Why are none of the larger particles visible in the Cassini photos?

When a star in a binary system goes supernova, what happens to its companion? Is it destroyed?

What are asteroids made of?

Could there be large amounts of water in deep aquifers on Mars?

Could the nearby star Sirius B explode as a type Ia supernova, like some other white dwarf stars in binary star systems have?

The surface of Neptune's moon Triton is partially made up of methane. If an asteroid were to collide with it, would that touch off an explosion?

Who was the first person in history to accurately predict a solar eclipse?

What is the center of a galaxy made of that makes it so bright?

How do scientists determine the elevations of mountains and other topography on the Moon, Mars, and other places where there is no sea level as the reference point?

Do astronomers name and catalog individual stars in other galaxies?

If a black hole crossed in front of a background star, could we see the black hole and determine its size?

Who first measured the speed of light? What method was used?

What determines the particular shape of nebulae like the Dumbbell, Trifid, and Lagoon?

Is it possible to combine digitized observations of an object from widely separated telescopes, creating a virtual interferometer?

How much water covered Mars' surface, and which areas had the most water? Why did it disappear?

Is there an equivalent to the North Star in the Southern Hemisphere?

When galaxies merge, what happens to their central black holes?

What would our solar system look like if aliens observed it with the same telescope used to image three planets around the star HR 8799?

Could the meteorite that formed the Hellas Basin on Mars have triggered the volcanic eruptions on the other side of the planet that created Olympus Mons?

Could large numbers of black holes account for much of the "dark matter" estimated to exist in our universe?

How do the bars in barred spirals form?

The constellation Ophiuchus lies partly on the ecliptic, so why isn't it considered one of the zodiacal signs?

In stellar magnitude estimates, why do larger numbers represent dimmer objects? Shouldn't it be the other way around?

If Io continues to erupt at the fast rate it does now, could all of the moon's insides end up on the surface, causing it to collapse onto itself and start the process all over again?

How many black holes have astronomers discovered in our Milky Way Galaxy? How do they find a black hole?

What is a parsec? Why do scientists use this unit instead of light-years?

With temperatures being so low in space, why don't the gases condense into liquids or freeze as they do here on Earth?

What is a Wolf-Rayet star?

How do stars move inside a globular cluster? Do they orbit a black hole, like the stars in the Milky Way Galaxy?

Can a black hole swallow an entire galaxy?

Is there a simple way for amateur collectors to determine if meteorites for sale are authentic?

Would it be possible for an amateur astronomer to combine multiple telescopes into a long-baseline interferometer?

Why do images of stars often show four spikes radiating from the stars' centers?

I've read about stellar explosions propagating at "supersonic speed." On Earth, supersonic means faster than the speed of sound. How does this idea apply to stars?

After the Andromeda and Milky Way galaxies finish merging in a few billion years, what will happen to the other galaxies in the Local Group?

What bodies beyond Pluto will the New Horizons probe investigate?

What would we see if the solar system were located in a dark nebula like the Coal Sack? Would things look much different from inside a bright nebula such as the Orion?

What was the most powerful supernova blast ever seen?

How many stars does our galaxy contain?

Does dark matter clump together like ordinary matter?

What are the heliosphere, the termination shock, and the heliopause?

Why are there no stone carvings or evidence of writing at Stonehenge? Didn't the builders have some written language or symbols?

What do radio telescopes observe, and how does it relate to the visible sky?

In what direction and at what speed is the solar system moving through the Milky Way?

Is it possible to do solar observing and imaging with a large-aperture Newtonian or Dobsonian telescope?

Astronomers have estimated star diameters from their colors and brightnesses, but modern interferometers can measure them directly. How do the two methods compare?

As the Sun set one evening, I observed a sword-like column of light pointing straight up from the horizon. What was it?

Astronomers say our galaxy hasn't seen a supernova in 400 years. Why don't they count the 1987 supernova?

Why does a space shuttle rotate shortly after launch?

Could a dwarf galaxy merging with the Milky Way have triggered the solar system's formation?

People across town heard a window-rattling sonic boom from the February 19 fireball here, but we heard nothing. Why?

How do you draw the line between a large globular cluster and a dwarf spherical galaxy?

When will the New Horizons spacecraft overtake Voyager 1 as the most distant human-created object in space?

Why doesn't Astronomy magazine recognize Messier 110?

Why do colliding spiral galaxies make an elliptical galaxy instead of a larger spiral?

Can telescopes see a star's disk?

Could you recommend an astronomy book for beginners?

How many supernovae explode each second in the universe?

In Chicago, shortly before World War II began, I saw the evening sky turn blood red. What caused this?

How do astrophysicists measure a black hole's mass?

What is the difference between comets and asteroids?

What is a Bok globule?

With lots of elements like uranium, how can the star HE 1523-0901 be 13.2 billion years old?

Do all spiral galaxies eventually become elliptical galaxies?

How do planets, moons, and asteroids become round?

How can astronomers know details about the appearances ancient comets?

If photons have no mass, how can gravity bend light?

What's the best way to observe a meteor shower?

Would astronauts on the Moon observe Earth occulting background stars?

Does a black hole stay a black hole for life?

Could nearby stars Alpha Centauri A and B harbor earthlike planets in stable, circular orbits?

What is a planemo?

Where can I get information about impact sites on Earth?

How are comets named?

Because the universe was smaller at earlier times, do we see larger volumes of it as we look back in time?

Have we ever seen any star as more than a point of light?

How do right ascension and declination relate to longitude and latitude?

Where's the best place to view aurorae?

If time stops at a singularity, how can massive black holes grow?

Does the Moon wobble?

Should I purchase a 1¼" or 2" star diagonal for my new telescope?

What is the theory of panspermia?

What is the Stardust spacecraft's mission now that it has returned comet samples to Earth?

Does the debris left behind by comets pose a hazard to the International Space Station?

Does the Hubble constant change over time?

What is the difference between a planetesimal and a dwarf planet?

Why is the Sun's corona so hot?

Do any two photos taken of the same location on Mars at two different times show surface changes?

Does a black hole gobble up neutrinos, or do the neutrinos just breeze through the black hole as they do through everything else?

If galaxies are expanding away from one another, why are Local Group galaxies like the Milky Way and Andromeda moving toward each other?

Why are neutron stars constrained to a diameter of 10 to 12 miles?

How would a supernova explosion near our solar system affect Earth?

What will happen when the 100-plus-solar-mass star Eta Carinae dies?

What would happen if a black hole met a 1-solar-mass star made of antimatter?

Where do comets come from and why are their orbits so elongated?

Does redshift increase over time?

Does the "green flash" ever occur during sunrise, or is sunset the only time one can occur?

How do gravity assists work?

Why do all planets in our solar system orbit in approximately the same plane?

Could an asteroid the size of the one that killed the dinosaurs knock Earth out of its orbit and create a global climate shift?

Can you see the American flag Apollo 11 astronauts left on the Moon in 1969?

Do black holes stretch or rip the fabric of space-time?

Why can the Hubble Space Telescope see distant galaxies but not exoplanets?

How does the "Planets this month" foldout map work? I don't see any time given for the map, unlike the circular star map on the following page.

How do astronomers measure the speed of dust and gas carried away by stars' stellar winds?

What is a comet?

What is a solar prominence, and what equipment do you need to photograph one?

Will Pluto ever collide with Neptune or Uranus?

Could a nuclear explosion break apart an earthbound asteroid?

Do black holes ever disappear?

Does the fact that our universe is accelerating imply that it will expand forever?

Would water released from a spacecraft into space boil because of the low pressure or freeze due to the low temperature?

Could black holes be the source of dark energy in our universe?

Could particles of Comet Wild 2, collected and returned to Earth by the Stardust probe, have been picked up in the outer solar system after the comet formed?

What causes the difference in the amount of sunlight reflected to Earth by the Moon's highlands and maria?

What is the difference between the expansion of space and time following the Big Bang and matter expanding from an infinitely dense point?

Does time slow down as a spacecraft approaches the speed of light, thus allowing the craft to travel back or forward in time?

If gravity can leak out of our universe, is it possible gravity is leaking into our universe in a similar manner, causing effects attributed to dark matter?

With the Moon gradually moving away from Earth, what will happen to people and objects when the Earth-Moon center of mass reaches Earth's surface?

If the universe is expanding, are atoms, particles, and people also expanding?

Why do some meteor showers, such as the Geminids, have slower-moving meteors, while others, like the Perseids, have faster-moving meteors?

Why is the Moon's orbital distance increasing over time?

How big of a meteor impact would it take to send the Moon careening into Earth?

What is a "Blue Moon," and does the Moon actually look blue when it happens?

Will the MESSENGER probe be able to spot Vulcanoid asteroids inside Mercury's orbit?

Can a 0.2-second pulse from a magnetar put out as much energy as the Sun does in 250,000 years?

Can you explain why irregular constellation boundaries appear in various star atlases?

If nothing, not even light, can escape a black hole, then why do nearly all artist renderings of black holes show an accretion disk and a jet that appears to spew out particles perpendicular to the disk?

Why are there no green stars?

Is there a simple way to explain the equatorial coordinate system so an amateur can understand it?

What kind of education does a person need to become an astronomer?

Where in space did the Big Bang happen?

How do neutron stars generate steady magnetic fields?

What is dark matter?

What would happen if you fell into a black hole?

Do the planets ever transit or occult each other?

How do the conservation laws of physics relate to the expansion and fate of the cosmos?

Why didn't Jupiter become a star?

How do spacecraft safely navigate the asteroid belt?

If the universe began in an infinitely small volume, why isn't the universe a single supermassive black hole?

What does it mean when you say things like "The Moon passes 4° south of Mars"?

Does the Moon follow the same path across the sky as the Sun?

In "Who knows what dangers lurk in space?" (March 2006), you stated, "Cerenkov radiation occurs when certain high-energy particles move through water or the atmosphere faster than the speed of light." How can anything exceed the speed of light?

How do astronomers determine the ages of stars?

Is it a coincidence that most of the planets fall within the Titius-Bode law's boundaries?

What is meant by the term "flat universe"? How is this flatness supported by measurements of the cosmic microwave background?

If nothing can travel faster than light, and the universe is 13.7 billion years old, how can two galaxies lie 10 billion light-years from us in opposite directions?

Is there any way photons could contribute to the mysterious dark matter?

What typical magnifications did 19th-century observational astronomers use for lunar and planetary viewing?

How can I recognize meteorites among other stones?

How would astronomers in another solar system know by observing our Sun's wobble that our Sun has not just one big planet, but nine, each with a different mass?

I read photons have energy but no mass, so how can massless photons produce a force upon striking a solar sail?

	Sun	Sun	Moon
RISE	—		—	PHASE	—
SET	—		—	DISK	—

	Mercury	Mercury	Venus	Venus
RISE	—		—
SET	—		—
		—		—

	Mars	Mars	Jupiter	Jupiter	Saturn	Saturn
RISE	—		—		—
SET	—		—		—
		—		—		—