juliensimon/nasa-apod huggingface dataset:
Callisto in True Color (1998-05-11)

Callisto's surface has many stories to tell.  The most distant of Jupiter's Galilean Moons, Callisto shows the highest density of impact craters in the Solar System, but harbors no volcanoes or even any large mountains.  Callisto's surface is laced with cracks and craters from billions of years of collisions with interplanetary debris. This image shows Callisto's true colors, and was taken in November 1997 by the robot spacecraft Galileo currently orbiting Jupiter.
Callisto Enhanced (1998-05-12)

Callisto is half rock and half ice. This moon of Jupiter is approximately the size of the planet Mercury, making it the third largest moon in the Solar System, after Ganymede and Titan. Callisto's icy surface is billions of years old, lacks any sign of volcanic activity, and is densely covered with rifts and craters.  These features are particularly apparent in this contrast-enhanced image taken by the Galileo spacecraft, and released last week.  Visible near the image center is Valhalla, one of the largest impact craters in the Solar System, measuring about 4,000 kilometers across. The rings and size of Valhalla make its appearance similar to the Caloris Impact Basin on Mercury.
Occultations and Rising Moons (1998-05-13)

On April 23, the rising crescent Moon occulted (passed in front of) Venus and Jupiter. The double occultation was a rare event and only visible from certain locations tracing a path across Earth's surface. This dramatic telephoto picture was taken at one such location, Ascension Island in the South Atlantic. The sunlit crescent is over-exposed revealing the rest of the lunar surface illuminated by faint earthshine. Venus is emerging just beyond the crescent's tip and Jupiter is trailing above the dark lunar edge with a spot of light, Jupiter's moon Ganymede, between the lunar limb and the planet's disk. Look closely at Jupiter and you can see yet another Jovian moon, Io, just visible against Jupiter's glare!

Image credit: Olivier Staiger
Comet Stonehouse (1998-05-14)

Comets move against a field of background stars. Their apparent motion is slow but carefull tracking reveals their orbits, allowing these visitors to the inner solar system to be identified as old or new acquaintances. Recently a new comet, designated 1998 H1, was discovered by observer Patrick L. Stonehouse of Wolverine, Michigan, USA, and announced on April 26. At 10th magnitude, comet Stonehouse is too faint to be seen by the unaided eye, but it is presently a popular object for telescope-equipped comet watchers. This false color picture of comet Stonehouse was taken on May 1st at Limber Observatory and is a composite of 8 exposures each 60 seconds long. The sequence of exposures was made with the telescope following the background stars. The individual pictures were then aligned on the comet and added together. Because of the comet's relative motion, the combined multiple-exposure shows trails of progressively offset star images but nicely captures the comet's coma and faint, tenuous tail.

Image credit: D. McDavid and G. Dowdle,
Limber Observatory
TRACE and the Active Sun (1998-05-15)

This dramatic high resolution picture looking across the edge of the Sun was taken April 24th by a telescope on board the newly launched Transition Region and Coronal Explorer (TRACE) satellite. It shows graceful arcs of intensely hot gas suspended in powerful looping magnetic fields which soar above a solar active region. The colorized image was made in the extreme ultraviolet light radiated by highly ionized Iron atoms. With a temperature of a mere 6,000 degrees Celsius, the sun's surface is relatively cool and dark at these wavelengths, but the million degree hot plasma loops glow strongly! Such TRACE images follow the plasma and magnetic structures arising from the surface of the sun as they merge with the tenuous, hot solar Corona or outer atmosphere. By operating the TRACE instruments during the Sun's increasingly active phase, scientists hope to explore the connections between complex solar magnetic fields and potentially hazardous solar eruptions.
Helios Helium (1998-05-16)

This image of the relatively quiet Sun was made using ultraviolet light emitted by ionized Helium atoms in the Solar chromosphere. Helium was first discovered in the Sun in 1868, its name fittingly derived from from the Greek word Helios, meaning Sun. Credit for the discovery goes to astronomer Joseph Norman Lockyer (born May 17, 1836). Lockyer relied on a then recently developed technique of spectroscopy, dissecting sunlight into a spectrum, and the idea that each element produces a characteristic spectral pattern of bright lines. He noticed a yellow line in a solar spectrum made during an eclipse which could not be accounted for by elements known on Earth. Almost 27 years later terrestrial Helium was finally discovered when the spectrum of a Helium bearing mineral of Uranium provided an exact match to the previously detected element of the Sun. Helium is now known to be the second most abundant element (after Hydrogen) in the Universe.
Our Solar System from Voyager (1998-05-17)

After taking spectacular pictures of our Solar System's outer planets, Voyager 1 looked back at six planets to take our Solar System's first family portrait. Here Venus, Earth, Jupiter, Saturn, Uranus, and Neptune, were all visible across the sky. Each, however, was now just a small speck of light, dimmer than many of the stars in the sky. Voyager 1 is only one of four human-made objects to leave our Solar System, the other three being Voyager 2, and Pioneer 10 and Pioneer 11.
NGC 6369: A Donut Shaped Nebula (1998-05-18)

Why isn't the star in the center of the nebula?  NGC 6369 appears to be a fairly ordinary planetary nebula. It can be seen with a good telescope in the constellation of Ophiuchus. The gas expelled by the central star is bunched in the shape of a donut or cylinder.  During the planetary nebula phase, the central star sheds its outer atmosphere as it is evolving to become a white dwarf star. The above image was taken with the Hubble Space Telescope.  A closer look at NGC 6369 indicates that the central star is closer to a dimmer edge of the nebula than to the opposing brighter edge.
Apollo 11: Onto a New World (1998-05-19)

A human first set foot on another world on July 20, 1969. This world was Earth's own Moon. Pictured above is Neil Armstrong preparing to take the historic first step.  On the way down the Lunar Module ladder, Armstrong released equipment which included the television camera that recorded this fuzzy image. Pictures and voice transmissions were broadcast live to an estimated world wide audience of one billion people. The Apollo Moon landings have since been described as the greatest technological achievement the world has known.
Discovery Image: Comet SOHO (1998 J1) (1998-05-20)

Staring at the Sun from a vantage point in space (Kids, don't try this at home!), the Solar and Heliospheric Observatory (SOHO) has enabled the discovery of much about our closest star. It has also been used to discover about 50 comets. While not competing with Hale-Bopp, one of SOHO's recently discovered comets has proved to be bright enough to see with the unaided eye. The May 4th discovery image is shown above with an enlarged inset of the comet. This colorized image is from SOHO's solar coronagraph (LASCO) which views the region around the Sun by blocking out the overwhelming sunlight with an occulting disk. The disk is visible near the bottom left, with the Sun's size and position indicated by the white circle. Bright solar wind regions can also be seen along with the the planet Mars and a background of stars. The comet itself is just entering the field of view at the upper right. Observers report that Comet SOHO (1998 J1) has now been seen low in the western sky following sunset and is moving south and east becoming more visible as this month progresses, particularly from the Southern Hemisphere.
Bright Comet SOHO (1998-05-21)

Discovered this month with an orbiting solar observatory, bright Comet SOHO has now emerged from the Sun's glare. This telephoto picture of the new naked-eye comet was taken by astrophotographer Michael Horn after sunset in the western twilight above Lake Samsonvale, Brisbane, Australia on May 18. The comet is seen in the constellation Orion. Its long lovely tail stretches nearly 5 degrees to the bright star Bellatrix, near the top of the image. For Southern Hemisphere comet watchers, views of Comet SOHO (1998J1) will improve as this month draws to a close and the comet climbs to the south and east on its journey outward bound. In February 1999, NASA plans to launch the Stardust mission to fly close to a comet and return samples of dust from a comet's tail.

Image credit: Michael Horn
The Center of Centaurus A (1998-05-22)

A fantastic jumble of young blue star clusters, gigantic glowing gas clouds, and imposing dark dust lanes surrounds the central region of the active galaxy Centaurus A. This mosaic of Hubble Space Telescope images taken in blue, green, and red light has been processed to present a natural color picture of this cosmic maelstrom. Infrared images from the Hubble have also shown that hidden at the center of this activity are what seem to be disks of matter spiraling into a black hole with a billion times the mass of the Sun! Centaurus A itself is apparently the result of a collision of two galaxies and the left over debris is steadily being consumed by the black hole. Astronomers believe that such black hole "central engines" generate the radio, X-ray, and gamma-ray energy radiated by Centaurus A and other active galaxies. But for an active galaxy Centaurus A is close, a mere 10 million light-years away, and is a relatively convenient laboratory for exploring these powerful sources of energy.
7,000 Stars And The Milky Way (1998-05-23)

This panorama view of the sky is really a drawing. It was made in the 1950s under the supervision of astronomer Knut Lundmark at the Lund Observatory in Sweden. To create the picture, draftsmen used a mathematical distortion to map the entire sky onto an oval shaped image with the plane of our Milky Way Galaxy along the center and the north galactic pole at the top. 7,000 individual stars are shown as white dots, size indicating brightness. The "Milky Way" clouds, actually the combined light of dim, unresolved stars in the densely populated galactic plane, are accurately painted on, interrupted by dramatic dark dust lanes. The overall effect is photographic in quality and represents the visible sky. Can you identify any familiar landmarks or constellations? For starters, Orion is at the right edge of the picture, just below the galactic plane and the Large and Small Magellanic Clouds are visible as fuzzy patches in the lower right quadrant.
A High Energy Fleet (1998-05-24)

Looking like a fleet of futuristic starcruisers, NASA's highly successful series of High Energy Astrophysical Observatory (HEAO) spacecraft appear poised over planet Earth. Labeled A, B, and C in this vintage illustration, the spacebased telescopes were known as HEAO-1, HEAO-2, and HEAO-3 respectively. HEAO-1 and HEAO-2 were responsible for revealing to earthlings the wonders of the x-ray sky, discovering 1,000s of celestial sources of high-energy radiation. HEAO-2, also known as the Einstein Observatory, was launched near the date of the famous physicist's 100th birthday (November, 1978) and was the first large, fully imaging x-ray telescope in space. HEAO-3, the last in the series, was launched in 1979 and measured high energy cosmic-ray particles and gamma-rays. These satellite observatories were roughly 18 feet long and weighed about 7,000 pounds. Their missions completed, all have fallen from orbit and burned up harmessly in the atmosphere.
M83: A Barred Spiral Galaxy (1998-05-25)

M83 is a bright spiral galaxy that can be found with a small telescope in the constellation of Hydra. M83 is a member of the Centaurus group of galaxies, a nearby group dominated by the massive galaxy Centaurus A.  It takes light about 15 million years to reach us from M83. The spiral arms are given a blue color by the many bright young stars that have recently formed there.  Dark dust lanes are also visible.  Stars and gas in spiral arms seem to be responding to much more mass than is visible here, implying that galaxies are predominantly composed of some sort of dark matter. Finding the nature of this dark matter remains one of the great challenges of modern science.
A Seemingly Square Sun (1998-05-26)

Isn't the Sun round? Yes, but in the above picture, the Earth's atmosphere makes it appear almost square.  Here a layer of air near the Earth was so warm it acted like a giant lens, creating increasingly distorted paths for sunlight to reach the camera.  Similarly, on a long flat highway, it may appear that the road in the distance is covered with water. In this case, light from the blue sky is being unusually refracted by warm air just above the dry road.  No matter how the Earth's atmosphere makes the Sun appear, the Sun will always be spherical. This setting Sun was photographed over Lake Michigan in Muskegon, MI.

Image credit: T. Polakis
Magnetar (1998-05-27)

What do you call a neutron star with a super-strong magnetic field? You guessed it ... a Magnetar. Imagine a star with more mass than the sun, the density of a neutron, and a magnetic field about a thousand trillion (a one followed by 15 zeroes) times stronger than Earth's. It sounds exotic and theoretical, but strong evidence for the existence of magnetars has recently been announced based on data from orbiting X-ray and Gamma-ray observatories. Neutron stars are formed in the violent crucibles of stellar explosions. Some become pulsars with relatively weak magnetic fields, spinning and emitting pulses of electromagnetic radiation as their rotation slows. However, astronomers now believe that some become magnetars, with magnetic fields so intense that the solid neutron star crust buckles and shifts under its influence. The resulting star quakes could repeatedly generate brief flashes of hard X-rays and soft gamma-rays giving rise to the rare but mysterious "soft gamma repeaters" (not to be confused with " gamma-ray bursters"!). This still frame from an animation illustrating a spinning, flashing magnetar emphasizes the looping magnetic field lines embedded in the X-ray hot neutron star surface.
Afterglow (1998-05-28)

This sequence of three false color X-ray pictures from the Italian/Dutch BeppoSAX satellite follows the fading glow from a gamma-ray burster. This burster triggered orbiting gamma-ray observatories on December 14, 1997 and within 6.5 hours the sensitive X-ray cameras onboard BeppoSAX had been turned to record the first image (left) of the afterglow. Each image covers a field about the size of the full moon with the position of the afterglow indicated by the white circle. The first two pictures were taken 6 hours apart, while the final picture was made 2 days after the gamma-ray burst. Initiated by an unknown but immensely powerful explosive event, gamma-ray bursts are thought to be caused by blast waves of particles moving at nearly the speed of light. The expanding cosmic fireball produces seconds-long bursts of gamma-rays and then as it slows and sweeps up surrounding material, generates an afterglow visible for many days at X-ray, optical, and radio energies. Evidence indicates that this burst originated at a distance of 12 billion light-years requiring a fantastic and extreme energy source. What could power a gamma-ray burst?
An Extrasolar Planet? (1998-05-29)

This infrared Hubble Space Telescope view may contain the first ever direct image of a planet outside our own solar system. The picture shows a very young double star located about 450 light-years away toward the constellation of Taurus. Cataloged as TMR-1 (Taurus Molecular Ring star 1), the binary system is still embedded in the dust cloud that formed it. This double star and dust cloud are the brightest grouping in the picture, glowing strongly at infrared wavelengths. A filament extends from the binary system toward the lower left and points toward the spot of light representing the candidate planet. Astronomers believe this planet is a "runaway" object which was gravitationally ejected, the filament tracing the path to its present location at about 1500 times the Earth-Sun distance from the parent star system. Models suggest that the planet and binary system are a mere 300,000 years old, with the planet having a mass of about 2 to 3 Jupiters. Future observations to look for the planet's continued runaway motion and spectral signatures should be able to confirm the nature of this object. While this and other tantalizing discoveries of extrasolar planetary objects and protoplanetary disks don't seem to offer direct examples of solar systems like our own, they do strongly hint that planet formation is a varied and common process.   Update: TMR-1 likely just a background star.
Water World (1998-05-30)

Water (Dihydrogen Oxide, H2O) is a truly remarkable chemical compound, fundamental to life on Earth. Earth is the only planet in the Solar System where the present surface temperature and pressure allow the three forms of water, solid (ice), liquid (ocean), and gas (water vapor condensing in clouds) to exist simultaneously. Water in one of these forms accounts for everything visible in this view of Earth from space looking north at the Bering Sea and the coast of Alaska, USA, around Bristol Bay.
Phobos: Doomed Moon of Mars (1998-05-31)

Phobos is doomed. Mars, the red planet named for the Roman god of war, has two tiny moons, Phobos and Deimos, whose names are derived from the Greek for Fear and Panic. These Martian moons may well be captured asteroids originating in the asteroid belt between Mars and Jupiter or perhaps from even more distant reaches of the Solar System. In this 1977 Viking orbiter image, the largest moon, Phobos, is seen to be a heavily cratered asteroid-like object. It is about 17 miles across and zips through the Martian sky completing an orbit in less than 8 hours. Phobos orbits so close to Mars, (about 3,600 miles above the surface compared to 250,000 miles for our Moon) that gravitational tidal forces are dragging it down. In 100 million years or so it should crash into the surface or be shattered by stress caused by the relentless tidal forces, the debris forming a ring around Mars.
Solar Flares Cause Sun Quakes (1998-06-01)

An 11th magnitude quake has been recorded on the Sun, immediately following a moderate solar flare.  The quake was the first ever recorded on the Sun, but only because astronomers have only recently figured out when and how to find them using the orbiting SOHO spacecraft. Dark waves from the quake can be seen in the above picture spreading out from an explosive bright flare. The solar ripples are similar in appearance to waves caused by a rock thrown into a pond.  The magnitude and evolution of these quakes gives information about the physical nature of solar flares, the surface of the Sun, and even the Sun's interior.
NGC 6302: The Butterfly Nebula (1998-06-02)

The Butterfly Nebula is only thousands of years old.  As a central star of a binary system aged, it threw off its outer envelopes of gas in a strong stellar wind. The remaining stellar core is so hot it ionizes the previously ejected gas, causing it to glow.  The different colors of this planetary nebula are determined by small differences in its composition.  This bipolar nebula will continue to shine brightly for only a few thousand more years, after which its central star will fade and become a white dwarf star. The above picture is one of the first ever taken by the Very Large Telescope (VLT), a new 8.2-meter telescope located in Chile.

Image credit: Very Large Telescope, 
ESO
Martian Crater Shows Evidence of Dried Pond (1998-06-03)

Did a pond once exist in this Martian crater? Recent photographs by the spacecraft Mars Global Surveyor, currently in orbit around Mars, show features unusual for Mars yet similar to a dried pond on Earth.  Previously, much evidence suggested the effects of ancient channels of flowing water on Mars, but less evidence had been found for dried pools of water.  Islands and bays on this crater floor indicate an accumulation of some liquid, however, a hypothesis consistent with channels found on the (inset) crater walls.   As it is also possible the features were formed by other mechanisms including flowing lava, future observations and analysis will be needed to say for sure.
Comet SOHO and Nebulae in Orion (1998-06-04)

Astrophotographer Michael Horn captured this gorgeous view of comet SOHO in the dark night sky above Wandibindle, Queensland, Australia on May 23rd. On this date, comet SOHO was moving against the background of the nebula-rich constellation of Orion. South is up in the picture which shows SOHO's bright head or coma and long tail extending past the glowing gas clouds and dark dust lanes of the Flame and Horsehead nebulae.  Alnitak, the bright star above and to the right of the cometary coma, is also known as Zeta Orionis, the eastern-most of the three stars in Orion's belt. Southern Hemisphere observers report that comet SOHO has recently undergone a dramatic increase in brightness.

Image credit: M. Horn
Neutrinos in the Sun (1998-06-05)

Neutrinos, along with things like electrons and quarks, are fundamental pieces of matter according to physicists' Standard Model. But neutrinos are hard to detect. Readily produced in nuclear reactions and particle collisions, they can easily pass completely through planet Earth without once interacting with any other particle. Constructed in an unused mine in Japan, an ambitious large-scale experiment designed to detect and study neutrinos is known as Super-Kamiokande or "Super-K". Only(!) 500 days worth of data was needed to produce this "neutrino image" of the Sun, using Super-K to detect the neutrinos from nuclear fusion in the solar interior. Centered on the Sun's postion, the picture covers a significant fraction of the sky (90x90 degrees in R.A. and Dec.). Brighter colors represent a larger flux of neutrinos.  News: In a tantalizing recent announcement, an international collaboration of Super-K researchers has now presented evidence that the ghostly neutrinos undergo quantum mechanical oscillations, changing their particle identities and quantum properties over time. Theorists have considered neutrinos to be massless particles but these oscillations would imply that they have a very small (but nonzero) mass. Astrophysicists are taking note because even a small mass for ubiquitous, nearly undetectable neutrinos would make them accountable for a substantial fraction of the total mass of our Universe, influencing and perhaps determining its ultimate fate! A measurable mass for neutrinos would also make them candidates for the mysterious dark matter known to affect the motions of stars and galaxies, while proof of neutrino oscillations would be a step toward resolving the decades old Solar Neutrino Problem. Even skeptical scientists will be waiting impatiently to see if these results are independently confirmed.
M100: A Grand Design (1998-06-06)

Majestic on a truly cosmic scale, M100 is appropriately known as a Grand Design spiral galaxy. A large galaxy of over 100 billion or so stars with well defined spiral arms, it is similar to our own Milky Way. One of the brightest members of the Virgo Cluster of galaxies , M100 (alias NGC 4321) is 56 million light-years distant in the spring constellation of Coma Berenices. This Hubble Space Telescope image of the central region of M100 was made in 1993 with the Wide Field and Planetary Camera 2. It reveals the bright blue star clusters and intricate winding dust lanes which are hallmarks of this class of galaxies. Studies of stars in M100 have recently played an important role in determining the size and age of the Universe.