NASA Astronomy Picture of the Day 1997-4

Hale-Bopp and Andromeda Credit and Copyright:

Which is closer: the comet or the galaxy? Answer: the comet. In its trek through the inner Solar System, Comet Hale-Bopp has passed nearly in front of the Andromeda Galaxy (M31), seen on the lower left. At the time of this picture, March 27th, Comet Hale-Bopp was about 10 light-minutes from the Earth, while M31 remained about 3 million light-years distant. By contrast, light can cross the Earth in about 1/20th of a second, and light takes about one second to reach Earth's Moon. Comet Hale-Bopp is one of the largest comets ever recorded, and although its' nucleus has never been photographed, it is estimated from brightness and spin measurements to be about 40 kilometers across. In contrast, Comet Halley in 1987 was measured to be 15 km, and Comet Hyakutake in 1996 was estimated to be no more than 10 km.

A Complete Aurora Credit:

Aurora frequently make complete rings around a pole of the Earth. This particular "crown", visible in orange near the top of this image, was taken by the orbiting Polar spacecraft about one year ago and released by NASA last month. A complete auroral oval is normally hard to photograph because part of it usually occurs over a brightly sunlit portion of the Earth. Polar's Earth Camera, however, can be programmed to filter out all but a certain type of ultraviolet light. In this "color", atmospheric oxygen can glow brighter than reflected sunlight. People with normal eyesight living near the Great Lakes in North America would have been able to see this aurora, were it not daytime!

Earth, Clouds, Sky, Comet Credit and Copyright:

Does a comet's dust tail always orbit behind it? Since comets rotate, they shed gas and dust in all directions equally. Small ice and dust particles expelled by the comet, however, are literally pushed around by sunlight. The smaller the particle, the greater the effect. When the comet is headed inward, sunlight slows down small particles so they orbit behind the comet. When the comet is headed back out though, sunlight speeds them up, so small particles orbit in front of the comet. Comet Hale-Bopp itself is too big to have its orbit affected by the momentum of sunlight. Therefore, since Comet Hale-Bopp started back out to the outer Solar System two days ago, we can expect the dramatic dust tail shown above to shift in front in the coming days.

Hale-Bopp In Stereo Credit and Copyright:

This stereo pair of Hale-Bopp images combines two pictures from slightly different viewing angles. Simulating stereo vision, the difference was generated by the comet's apparent motion as it cruised through the inner Solar System. The camera was located in the Shoshone National Forest, Wyoming, USA, Planet Earth, and the two pictures were taken about 45 minutes apart on March 25. Digitized versions were then carefully cropped and adjusted so that the background stars matched. The product of an internet collaboration between J. Modjallal and M. Frost, this pair is meant to be viewed from a comfortable distance by gently crossing your eyes until the images merge.

A Black Hole in M87? Credit:


Mercury Astronauts and a Redstone Credit:


GRB970228: What's There? Credit:

Could this fuzzy blob be the key to the whole gamma-ray burst (GRB) mystery? Astronomers the world over are now scrambling to determine the true nature of the extended emission seen to the lower right of the bright source in the above image. The bright object in the center is rapidly fading - and thought to be the first true optical counterpart to a GRB. But is it housed in a galaxy? If so, after the central emission has faded, this galaxy should be identifiable. Today, follow up observations of this blob are planned with the Hubble Space Telescope. If the extended emission does come from a galaxy it would bolster indications that the February 28th GRB occurred in that galaxy, across the universe from us. This, in turn, would imply that GRBs are truly the most powerful explosions ever known.

Comet Hale-Bopp Over New York City Credit and Copyright:


Oceans Under Jupiter's Europa? Credit:

Is there life beneath Europa? Today, new results are being announced (at about 2 pm EST) about the possibility of oceans under Jupiter's moon Europa. The existence of such oceans increases the likelihood that some sort of life exists beneath the fractured ice planes of Jupiter's smoothest satellite. Results from the February 20th flyby of the robot spacecraft Galileo past Europa now clearly indicate that either large bodies of water or slush exist under a relatively thin ice sheet covering the moon's surface, at least in some regions. Areas on the surface are marked by very few craters, indicating that water flowed there in the time since most craters formed.

Europa's Ice Rafts Credit:

The surface of Jupiter's moon Europa has shifted. Photographs released yesterday of Europa's surface taken by the spacecraft Galileo show that the smooth moon's icy surface is sometimes like a giant scrambled jigsaw puzzle. Pieces of Europa's surface, identifiable above, have rafted to new locations. Also visible are larger areas that are clearly shifted from their original alignment. What could cause this surface shuffle? One leading explanation is water - oceans of water just beneath Europa's ice plains. This discovery again fuels speculation that life might exist far from the comfort of Earth.

The Sun Puffs Credit:


Arecibo: The Largest Telescope Credit:

The Arecibo radio telescope is currently the largest single-dish telescope in the world. First opening in 1963, this 305 meter (1000 foot) radio telescope resides in a natural valley of Puerto Rico. The Arecibo telescope has been used for many astronomical research projects, including searches and studies of pulsars, and mapping atomic and molecular gas in the Galaxy and the universe. As the Arecibo dish can also be used to send radio waves, it has bounced and recorded radiation off of planets in our Solar System, and has even broadcast messages to areas of the Galaxy that might contain intelligent extra-terrestrial life. Any person in the world may use the telescope, providing their proposal is selected by a review committee.

Jets From SS433 Credit:

SS433 is one of the most exotic star systems known. Its unremarkable name stems from its inclusion in a catalog of stars which emit radiation characteristic of atomic hydrogen. Its very remarkable behavior stems from a compact object, a black hole or neutron star, which has produced an accretion disk with jets. As illustrated in this artist's vision of the SS433 system based on observational data, a massive, hot star (left) is locked in a mutual orbit with a compact object. Material transfers from the massive star into an accretion disk surrounding the compact object blasting out two jets of ionized gas in opposite directions - at about 1/4 the speed of light! Radiation from the jet tilted toward the observer is blueshifted, while radiation from the jet tilted away is redshifted. The binary system itself completes an orbit in about 13 days while the jets precess (wobble like a top) with a period of about 164 days. Are the jets from SS433 related to those from black holes at the centers of galaxies?

Hale-Bopp's Hoods Credit:


Hale-Bopp and Plateau de Bure Interferometer Credit and Copyright:

Comet Hale-Bopp is being observed by many different telescopes. Here the comet is pictured behind the array of radio telescopes which compose the Plateau de Bure Interferometer. These telescopes are being used to detect the presence of different molecules in the coma and tail of Comet Hale-Bopp. Molecules detected in the comet include carbon monoxide and sulfur dioxide. The abundance of different types of molecules in Comet Hale-Bopp's coma give clues to its composition and history, as well as clues to the composition and history of our Solar System. Comet Hale-Bopp has now rounded the Sun and is headed back out. It can still be seen by northern observers for several weeks in the northwest sky after sunset.

A Star Cluster Through Hale-Bopp's Tail Credit and Copyright:

Comet Hale-Bopp continues to look impressive. The photograph above captured the comet on April 7th passing nearly in front of M34, a star cluster in the constellation of Perseus. Many of the stars in this open cluster can be seen through Comet Hale-Bopp's white dust tail. The bright blue ion tail now shows several streams. Now receding from both the Sun and the Earth, Comet Hale-Bopp should still remain an impressive sight for weeks to come as it slowly fades.

Pwyll: Icy Crater of Europa Credit:

The impact crater Pwyll (a name from Celtic Mythology) is thought to represent one of the youngest features on the surface of Jupiter's moon Europa. A combination of color and high resolution black and white data from NASA's Galileo spacecraft was used to produce this view looking down on the crater with the sun illuminating the scene from the right. Pwyll's visible dark central region is about 24 miles in diameter, while brilliant white rays of debris blasted from the impact site extend outward for hundreds of miles. The white debris or ejecta clearly overlays everything else on the surface - indicating that this formation is younger than all surrounding features. Interestingly, the bright white color suggests a composition of fresh water ice particles. Does water in liquid form exist below Europa's icy surface?

Solar Storm Causes X-Ray Aurora Credit:


Spiral Galaxy M83 Credit and Copyright:


Moon Robot: Lunokhod 1 Credit:


Big Sky Comet Credit and Copyright:


Historic Optical Flash Fades Credit and Copyright:


Antlia: A New Galactic Neighbor Credit and Copyright:

Meet Antlia, a newly discovered neighbor of our Milky Way Galaxy. Announced just last week, this faint member of our Local Group of Galaxies has only about one million stars. It is late in being noticed because it is so hard to see against a bright sky. This faint galaxy is seen here in the background - the bright stars visible are in our own galaxy. The Antlia Dwarf Galaxy is actually a dwarf spheroidal galaxy, similar to nine known dwarf spheroidals that orbit our Milky Way Galaxy. Antlia, named for the constellation in which it resides, is unusual because it does not hover near either of the two major galaxies of the Local Group: our Milky Way Galaxy and M31.

The Frothy Milky Way Credit:

Astronomers have recently discovered that looking at dust along the plane of our Milky Way Galaxy is a bit like looking into a frothy glass of beer. The dust between stars in our galaxy is arranged like a foam with bubbles and voids -- apparently churned by shocks and winds generated as stars cycle through their lives. This processed infrared image, based on data from NASA's IRAS satellite, maps the radiation from the edges of galactic dust clouds and reveals the complex distribution. The image covers an area of about 40x60 degrees centered on the galactic plane near the Cygnus region. It shows bright bubble-shaped and arc-like dust clouds around the supernova remnants and starbirth regions embedded in the galactic disk.

Hale-Bopp Polarized Credit and Copyright:

Light polarization is familiar to many outdoor enthusiasts who use polarizing sunglasses to cut the glare of reflected light. These two views of comet Hale-Bopp also demonstrate the effect of polarization. At left is an "ordinary intensity image" of Hale-Bopp's coma taken April 14, while on the right a similar image made with polarizing filters represents the intensity of polarized light. The arcs visible in the polarized view probably correspond to concentrations of ejected cometary dust that produce polarization by reflecting sunlight. Any sort of reflection - from clumps of comet dust or the surface of your favorite lake or ski slope - can polarize light by causing the light waves to vibrate in a plane defined by the reflecting surface.

The Perseus Cluster of Galaxies Credit:


Sputnik: Traveling Companion Courtesy:


Io's Sodium Cloud Credit:


Hale Bopp and Orion Credit and Copyright:


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