The first US spacecraft was Explorer 1. The cylindrical 30 pound satellite was launched (above) as the fourth stage of a Jupiter-C rocket (a modified US Army Redstone ballistic missile) and achieved orbit on January 31, 1958. Explorer I carried instrumentation to measure internal and external temperatures, micrometeorite impacts, and an experiment designed by James A. Van Allen to measure the density of electrons and ions in space. The measurements made by Van Allen's experiment led to an unexpected and startling discovery - an earth-encircling belt of high energy electrons and ions trapped in the magnetosphere - now known as the Van Allen Belt. Explorer I ceased transmitting on February 28 of that year but remained in orbit until March of 1970.

What's happening in the eye-like center of this planetary nebula? The geometry revealed in this Hubble Space Telescope view of the central part of an "etched hourglass nebula" known as MyCn 18 presents a puzzle. First, the axis of this central region does not line up well with the outer hourglass structure. Next, the hot star (the white spot to the left of center) which ejected this gaseous nebula as it evolved towards its white dwarf phase, is not exactly at the center of the "eye". These unexpected results leave astronomers looking for the missing pieces of the ejection process that created this unusual and beautiful structure.

Get ready for one of the most impressive but least anticipated light shows in modern astronomical history. Next month, newly discovered Comet Hyakutake will pass closer to the Earth than any recent comet. Unknown before its discovery by Yuji Hyakutake on 30 January 1996, the fuzzy spot in the above photograph is a comet now predicted to become bright enough to see without a telescope. Although comets act in such diverse ways that predictions are frequently inaccurate, even conservative estimates indicate that this comet is likely to impress. For example, even if Comet Hyakutake remains physically unchanged, its close pass near the Earth in late March 1996 should cause it to appear to brighten to about 3rd magnitude - still bright enough to see with the unaided eye. In the next two months, though, the comet will continue to approach the Sun and hence should become brighter still. Optimistic predictions include that Comet Hyakutake will change physically, develop a larger coma and tail, brighten dramatically, move noticeably in the sky during a single night, and may ultimately become known as the "The Great Comet of 1996." Move over Hale-Bopp!

If you could stand on Mars - what would you see? Viking 1 robot landers answered this question in 1976 with pictures like the one shown above. The dark rocks, red soil, and green-tinged sky grace this rendition of a normal Martian afternoon. At the bottom corners of the picture are portions of Viking spacecraft. The red color of the rocks is caused by an abundance of iron in the soil. The Martian surface is covered by rocks, huge craters, fantastic canyons, and gigantic volcanoes that dwarf any on Earth. No life has been found, but some speculate that since not all spacecraft reaching Mars from Earth had been fully decontaminated, Earth born microbes might live there now.

Above are two microwave images of the sky, looking north and south of our galaxy's equator, based on data from NASA's COBE satellite. After computer processing to remove contributions from nearby objects and the effects of the earth's motion, they show "spots". These spots are the oldest structures known - probably the oldest structures humanity will ever know. They are also the most distant. As our universe expanded and cooled, conglomerations of mass formed - these are some of the first. They confirm that only a million years after the big-bang - which occurred roughly 15 billion years ago - parts of the universe were visibly hotter than other parts. By studying the size and distribution of the spots found with COBE and future missions, astronomers hope to learn what matter and processes caused the spots to form - and hence determine the composition, density, and future of our universe.

Our Earth is not at rest. The Earth moves around the Sun. The Sun orbits the center of the Milky Way Galaxy. The Milky Way Galaxy orbits in the Local Group. The Local Group falls toward the Virgo Cluster of Galaxies. But these speeds are less than the speed that all of these objects together move relative to the microwave background. In the above all-sky map, radiation in the Earth's direction of motion appears blueshifted and hence hotter, while radiation on the opposite side of the sky is redshifted and colder. The map indicates that the Local Group moves at about 600 kilometers per second relative to this primordial radiation. This high speed was initially unexpected and its magnitude is still unexplained. Why are we moving so fast? What is out there?

What's the closest galaxy to our Milky Way? For many years astronomers thought it was the Large Magellanic Cloud (LMC). But the seemingly insignificant fuzzy patch shown above turned out to be part of a galaxy that is even closer. Deemed the "Sagittarius Dwarf", this small galaxy went unnoticed until its discovery in 1994 by R. Ibata, G. Gilmore and M. Irwin (RGO). The reason the Sagittarius Dwarf hadn't been discovered earlier is because it is so dim, it is so spread out over the sky, and there are so many Milky Way stars in front of it. The distance to the Sagittarius Dwarf was recently measured to be about one third of the distance to the LMC. Astronomers now believe that this galaxy is slowly being torn apart by the vast gravitational forces of our Galaxy.

What hit Mars? The impact crater Schiparelli near the center of the above image was likely caused by a collision with an object the size of an asteroid. Also evident in this full face mosaic of Mars are numerous craters from many other impacts with smaller objects over billions of years. At the lower right, white carbon dioxide frost can be seen in the Hellas basin. The frost forms because temperatures can drop as low as -140 degrees Celsius on Mars. Some Martian regions, however, occasionally reach as high as 20 degrees Celsius - a typical room temperature here on Earth.

What is our Galaxy made of? Stellar motions indicate there is much more mass than just stars and gas. Photographs like the two shown above may be yielding a clue about the dark matter, however. Pictured is the first recorded instance of a dim star in our Galaxy moving in front of a bright background star, shown by the arrow, deflecting light around it, and causing the background star to appear much brighter (right frame). Were our Galaxy made predominantly of MAssive Compact Halo Objects (MACHOs), many similar such gravitational lensing events would be expected when photographing the Large Magellanic Clouds (LMC) - hence indicating the presence of MACHO lenses in our Galaxy. A research team led by Charles Alcock this month claimed enough LMC gravitational lensing events to indicate at least half of the dark matter in our Galaxy is composed of MACHOs. This spectacular claim may well be correct - but awaits crucial testing with future observations and modeling.

What is the shape and composition of our Milky Way Galaxy? This question would be easier to answer if there wasn't so much obscuring dust! In the 1940s, however, astronomer Walter Baade identified a "window" near the center of our Galaxy where there is comparatively little opaque dust. Now called "Baade's Window", this sky region contains millions of stars and is used for many studies of the distant Milky Way. One clever use, devised by Bohdan Paczynski, is to monitor millions of stars in our Galactic Bulge - many through Baade's window - for sudden brightening due to gravitational lensing. Current observations by the OGLE and MACHO collaborations have now identified dozens of gravitational amplification events. This unexpectedly large number supports previous claims that our Galaxy has a "bar" of stars across the central nucleus, pointed nearly at the Sun.