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Adventures in space astronomy.

Astronomers are continually confronted with the delicious dilemma of "so many objects, so little time" to study them. While there are ground-based telescopes available, many objects (such as planets) cry out for close-up explorations. Others radiate at wavelengths invisible to the human eye and often require specialized sensors operating beyond Earth's atmospheric blanket.

There are a number of space-astronomy missions scheduled for 1998 and 1999. While all are impressive, the highest-profile missions include a small armada of Mars-bound spacecraft (joining the missions already at the red planet) and a set of new Earth-orbiting satellites that will gather X-ray emissions from such dim, distant objects as newborn galaxies.

Mars

It looks as if 1999 will be the year of Mars. Planetary scientists should have six spacecraft operating on or around the red planet by early in 2000--if all the planned launches are successful.

That's a big "if." Mars has been a target of exploration from the earliest days of the Space Race, but many missions have ended in disappointment. In 1960, only three years after the first Sputnik satellite launch, the Soviet Union attempted to send the first two probes to Mars. Both failed shortly after liftoff. Success didn't come until 1965, when NASA's Mariner 4 reached the planet and sent back a few pictures of its barren, cratered surface. This first wave of Mars explorations culminated in the 1976 landing of two Viking probes. These and their orbiting "mother ships" operated successfully into the early 1980s, providing lots of pictures and information about Martian geology and weather.

The next visitors to Mars--two Soviet Phobos missions--arrived in 1988 and 1989, but both failed. Four years later, NASA's ambitious Mars Observer fell silent only days before it reached Mars. An investigation found problems with its onboard engine, which probably exploded when it was switched on.

Mars exploration lagged until 1997, when Pathfinder and its tiny rover, Sojourner, captured our attention with spectacular images from the Martian surface.

Following on Pathfinder's heels was Mars Global Surveyor, which is dipping into the thin Martian atmosphere. It's cruising at nearly 4.6 kilometers (2.8 miles) per second through the thin carbon dioxide. On each orbit, its friction with the atmosphere bleeds off a little bit of speed so that MGS doesn't fly quite as high on following orbits. This maneuver is called "aerobraking" and gets MGS to its final polar orbit using almost no fuel. Global Surveyor will reach that orbit in March 1999, sailing about 378 km (235 miles) above the planet.

NASA's ongoing and forthcoming missions to the red planet are part of the agency's Mars Surveyor program. Instead of building a complex and expensive probe like Mars Observer, the agency is planning a series of simple and inexpensive spacecraft. Each mission is set to carry a small number of instruments aimed at performing a variety of very specific tasks such as surface mapping, atmospheric sampling, or soil testing. Chief among them is the Mars Orbiter Camera, which is sending back the best pictures of the surface ever taken from orbit. Global Surveyor's other instruments include an altimeter that measures the heights of Martian mountains and the depths of valleys. A magnetometer measures the strength of the planet's magnetic field, which is very weak compared to Earth's. Another device looks at infrared radiation from the atmosphere, searching for the spectral fingerprints of water and other chemicals.

July 3, 1998, heralded the launch of Japan's Mars mission, called Nozomi (Planet-B). It joins Global Surveyor in early 1999 and focuses on the interaction of the Martian atmosphere with the solar wind (the steady stream of charged particles that flows from the Sun). Mars doesn't have the same strong radiation belts and auroras that we see on Earth, Jupiter, and Saturn, so the planet is more directly exposed to the harsh interplanetary environment.

Nozomi's scientific instruments have been provided by different countries. A plasma analyzer from Canada will study the temperature and density of hot gas in the solar wind as it interacts with the outer atmosphere, while mass spectrometers from NASA and Sweden will determine the chemical composition of the gas. A German device will sample dust and micrometeoroids. Finally, two Japanese cameras will add another pair of eyes to those staring at Mars.

More Mars

In September 1999 the two-part Mars Surveyor '98 mission begins. The first part, called Mars Climate Orbiter (MCO), joins Global Surveyor at the red planet after a 10-month voyage. Like Global Surveyor it will use aerobraking to get into the right orbit. By December the orbiter will be recording images of Martian clouds using a wide-angle color camera. It also will map the surface at high resolution and study the distribution of water vapor and ozone. MCO also will study the climatic effects of solar heating, which is only half as intense as on Earth because Mars is farther from the Sun.

Researchers also want to understand how dust storms and weather systems move material from one latitude to another. Because there are no oceans on Mars, large-scale weather patterns are very different from those on Earth. MCO should help us learn not only what the Martian climate is like today but also what it was like in the past. Although the orbiter can't search directly for evidence of Martian life, it will help determine whether conditions on Mars were previously more favorable for life.

The other half of Mars Surveyor '98 is the Mars Polar Lander, due to arrive at the red planet just as MCO is settling into its orbital routine. A few hundred kilometers above the surface, the lander will separate from its cruise stage and descend toward the Martian south polar cap. After two scorching minutes plummeting through the atmosphere, the craft will jettison its heat shield and deploy a large parachute. As the lander floats downward, a camera will switch on to provide the first-ever pictures of a Mars landing. Finally, the parachute will be cut loose and powerful rocket engines will fire to bring the craft in for a landing.

The planned landing site lies just outside the south polar cap, in a hilly, cratered area between two high plains called Planum Chromium and Planum Australe. The craft doesn't have airbags like those used on Pathfinder, so it could be in trouble if the landing site is rockier than expected.

On the surface a second camera, similar to the one used on Pathfinder, will provide panoramic views of the area. A Russian-built instrument will beam laser light up into the atmosphere to probe clouds and dust. A third camera will help steer a robot arm to scoop up soil in a search for ice.

It will be spring at the south pole, the land of the Martian midnight Sun. If we're lucky, Polar Lander will survive for many months, but as winter sets in and night begins to fall, the spacecraft's batteries will freeze, ending the mission.

During its descent and landing, a few minutes before the cruise stage burns up in the Martian atmosphere, Polar Lander will eject two Deep Space 2 penetrator rockets. Each will smash into the ground near the polar cap at 760 km (465 miles) per hour and break into two pieces. One half will stay on the surface to act as a radio relay while the other half burrows underground to probe the frozen soil. The first penetrators scheduled to be used in space are aboard the Japanese Lunar A probe, which will study our Moon's surface early in 1999.

If all goes well, as 1999 comes to a close three orbiting monitors and a south polar station will have inaugurated a research program that will carry on until the next wave of Mars explorers sets out in 2001.

From the Earth to the Universe

In Earth orbit this year, there's plenty of action planned. Early in 1998, the Near Earth Asteroid Rendezvous spacecraft flew past Earth, giving observers in the United States the first-ever chance for a naked-eye glimpse of a passing interplanetary spacecraft. The probe is headed toward a January 10, 1999, encounter with the near-Earth asteroid 433 Eros, which orbits the Sun at an average distance of 218 million km (136 million miles)--the Earth orbits the Sun at about 150 million kilometers (93 million miles). The spacecraft, which encountered the asteroid Mathilde in 1997, will study the surface properties, composition, and structure of Eros in an effort to help scientists understand more about asteroid formation.

The Hubble Space Telescope continues to peer deep into the universe, returning images of everything from planetary storms and moons to starbirth, stardeath, early galaxies, and black holes. The telescope's visual range extends from the infrared into the ultraviolet, giving scientists multiwavelength eyes on the universe.

An especially prominent theme in astronomy for 1999 is X-ray astronomy. Three X-ray observatories are set for launch. Germany's ABRIXAS (A BRoad-Band Imaging X-ray All-Sky Survey) will carry out a census of cosmic X-ray sources to complement the detailed images and spectra that will come down from its two larger cousins. Europe's X-ray Multi Mirror (XMM) telescope will be launched on a giant Ariane 5 rocket. Its three co-aligned X-ray telescopes are the biggest ever built and will let astronomers study X-ray emissions from distant sources in very fine detail. The most detailed information, though, will come from NASA's Advanced X-ray Astrophysics Facility. AXAF's total collecting area is smaller than XMM's, but the mirrors are the highest quality ever built and will return pictures as sharp as those from a ground-based optical telescope. After AXAF's January 1999 launch, the control center will spend a couple of months checking out the telescope, and regular observing should begin in the spring.

Early on, AXAF will mostly be staring at bright stars to test its focusing. It will also look at the Cassiopeia A supernova remnant and the Coma Cluster of galaxies, both of which have been well studied by other satellites. Two more months of scientific observations will be made by scientists directly involved with the AXAF program before the telescope is available to astronomers worldwide. The longest single observation in the early months will be a study of the so-called Lockman Hole. This region of space is relatively free of X-ray-absorbing dust and gas, which will allow AXAF to search for the faintest, most distant galaxies. AXAF will also take a look at the same patches of sky used for the Hubble Deep Field images.

Closer to home the Virgo Cluster, the nearby galaxies M51 and M33, and the Andromeda Galaxy are all targets for AXAF's X-ray eyes. Clusters of galaxies and supernova remnants fill out much of the rest of the early program. AXAF's data will be radioed down to the control center in Cambridge, Massachusetts, and sent out to the observers. After a year the data will be made available on the Internet.

Finally, 1999 ends with a third servicing mission to visit the Hubble Space Telescope. Astronauts will swap out instruments and refurbish any parts that may need it. With HST and its sister satellites in orbit or near launch, the next few years will be an exciting time to be doing space astronomy.
Astronomy and Planetary-Science Highlights in 1999

Date        Location               Event

January     Cape Canaveral, FL     Launch of AXAF mission

January     Cape Canaveral, FL     Launch of Mars Polar Lander

January     433 Eros               Near Earth Asteroid Rendezvous at
                                   Eros

January     Vandenberg AFB, CA     Launch of Submillimeter Wave
                                   Astronomy Satellite

February    Lunar orbit            Lunar A fires penetrators at Moon's
                                   surface

February    Cape Canaveral, FL     Stardust launched toward Comet
                                   Wild 2

February    Kapustin Yar, Russia   Launch of ABRIXAS X-ray satellite

March       Cape Canaveral, FL     AXAF X-ray telescope begins regular
                                   observing

June        Venus                  Cassini makes second Venus flyby

August      Earth                  Cassini makes Earth flyby

August      Kourou, Guyana         Launch of X-ray Multi Mirror
                                   telescope

September   Mars orbit             Mars Climate Orbiter enters orbit

October     Mars orbit             Nozomi reaches Mars

October     Io                     Galileo makes Io flyby

October     Wallops Island, VA     Launch of High Energy Transient
                                   Experiment 2

December    Earth orbit            Shuttle Columbia services Hubble
                                   Space Telescope

December    Martian south pole     Mars Polar Lander touchdown

Note: If 40 years of space flight have taught us anything, it's that
space flight is unpredictable. Thus the dates in this table are
tentative. More information about almost all the listed missions
is available on the World Wide Web. Links to the appropriate sites
are available on SKY Online at http://www.skypub.com/.


Jonathan McDowell is an astronomer at the Harvard-Smithsonian Center for Astrophyics and writes a weekly electronic newsletter about the space program (http://hea-www. harvard.edu/QEDT/jcm/space/jsr/jsr.html).
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Author:McDowell, Jonathan
Publication:SkyWatch
Date:Jan 1, 1999
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