Like a miner prospecting for gold, NASA hopes its latest robot to Mars hits paydirt when it lands Sunday near the red planet's north pole to conduct a 90 day digging mission. The three-legged Phoenix Mars lander fitted with a backhoe arm is zeroing in on the unexplored artic region where a reservoir of ice is believed to lie beneath the Martian surface. Phoenix lacks tools to detect signs of alien life, either now or in the past. However, it will study whether the ice ever melted and look for traces of organic compounds in the permafrost to determine if life could have emerged at the site.

Before this robotic geologist can excavate the soil, it must first survive a nail-biting plunge through the martian atmosphere. Despite the rousing success of NASAs twin Mars rovers, which landed in 2004, more than half of the world's attempts to land on the planet have failed. It's kind of like first-day jitters, there's a lot of excitement, but there's also some nervousness. Launched last summer from Cape Canaveral, Fla., Phoenix has traveled 422 million miles for Sunday 25th's touchdown.

The spacecraft's main tool is an 8 foot aluminum and titanium robotic arm capable of digging trenches 2 feet deep. Once ice is exposed, believed to be anywhere from a few inches to a foot deep, the lander will use a powered drill bit at the end of the arm to break it up. It'll be a construction zone.. They predict the ice will be as hard as a sidewalk. The excavated soil and ice bits will then be brought aboard Phoenix's science lab. It will be baked in miniature ovens and the vapors analyzed for organic compounds, the chemical building blocks of life.

The last time NASA did tests for organics it was on a hunt for extraterrestrial life in 1976 with the twin Viking spacecraft. No conclusive signs of life were found. On this new mission, Phoenix will also probe whether the underground ice ever melted during a time when Mars was warmer and wetter.

On Sunday, Phoenix will punch through the Martian atmosphere at more than 12,000 mph. Over the next seven minutes, it will use the atmosphere's friction and a parachute to slow to 5 mph. Seconds before touchdown, Phoenix will fire its thrusters for what scientists hope will be a soft landing. If all goes well, ground controllers expect to hear a signal at 7:53 pm. EDT.

If successful, Phoenix would join the twin rovers Spirit and opportunity on the Martian surface. Together, the rovers have traveled more than 10 miles in their four years exploring opposite sides of the equator. They have uncovered geologic evidence that water once flowed at or near the surface of ancient Mars..

Unlike the six wheeled rovers, Phoenix will stay in one spot. The cost of the mission is $420 million. Phoenix will communicate with Earth through the two NASA orbiters circling the planet.

OBSERVATION HISTORY. The earliest recorded supernova, SN 185, was viewed by Chinese astronomers in 185 CE. The widely observed supernova SN 1054 produced the Crab Nebula.

Supernova SN 1572 and SN 1604, the last to be observed in the Milky Way galaxy, had notable effects on the development of astronomy in Europe because they were used to argue against the Aristotelian idea that the world beyond the moon and planets was immutable.Since the development of the telescope, the field of supernova discovery has enlarged to other galaxies, starting with the 1885 observation of supernova S Andromedae in the Andromeda galaxy. Supernovae provide important information on cosmological distances. During the twentieth century, successful models for each type of supernova were developed, and scientists comprehension of the role of supernovae in the star formation process is growing.

Some of the most distant supernovae recently observed appeared dimmer than expected. This has provided evidence that the expansion of the universe may be accelerating.

DISCOVERY

Because supernovae are relatively rare events, occurring about once every 50 years in a galaxy like the Milky Way, many galaxies must be monitored regularly in order to obtain a good sample of supernovae to study.

Supernovae in other galaxies cannot be predicted with any meaningful accuracy. When they are discovered, they are already in progress. Most scientific interest in supernovae, as standard candles for measuring distance, for example, require an observation of their peak luminosity. It is therefore important to discover them well before they reach their maximum. Amateur astronomers, who greatly outnumber professional astronomers, have played an important role in finding supernovae, typically by looking at some of the closer galaxies through an optical telescope and comparing them to earlier photographs.

Towards the end of the 20th century, astronomers increasingly turned to computer controled telescopes and CCDs for hunting supernovae. While such systems are popular with amateurs, there are also larger installations like the Katzman Automatic Imaging Telescope. Recently, the Supernova Early Warning System (SNEWS) project has also begun using a network of neutrino detectors to give early warning of a supernova in the Milky Way galaxy. A neutrino is a particle that is produced in great quantities by a supernova explosion, and it is not obscured by the interstellar gas and dust of the galactic disk.

Supernova searches fall into two classes, those focused on relatively nearby events and those looking for explosions farther away. Because of the expansion of the universe, the distance to a remote object with a known emission spectrum can be estimated by measuring its Doppler shift (or redshift), on average, more distant objects recede with greater velocity than those nearby, and so have a higher redshift. Thus the search is split between high redshift and low redshift, with the boundary falling around a redshift range of z = 0.1 - 0.3, where z is a dimensionless measure of the spectrum's frequency shift.

High redshift searches for supernovae usually involve the observation of supernova light curves. These are useful for standard or calibrated candles to generate Hubble diagrams and make cosmological predictions. At low redshift, supernova spectroscopy is more practical than at high redshift, and this is used to study the physics and environments of supernovae. Low redshift observations also anchor the low distance end of the Hubble curve, which is a plot of distance versus redshift for visible galaxies.