Earth as You've Never Seen it

Awe-inspiring images of our planet erupting, melting and more

A Collision Created the Andes: The world’s longest mountain range, the Andes, flanks the western coast of South America, which is colliding with the ocean floor. Because the rocks forming the continents are lighter than the oceanic crust, the Andes rise up as the floor of the Pacific Ocean slides beneath South America at a rate of about four inches a year. In this image, the colors reflect altitude. The highest mountains are white, while the black bands along the coast represent the deepening sea. Photo by Various Stallites

For centuries, explorers have risked their lives to reach far-flung corners of the planet. Today, satellites provide incredibly sharp images of nearly every spot on Earth, so anyone can sit back and view hard-to-reach places from the comfort of their own home.

Harvesting data from high-tech sensors on various international satellites, Germany’s space-research agency, DLR, compiled these awe-inspiring and enlightening images of islands, mountains and glaciers across the seven continents. Together they are a record of our planet in action—erupting, melting, colliding, cracking. Here, a look at some of the most impressive satellite images of the past 20 years.

This article was originally published in the inaugural issue of Science Illustrated, a recently launched sister publication of Popular Science. Like what you see? Subscribe today at scienceillustrated.com.

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Why Stars Have Seasons

Have you ever wondered why most star patterns are associated with specific seasons of the year? Just why, for instance, can evening sky watchers in the Northern Hemisphere enjoy Orion the Hunter only during the cold wintry months? During balmy summer evenings it is not Orion, but the stars of Scorpius, the Scorpion, that dominate the southern sky. Spring evenings provide us with a view of the sickle of Leo, the Lion. Yet on fall evenings, it's the Great Square of Pegasus that vies for the stargazer's attention.

The change is subtle. Were we to watch the night sky on any one night from dusk to dawn we would notice certain stars rising from above the eastern horizon in the evening hours. They would sweep across the sky during the night, finally setting beneath the western horizon by dawn. No big deal here, since, after all, the sun does the same thing during the daylight hours. It's caused by Earth's rotation.

But with the passage of time, we would notice something rather puzzling.

Those stars that were low over the western horizon during the early evening hours would, within a matter of a few weeks, disappear entirely from our view, their places being taken up by groups which a few weeks earlier were previously higher up in the sky at sundown. In fact, it would seem that with the passage of time, all the stars gradually shift westward while new stars move up from the eastern horizon to take their places.

But just why is this shift happening?

Four minutes a day

If we were to synchronize our clocks using the motions of the stars as a reference we would discover that the Earth would complete a single turn on its axis not in 24 hours, but actually 23 hours and 56 minutes, or four minutes shy of 24 hours. This would be a day based upon the apparent movement of the stars in our sky, which astronomers call a "sidereal" day from the Latin word for star.

While this is happening, all of us are being carried around the sun on an annual journey almost 600 million miles long. Our orbit is almost a circle and as seen from the sun the Earth would move about one degree each day, since we take about 365 days to go around a circle of 360 degrees. As seen from Earth — from our vantage point — the sun seems to move and it changes its place in the sky by that one-degree per day, as measured against the background of stars.

Of course, we can't see the stars in the daytime but astronomers can measure the position of the sun. The direction of the sun's apparent motion is eastward among the stars. Since the daily turning of the sky (caused by the Earth's rotation) appears to move westward, this slight motion of the sun is what makes a day as measured by the sun (called a solar day) longer: the Earth must turn about one degree (or about 4 minutes) more than a full circle to complete a 24-hour day as measured by the sun.

That slight shift each day is what makes the different stars and constellations appear at different times of the year. The sun slowly changes its position, but so slowly that the stars which are up when the sun is down also change.

If you want to try an experiment, look outside some clear evening from a location you can find again. Notice the exact time that a particular star is directly aligned with some object, like a telephone pole or a roof. Look the very next night; stand in the very same place and the star will be there four minutes before the time it was the previous night (of course your clock must be set accurately each night).

You are observing the effects of the Earth's motion around the sun.

Star time versus sun time

At this point you might be a bit confused. If the Earth takes 23 hours 56 minutes to turn on its axis, why do we say that a day is 24 hours long?

Astronomers have devised special clocks adjusted to keep time solely by the stars. These astronomical clocks keep sidereal time. There is no a.m. or p.m. in a sidereal day.

With the clocks that we use every day, the hour hand goes completely around 12 hours twice a day. But with a sidereal clock, there are 24 hourly numbers on the dial instead of 12 and the hour hand goes around only once in a sidereal day. The hours start at 00 hour (zero hour) and are numbered straight through to 23 hours and then starts at the zero hour again. The other difference is that the sidereal clock runs four minutes fast as compared to a regular clock.

Now, if our daily lives were governed by the sidereal clock, there would be times during the year when the sun would appear highest in the sky at noontime, but at other times of the year it would appear highest at midnight or setting at 6 a.m. (or something else strange). We're accustomed, of course, to being awake when it's light and asleep when it's dark, so astronomers also have developed a "mean" sun — which is fictitious and for most of the year deviates somewhat from the sun's actual position in the sky.

Yet, the mean sun governs our ordinary clocks and results in the 24-hour time scale of which we have become accustomed to all of our lives.

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Spacecraft at Mars Prepare to Welcome New Kid on the Block

Three Mars spacecraft are adjusting their orbits to be over the right place at the right time to listen to NASA's Phoenix Mars Lander as it enters the Martian atmosphere on May 25.

Every landing on Mars is difficult. Having three orbiters track Phoenix as it streaks through Mars' atmosphere will set a new standard for coverage of critical events during a robotic landing. The data stream from Phoenix will be relayed to Earth throughout the spacecraft's entry, descent and landing events. If all goes well, the flow of information will continue for one minute after touchdown.

"We will have diagnostic information from the top of the atmosphere to the ground that will give us insight into the landing sequence," said David Spencer of NASA's Jet Propulsion Laboratory, Pasadena, Calif., deputy project manager for the Phoenix Mars Lander project. This information would be valuable in the event of a problem with the landing and has the potential to benefit the design of future landers.

Bob Mase, mission manager at JPL for NASA's Mars Odyssey orbiter, said, "We have been precisely managing the trajectory to position Odyssey overhead when Phoenix arrives, to ensure we are ready for communications. Without those adjustments, we would be almost exactly on the opposite side of the planet when Phoenix arrives."

NASA's Mars Reconnaissance Orbiter is making adjustments in bigger increments, with one firing of thrusters on Feb. 6 and at least one more planned in April. The European Space Agency's Mars Express orbiter has also maneuvered to be in place to record transmissions from Phoenix during the landing. Even the NASA rovers Spirit and Opportunity have been aiding preparations, simulating transmissions from Phoenix for tests with the orbiters.

Launched on Aug. 4, 2007, Phoenix will land farther north than any previous mission to Mars, at a site expected to have frozen water mixed with soil just below the surface. The lander will use a robotic arm to put samples of soil and ice into laboratory instruments. One goal is to study whether the site has ever had conditions favorable for supporting microbial life.

Phoenix will hit the top of the Martian atmosphere at 5.7 kilometers per second (12,750 miles per hour). In the next seven minutes, it will use heat-shield friction, a parachute, then descent rockets to slow to about 2.4 meters per second (5.4 mph) before landing on three legs.

Odyssey will tilt from its normally downward-looking orientation to turn its ultrahigh-frequency (UHF) antenna toward the descending Phoenix. As Odyssey receives a stream of information from Phoenix, it will immediately relay the stream to Earth with a more capable high-gain antenna. The other two orbiters, Mars Reconnaissance Orbiter and Mars Express, will record transmissions from Phoenix during the descent, as backup to ensure that all data is captured, then transmit the whole files to Earth after the landing. "We will begin recording about 10 minutes before the landing," said JPL's Ben Jai, mission manager for Mars Reconnaissance Orbiter.

The orbiters' advance support for the Phoenix mission also includes examination of potential landing sites, which is continuing. After landing, the support will include relaying communication between Phoenix and Earth during the three months that Phoenix is scheduled to operate on the surface. Additionally, NASA and European Space Agency ground stations are performing measurements to determine the trajectory of Phoenix with high precision.

With about 160 million kilometers (100 million miles) still to fly as of late February, Phoenix continues to carry out testing and other preparations of its instruments. The pressure and temperature sensors of the meteorological station provided by the Canadian Space Agency were calibrated Feb. 27 for the final time before landing. "The spacecraft has been behaving so well that we have been able to focus much of the team's attention on preparations for landing and surface operations," Spencer said.

The Phoenix mission is led by Peter Smith of the University of Arizona, Tucson, with project management at JPL and development partnership at Lockheed Martin, Denver. International contributions are provided by the Canadian Space Agency; the University of Neuchatel, Switzerland; the universities of Copenhagen and Aarhus, Denmark; the Max Planck Institute, Germany; and the Finnish Meteorological Institute. Additional information on Phoenix is online at http://www.nasa.gov/phoenix and http://phoenix.lpl.arizona.edu . JPL, a division of the California Institute of Technology in Pasadena, manages Mars Odyssey and Mars Reconnaissance Orbiter for the NASA Science Mission Directorate, Washington. Additional information on NASA's Mars program is online at http://www.nasa.gov/mars .

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