The final year of the decade was an important year for NASA's Earth Science program, with more than $2 billion worth of Earth observing satellites launched into space.

1999 also kicked off NASA's most comprehensive Earth-monitoring program -- the Earth Observing System.

EOS is a 20-year program that will help scientists understand the process of global climate change and determine whether human activity is setting the stage for environmental disaster. Close to 20 satellites will be launched under EOS over a period of five years.

EOS is a 20-year program that will help scientists understand the process of global climate change and determine whether human activity is setting thestage for environmental disaster.

Terra, perhaps the most important of all environmental space projects launched this year, is the flagship of NASA's Earth Observing System.

The principal Earth science satellite launched in 2000 will be the Aqua satellite, the sibling of Terra.

"Each of these instruments gives us unprecedented, comprehensive long-term monitoring of Earth," said Jim Closs, an EOS support scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland.

The interest in observing Earth from space, a practice called remote sensing, has been a part of NASA since the space program began in the 1950s.

Those early satellites -- including the Tiros 1 satellite, which after its launch in 1960 became the first to provide accurate weather forecasts -- proved that remote sensing was a viable and useful practice.

In a way, we are only now seeing the culmination of those efforts with the launch of NASA's Earth Observing System flagship -- the $1.3 billion Terra satellite, perhaps the most important of all environmental space projects launched this year.

The Terra satellite

"EOS in general, and I suppose Terra in particular, will be the Earth science equivalent of the Hubble Space Telescope," said Michael D. King, senior project scientist EOS. "They will allow us to ask questions and see things in the global environment that will separate the causes and effects."

Observations from Terra will help scientists understand both the interactions between Earth's land, water, air and atmosphere -- as well as the effects of those interactions on climate. Terra was launched on December 18.

Terra's five remote sensors will image as much of Earth as the satellite travels in a polar orbit around the planet.

Despite an internal mathematical glitch that caused the craft to become temporarily non-operational after launch, Terra is expected to collect its first images in late January.

Terra, and the Earth Observing System overall, will also benefit from the operations of Acrimsat, launched on December 20. Acrimsat measures the total energy of the light that comes from the sun -- believed by scientists to be an important factor in global climate change.

The field of environmental monitoring took another important step forward this year with the launch of the satellite Landsat 7, the latest in a series of satellites to image the Earth's land and coastal areas from space.

Its ability to snap detailed pictures of large regions, sense heat and view light in the part of the spectrum invisible to the human eye makes Landsat 7 a boon to both business and the environment.

A view of California as seen by Landsat

Available free over the internet, Landsat data can be used to sense changes in population density, water quality in lakes, deforestation, snowpack and even the flow of lava from volcanoes.

Each day, Landsat 7 captures enough data to fill the hard drives of 250 home computers. The satellite, launched on April 15, was built at a cost of $666 million.

Another environmentally relevant satellite, the Quick Scatterometer, or QuikScat satellite, was lofted into orbit June 16 on a mission to measure which way, and how hard, winds blow on Earth.

In addition to helping meteorologists and oceanographers predict weather patterns and storms, QuikScat can identify subtle changes in the global climate -- a key factor in deciding whether a reduction in atmospheric ozone is warming Earth.

Winds play a key role in distributing the heat absorbed by Earth. Heat travels by through ocean movements, which are influenced by ocean winds.

QuikScat, a $71 million satellite, works by sending pulses of light to the ocean surface and measuring the echo. Its instrument calculates the speed and direction of winds by measuring wind ripples near the ocean surface.

The satellite is unique in that it can continuously observe the winds in any weather condition.

Probably the most important Earth science satellite launched in 2000 will be the Aqua satellite, the sibling of Terra.

An artist's impression of Aqua in space

Scheduled for a December launch, Terra will undertake a six-year mission to study atmospheric humidity and temperature, cloud properties and radiation flow. Aqua has instruments to observe land, oceans and the atmosphere.