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Last Updated on Sunday, 02 May 2010 12:10
Published on Friday, 25 February 2005 00:00

U.K. scientists will be watching with keen interest when NASA's giant Terra spacecraft lifts off from Vandenberg Air Force Base in California later this week.

The $1.3 billion TERRA is considered to be the flagship of NASA's long term Earth Observing System programme. With major contributions from the U.S., Japan and Canada, the polar-orbiting spacecraft is designed to spend the next six years studying our planet's land surface, oceans, clouds and atmosphere.


Under the leadership of Dr. Guy Peskett, a team from the Dept. of Atmospheric Physics, Clarendon Laboratory, Oxford University, has helped to build the Measurement Of Pollution In The Troposphere (MOPITT) instrument, one of the scientific experiments on board this major mission.

"This is the first attempt to carry out long-term measurements from space of pollutants in the lower atmosphere, " said Dr. Peskett, a Co-Investigator on MOPITT.

TERRA carries five major scientific instruments. One of these is MOPITT, which is designed to spend the next five years measuring the distributions of carbon monoxide (CO) and methane (CH4) in the troposphere (the region of the atmosphere closest to the Earth's surface which gives us our day-to-day weather).

Concentrations of these gases can be altered by both natural phenomena and human activity, such as cultivation of rice paddies, or burning of the rainforests.

Although they are not major constituents of the atmosphere, they are important players in the chemistry of our planet's atmosphere.

Carbon monoxide is interesting because of its potential for showing atmospheric motions as well as providing information about chemical reactions in the troposphere.

Methane is an important greenhouse gas, which is extremely efficient at trapping heat. Even a small rise in its atmospheric concentration could cause temperatures to rise, causing ice sheets to melt and increases in sea level.

The international MOPITT science team, which includes scientists from Canada, the United States and the United Kingdom, will be studying the global data and using it to help develop mathematical models of the chemical processes taking place in the troposphere.

MOPITT measures the concentrations of CO and CH4 molecules in the troposphere by detecting the infrared light they emit. It does this through a technique called correlation spectroscopy.

Infrared light emitted by the atmosphere enters a telescope and is sent through a cell which is filled with the gas to be measured (either carbon monoxide or methane). So light from atmospheric CO is partially absorbed as it passes through a cell containing CO, while light emitted by other atmospheric gases is not affected.

By varying the amount of CO in the cell (or changing its length), the light emitted by the atmospheric CO can be 'labelled' with an amplitude modulation. It then falls onto a detector which converts the light signal into an electrical signal. The modulated signal of the CO can then be picked out from the out put of the detector by the instrument's signal processing electronics.

In this way, MOPITT acts as an optical filter, selecting only the light emitted by the atmospheric CO. The same method is used to measure the infrared signal from atmospheric methane. Using existing knowledge of the molecules' spectroscopic properties, the distributions of CO and CH4 in the atmosphere can then be computed.

Continuous measurements will be sent back as the TERRA satellite moves along its orbit. Previous measurements of pollutants from space have always looked sideways at the upper atmosphere. MOPITT will look straight down at the planet, continuously scanning a swath about 600km wide.

This has the advantages of increasing the spatial coverage of the instrument and improving the chance of finding gaps in the clouds. Unfortunately, it means that MOPITT also "sees" the surface of the planet and so picks up unwanted background radiation.

"This makes the measurements extremely difficult," said Dr. Peskett.

"However, if all goes well, we hope to be able to see a lot of detail, such as plumes of pollution down wind of cities, as well as producing global maps of carbon monoxide and methane levels every four days," he added.

MOPITT's performance is enhanced by cooling its detectors and filters to 100K (-173C). This temperature is obtained by using Stirling Cycle coolers developed at Oxford University and supplied by British Aerospace (now Matra Marconi Space, Stevenage, U.K.). The use of such mechanical coolers, rather than stored supercooled gas or radiative cooling, allows the instruments to operate over a much longer lifetime.

TERRA (formerly EOS AM-1) is the flagship of the Earth Observing System, a series of NASA spacecraft that are designed to observe the Earth from space.

It carries a payload of five state-of-the-art sensors that will study the interactions among the Earth's atmosphere, lands, oceans, life, and radiant energy (heat and light).

Roughly the size of a small coach, TERRA is currently scheduled to be launched by an Atlas-Centaur rocket from Vandenberg Air Force Base, California on the evening of 16 December 1999 (U.K time). It will be placed into a 705 km, sun-synchronous orbit. This means it always crosses the equator from north to south at 10:30 am local time, when cloud cover is minimal and its view of the surface is least obstructed.

Since the satellite's orbit will be roughly perpendicular to the direction of Earth's spin, swaths observed during each overpass can be compiled into images covering the entire planet. Over time, these global images will enable scientists to investigate the causes and effects of global climate change.

The sensors on Terra will not actively scan the surface (such as with laser beams or radar pulses). Instead, they work much like a camera, detecting sunlight reflected by clouds and the Earth's surface, and the heat which is emitted from the atmosphere and the surface.

This radiant energy is passed through gas correlation cells and is then focused onto cooled detectors. These are sensitive to selected regions of the electromagnetic spectrum, ranging from visible light to heat. The information produced by these detectors will then be transmitted back to Earth and processed by computers into images.

The principal scientific investigator for MOPITT is
Prof. James R. Drummond,
Department of Physics,
University of Toronto,
(E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it. )

Detailed information on Terra is available via the Internet at: The Mopitt home page is at:

Issued by:
Peter Bond, RAS Press Officer (Space Science)
Office & home phone: +44 (0)1483-268672
FAX: +44 (0)1483-274047
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Dr. Guy Peskett,
Dept. of Atmospheric Physics,
Clarendon Laboratory,
Oxford University,
Oxford OX1 3PU.

Tel: +44 (0)1865-272883 (direct) or +44 (0)1865-272901 (messages).
Fax: +44 (0)1865-272923. E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.