At first glance this week’s image seems to show a red plain
which would not look out of place on Mars. However, we can tell that this is a
view of Earth The large white structure is clearly artificial, and the wiggling
line connecting it to the edge of the image has the characteristic shape of a
road. The red surroundings are not what they first appear, as this
is actually a false colour view image from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER)
instrument aboard the Terra Spacecraft in orbit around Earth. This instrument
captures images of the Earth’s surface, but unlike a conventional camera it
doesn’t just rely on the visible wavelengths which we are used to seeing. ASTER’s
camera also captures information in the infrared region of the spectrum, which can
tell us a lot about the temperature of the areas it surveys.
False colour images like this one are very common in remote
sensing. By comparing how the surface looks in different spectral bands we can
learn a lot about its properties which would literally be invisible in a visual
image. Different parts of the spectrum can be used to categorise land use, determine
the health of the environment, and assess the impact of human activity.
The man made features such as roads and buildings can give us
a sense of scale for this image, which according to the NASA’s photojournal
“was acquired April 14, 2013, covers an area of 6.2 by 8.2 km, and is located
at 25.7 degrees north, 106.9 degrees east”. This puts it in the Guizhou
Province of China. The object in the centre of the image is the dish of the Five-hundred-meter Aperture Spherical radio
Telescope (FAST). This
observatory has only recently been completed and is now one of the largest
radio telescopes in the world. Google Earth gives us a closer look at the structure under construction, and shows the scale of what has been built here.
Radio telescopes are
one of our best tools for studying the universe beyond our solar system. They
consist of massive receivers, which pick up radio signals from distant stars
and galaxies. This means that they are also using electromagnetic radiation to acquire
an image, but are using a section of the spectrum much further from the visible
than even an infrared camera. The information which they can acquire is also
very different. Radio astronomy is a good compliment to that achieved using
visible light, in the same way that using different spectral bands can
compliment visible wavelength images of the Earth.
Radio telescopes
take the form of large antennae, which receive and interpret the signals
produced by distant stars and galaxies. However these signals tend to be very
weak, as they have travelled across vast distances to reach the earth. This means
that ever larger telescopes are constantly being built to pick up ever fainter
signals from the distant reaches of the universe.
The FAST telescope takes the form of a vast,
circular dish, built into a natural depression in the Chinese countryside. This
google search contains numerous images of the array under various stages of
construction, as well as highlighting the real colour of the terrain around it. The FAST array will make
detailed observations of Pulsars, as well as surveying the interstellar
molecules to tell us more about the distribution of elements across the universe.
Image credits:
- NASA/METI/AIST/Japan Space Systems, and U.S./Japan ASTER Science Team https://photojournal.jpl.nasa.gov/catalog/PIA20986
- Google Earth
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