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Hubble to Use Moon as Mirror to See Venus Transit 05.04.12
 This mottled landscape showing the
impact crater Tycho is among the most violent-looking places on our moon. Astronomers
didn't aim NASA's Hubble Space Telescope to study Tycho, however. The image was
taken in preparation to observe the transit of Venus across the sun's face on June
5-6. Hubble cannot look at the sun directly, so astronomers are planning
to point the telescope at the Earth's moon, using it as a mirror to capture reflected
sunlight and isolate the small fraction of the light that passes through Venus's
atmosphere. Imprinted on that small amount of light are the fingerprints of the
planet’s atmospheric makeup. These observations will mimic a technique that
is already being used to sample the atmospheres of giant planets outside our solar
system passing in front of their stars. In the case of the Venus transit observations,
astronomers already know the chemical makeup of Venus's atmosphere, and that it
does not show signs of life on the planet. But the Venus transit will be used to
test whether this technique will have a chance of detecting the very faint fingerprints
of an Earth-like planet, even one that might be habitable for life, outside our
solar system that similarly transits its own star. , Venus is an excellent proxy
because it is similar in size and mass to our planet. The astronomers will
use an arsenal of Hubble instruments, the Advanced Camera for Surveys, Wide Field
Camera 3, and Space Telescope Imaging Spectrograph, to view the transit in a range
of wavelengths, from ultraviolet to near-infrared light. During the transit, Hubble
will snap images and perform spectroscopy, dividing the sunlight into its constituent
colors, which could yield information about the makeup of Venus's atmosphere.
Hubble will observe the moon for seven hours, before, during, and after the
transit so the astronomers can compare the data. Astronomers need the long observation
because they are looking for extremely faint spectral signatures. Only 1/100,000th
of the sunlight will filter through Venus's atmosphere and be reflected off the
moon. This image, taken with Hubble's Advanced Camera for Surveys, reveals
lunar features as small as roughly 560 feet (170 meters) across. The large "bulls-eye"
near the top of the picture is the impact crater, caused by an asteroid strike about
100 million years ago. The bright trails radiating from the crater were formed by
material ejected from the impact area during the asteroid collision. Tycho is about
50 miles (80 kilometers) wide and is circled by a rim of material rising almost
3 miles (5 kilometers) above the crater floor. The image measures 430 miles (700
kilometers) across, which is slightly larger than New Mexico. Because the
astronomers only have one shot at observing the transit, they had to carefully plan
how the study would be carried out. Part of their planning included the test observations
of the moon, made on Jan. 11, 2012, as shown in the release image. Hubble
will need to be locked onto the same location on the moon for more than seven hours,
the transit's duration. For roughly 40 minutes of each 96-minute orbit of Hubble
around the Earth, the Earth occults Hubble's view of the moon. So, during the test
observations, the astronomers wanted to make sure they could point Hubble to precisely
the same target area. This is the last time this century sky watchers can
view Venus passing in front of the sun. The next transit won't happen until 2117.
Venus transits occur in pairs, separated by eight years. The last event was witnessed
in 2004. The Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency. NASA's Goddard Space Flight Center in
Greenbelt, Md., manages the telescope. The Space Telescope Science Institute (STScI)
in Baltimore, Md., conducts Hubble science operations. STScI is operated by the
Association of Universities for Research in Astronomy, Inc., in Washington, D.C.
For images and more information about Hubble’s view of the Moon and the Venus
transit, visit: https://hubblesite.org/news/2012/22
For more information about the Hubble Space Telescope, visit:
https://www.nasa.gov/hubble
Cheryl Gundy, STScI
Posted May 14, 2012
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