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
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.
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.
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
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.
image, taken with Hubble’s Advanced Camera for Surveys, reveals lunar
features as small as roughly 560 feet (170 m) 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
km) wide and is circled by a rim of material rising almost 3 miles (5
km) above the crater floor. The image measures 430 miles (700 km)
across, which is slightly larger than New Mexico.
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.
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
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.
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.