The planets
spectrum and the use of an imaging blue filter, Wratten 47 (violet) or a UV
pass filter will reveal progressively more detail, assuming your imaging
system is capable of showing it.
One problem when working with a UV filter is that the image is often
incredibly dim, making it hard to achieve a decent focus. Another issue is
that modern optical coatings can interfere with short wavelengths, reducing
the amount of detail transmitted. This also applies to Barlow lenses used to
increase image scale and has led a number of amateurs to switch to using
fused silica Barlows when imaging Venus to limit UV loss.
Neither Mercury nor Venus has satellites. Both are solitary wanderers
within space.
Next in line our own Earth is the only planet suited for the development of
intelligent life. (Whether it actually appeared here is open to debate!) Beyond
comes Mars, the planet we have observed and explored in most detail
through the eyes of NASA’s surface rovers.Mars
Mars’s year is 687 Earth days, and it has a rotation period of 37 minutes
longer than our own. To us Mars has always held special interest, being the
only world where our type of life could exist; a few tens of years ago it was
believed Mars was the centre for established life, with canals visible through
our largest telescopes. The canals do not exist; they were tricks of the eye,
and Mars today is a world where we could not survive in the open.
The planet is a rocky desert and though it must once have had wide seas,
the water has dried up and today the surface of Mars is completely arid.
Small telescopes will show the main surface details, the red deserts, the
dark areas and the ice caps at the poles that wax and wane with the Martian
seasons. There is no doubt the caps are made of water ice, but when they
shrink in the Martian summer they do not melt but sublime – change –
directly from a solid into the gaseous state. Rainfall has been unknown
for millions of years. The atmosphere appears to consist mainly of carbon
dioxide with not much free oxygen.
Many spacecraft have now been sent to Mars. Some have been put into
orbit and mapped the entire surface. Others have crash-landed there, and
two, the US Spirit and Opportunity, were brought gently down and roved
around the surface sending back data. At the time of writing Opportunity is
still moving around and sending back all kinds of fascinating information. The
second probe, Spirit, is stuck in a drift and is unlikely to move again. But it did
its job well. The latest probe, Curiosity, was launched in 2011 and is the most
complicated vehicle ever to be sent to Mars.
Winds on Mars are quite strong, though in that thin atmosphere they have
very little actual force. It was once thought that the atmospheric pressure
at ground level was 85 millibars – the actual ground pressure is below 10
millibars everywhere, corresponding to what would be a laboratory vacuum
on Earth.
There are craters due to impact, not unlike those of the Moon. There are
also high mountains and huge volcanoes which must once have been very
active but are now, we believe, extinct. The highest mountain, Olympus
Mons, towers to a height of 25 thousand feet above the ground below, and
is crowned by a huge summit caldera. The orbiting probes have sent back
pictures which are very impressive. No doubt it will be climbed one day – not
too much of a problem because the slope to the summit is very gentle. A
spacesuit would not be an insuperable problem.
When will men reach Mars? One problem is radiation. In the journey
there, which will take several months, the travellers will be in their craft,today. Venus changed from a welcoming world into an immensely hostile
one. Unmanned probes have been there and even landed on the surface,
but whether manned flight will ever follow must be regarded as decidedly
dubious. Certainly this will not happen for a very long time.
To the naked eye Venus appears lovely, shining like an intensely bright
lamp in the sky, so it is no wonder the ancients named it after the goddess of
beauty. They had no way of knowing how hostile it was.
Imaging Venus
Venus, like Mercury, is an inferior planet, meaning that its orbit is smaller
than that of the Earth. However, Venus’s orbit is such that the planet can
achieve a reasonable separation from the Sun when it’s at elongation. Some
elongations, it has to be said, are better placed than others.
Unlike Mercury, Venus always looks bright, due to its highly reflective
cloud layer. Photographing Venus as a "dot", as described for Mercury,
works well and is somewhat easier due to the planet’s brilliance. Amazingly,
Venus is also bright enough to cast its own shadow and this too can be
photographed using an exposure length of several minutes' with the camera
set on high sensitivity. Both Mercury and Venus show phases. When on
the closest part of their orbits to the Earth, both show a slender, delicate
crescent. In the case of Venus, this can be quite large at almost an arc-
minute (approximately 1/30th the apparent diameter of the Moon in the sky)
across and is definitely a worthy target for any astrophotographer.
A rather ironic fact about Venus from an imager’s point of view is that
this most brilliant of planets is somewhat disappointing in terms of detail.
Visually its disc doesn’t give up its secrets easily and can appear rather bland.
The most successful methods for picking out subtle shaded detail within
Venus's clouds is to use a greyscale high-frame-rate camera fitted with a
selection of imaging filters. Most detail appears at the blue-UV end of the
5