The planets
Important work can be done in this field by measuring the position and
brightness of the object. Accurate brightness determination over short
exposures can be used to measure the rotation period of the asteroid.
We now take the next major step away from our home planet Earth, and
head into what is known as the Outer Solar System – where rocky planets
give way to the Giants – Jupiter, Saturn, Uranus and Neptune.Jupiter
Out this far we find a very different side to the Solar System. When the outer
planets formed, conditions were much cooler than nearer to the Sun, so
these planets evolved very differently. Jupiter has a core made up of ice and
silicates surrounded by a dense atmosphere containing hydrogen, helium
and other gases. A telescope shows Jupiter as a yellowish flattened disc.
Although the orbital period is nearly 12 years, the rotation period is less than
10 hours and this rapid spin causes the equator to bulge out. Normally the
disc shows streaks known as cloud belts. The most famous feature is the
Great Red Spot, an anti-cyclonic storm with a surface area greater than the
surface of the Earth. The cause of the red colour is still a matter for debate. It
is out of view for half of Jupiter’s day on the far side of the planet.
The joy of Jupiter from the observer’s point of view is that it is always
changing and one never knows what it will do next. Normally there are
several belts, in particular the two equatorial belts, one either side of the
planet’s equator. In 2010 the south equatorial belt suddenly vanished. It might
well have been snatched away like the “Hunters of the Snark” and Jupiter
observers were baffled. It has since reappeared but is not as regular as it
was and it will be some time before it reverts to normal. The disappearance
is thought to be due to the high atmospheric clouds which temporarily
obscured the belt. Jupiter is the ideal subject for the visual observer and also
for the photographer.Imaging Asteroids
Visually, an asteroid looks no different to a star and this makes it harder
to identify. The way to confirm that you’ve seen an asteroid is to record the
field in which you think it lies and do the same on a subsequent night. If you
compare the recordings and one of the "stars" has moved, that is probably
the asteroid.
Sketching the field is one way to do this, although a more up-to-date
method would be to image. Here, any camera that can take a photograph
of a star field can be used. It’s normal to couple the camera to a telescope
in order to record a field of view with a good image scale and with sufficient
light depth to record the asteroid convincingly. If this is done over several
nights the images can be brought together in an animation. For this to work,
the images need to be taken with the same equipment and cover the same
star field. Post capture, each image needs to be aligned with respect to its
stars before being added to the animation. Running the sequence in a loop
will "blink" between the frames, revealing the motion of anything moving.
For a typical asteroid, a separation of one, two, three or even more
days between shots is normally fine, but there is a class of asteroid which
appears to move so fast across the sky that they will show motion after
hours or, in extreme cases, minutes. These are the so-called Near Earth
Asteroids or NEAs for short. As their name suggests, these are bodies
which have orbits that take them relatively close to Earth, sometimes even
closer than the Moon. Consequently, these bodies can appear to move quite
rapidly across the sky, the apparent speed reaching a maximum as the
asteroid’s distance to Earth reaches a minimum.
Catching an NEA on camera requires skill and timing, but in practice the
procedure to follow is identical to that described above for "normal" asteroids.
Predictions of passes can be obtained from websites such as that provided
by the Jet Propulsion Laboratory at http://neo.jpl.nasa.gov/neo/. Accurate
positions can be obtained by entering the object's reference details along
with your own position etc. into the JPL Horizons ephemeris generator at
http://ssd.jpl.nasa.gov/?horizons.
Once you know the star field where the NEA is going to pass through, it’s a
case of setting up your camera and waiting for the appropriate time to catch
it. It is important to check the magnitude of the body and that your imaging
set-up can record stars at least down to that level, preferably a bit dimmer.
[12] Asteroid imaged using the
technique described below.
[13] Asteroid Vesta, seen from the
Dawn probe (NASA/JPL/Caltech).12 13