Introduction
Asteroids (from Greek ”star like”), are a class of small Solar System bodies
in orbit around the Sun. The term "asteroid" is applied to any astronomical
object orbiting the Sun that was not observed to have the characteristics of an
active comet or a planet. Common terminology is Asteroid or Minor Planet,
but the official term is Small Solar System Bodies, something that came out of
the same IAU meeting that attempted to redefine a Planet and "demote" Pluto to
Dwarf Planet status.
Of particular interest to Planetary Scientists today are the Near Earth
Objects (NEOs) and Binary Asteroids. NEOs are those objects that cross or
come very near to Earth orbit. That is, an Object that has the potential
to impact our planet at some point in the future. It is important to look
for and monitor these objects to determine accurate orbits and uncover whatever
physical characteristics we can. To that end, mankind has approached these
targets from 3 fronts. Optical observations through the traditional
telescope, Radar observations through earth based radio telescopes and planetary
missions from space craft either designed for the purpose or able to be tasked
to a close encounter on their way to their intended targets.
So what are they and why do we observe them?
So what do we know about Asteroids. Well we know that they are made up
primarily of rocky based material. They range in size from sub metre size
to well over 1000 km. There seems to be very little difference between
Asteroids and their NEO counterparts, however, the smaller objects, by and large
appear to be simply gravitationally bound rubble piles. That is, their
only physical strength is gravity and the friction between the particles that
make up the object. Asteroids, in the past, are believed to have evolved
through collision processes. If you look at asteroids today, you can see
that they are as pock marked as the moon. We also see that the smaller
objects are not spherical. In fact they are always elongated, potato
shaped if you will and that's what makes lightcurve analysis so important.
Each full rotation of the asteroid shows 2 long side and 2 short ends.
Obviously asteroids do not emit light, but they do reflect it, even though they
are as black as coal. The 2 long sides revealing peaks in their brightness
and the 2 short ends, providing the minima. The amplitude of the resulting
lightcurve, that is the relative brightness of the long sides and short ends,
reveals a little about the shape of the asteroid. The greater the
amplitude, the more elongated the asteroid. If we take a lot of ligthcurve
observations over many apparitions at varying phase angles we can actually build
up an accurate model of the asteroids shape and spin axis. If we combine
this analysis with other observations, such as occultation's that can derive a
profile and actual dimension of that profile we can derive the exact size of the
asteroid. If it's a binary (i.e. it has a moon) then we can also derive
it's mass. And all this with the relatively low cost of amateur
observations. Oh, didn't I mention it before..... most Asteroid
observations are actually undertaken by amateurs. There are very few
professional astronomers involved in the Planetary Sciences.
The rotation of (433) Eros
(Image supplied by NEAR)
What can we Amateurs do?
There are a number of observational techniques available to the Amateur
Astronomer - Photometry, Astrometry and Occultation's. Most amateurs who
buy their first CCD camera and have a desire to do some 'real' science, jump at
Minor Planet Astrometry. Astrometry is the means of taking pictures of an
asteroid and by comparing its position to catalogue stars, derive it's position
in the night sky. This is a pretty simple thing to do BUT it does require
the observer to have an accurately setup and reliable observational system.
Unfortunately there are far too many observers who do not understand the detail
and accuracy that is required and inevitably report results that will actually
increase the uncertainty errors in the MPC calculated orbits rather than reduce
it. As a result, most amateurs are discouraged from attempting this type
of work unless they can prove that they can always generate accurate (sub arc
second) precision results. The MPC will put new observers through their
paces and will not award an observatory code until the reported results are
consistently accurate enough.
Occultation's is the process of observing one body pass in front of another
object and accurately recording the time of disappearance and reappearance.
Asteroids are small, and the chance of them passing in front of a star that can
be easily observed by an amateur is relatively small and as such few events
occur each year and when they do occur, the amateur will need to travel, with
their equipment, to be in the expected shadow path. If the occultation is
successfully observed, the result is a series of 'chords' indicating the
'off'/'on' of the star that is being occulted. With observers spaced out
over the path of the shadow, the chords can map out a shape of the asteroids
profile as well as the size of that profile.
Photometry is somewhat more difficult. In this case, the observer must
observe the asteroid over a period of time and obtain a continuous series of
images. The brightness of the asteroid is measured and compared to
catalogued stars in the same images and a brightness profile is built up.
Over time, repeating patterns in the brightness profile are detected and when
the profile is 'phased' (replotted so that the repeating patterns overlay each
other) a lightcurve is generated and a synodic rotational period is derived.
The pattern can get 'confused' as a result of tumbling action (spin on more than
1 axis) or as a result of one or more orbiting moons (yes, asteroids, even the
very small ones, have moons). In this case we will detect patterns of more
than 1 overlaping period.
(Image courtesy of Dr Alan Harris, 2007)
Now we are looking at 'Real' science
So now I have outlined how amateurs, with backyard equipment, are capable of
making observations that provide data that scientists need. What's the
most obvious example? Well in 1978, when Dr Alan Harris started in the
asteroid game, only 157 asteroid lightcurves had been take, By the year
2000 that number had risen to 800 but in 2010, the number was well over 3600 and
that number continues to rise exponentially. A plot of asteroid size v's
period didn't reveal a great deal in the early days but now makes various
statements, the most obvious of which is commonly referred to as the spin
barrier, and it was the discovery of this barrier that resulted in the accepted
fact that most asteroids, particularly the small ones are gravitationally bound
rubble piles. So scientists are armed with another fact and with this
fact, new theories about asteroid formation and evolution are developed.
The next important discovery were the number of binary asteroids uncovered.
After nearly a decade, we have determined that approximately 25% of NEOs and
Inner Main Belt Asteroids are binaries. From this, we have uncovered the
theories related to the YORP effect and its subsidiary effect - BYORP. In
fact it has lead to new theories for the formation of binary systems as well as
asteroid pairs and the science will continue to evolve as we gather more and
more observational evidence - and we amateurs are at the leading edge.
Where are you?
