2004 FH flies past Earth about now from the distance of 43 000
kilometers. In the press it´s called an `asteroid´, but it´s diameter
is only 25 meters. Compare that with Ceres (934 km) or the newly
found objects of the Edgeworth-Kuiper Belt, like Quoaor, Ixion or
Sedna (1000-1700 km all of them).

It´s claimed that Pluto isn´t a planet because it has - expect it´s
spherical form - little common with the bigger planets. But how much
common 2004 FH has with Ceres? Not much. Ceres - which will be
visited (if everything goes well) by the NASA probe Dawn later this
decade - is a world, even if a small one. 2004 FH is a big boulder,
really a meteoroid and not an asteroid. Objects twice as big than it have
hit the Earth´s atmosphere during the last decades and exploded
without causing any harm.

So, it would be better call it an asteroid. But where to draw the line
between an asteroid and a meteoroid? Usually asteroid is considered
to be bigger than 50 meters. Nice round number, even if a meteroid 49
meters of diameter and an asteroid 51 meters of diameter are really
the same thing. But the line is have to be drawn somewhere, or so we
think.

We have the same problem with the newly found moons of the gas
planets. The number of their moons have doubled in the half decade
or so and astronomers are now finding objects only a few kilometers
of diameter around Jupiter. Can they be called moons? How small a
planetary satellite can be and still be considered a moon?

Certainly the objects that form the rings of the gas planets can´t all
be considered to be moons...

In the end, all our classifications are arbitrary. We can easily distinguish
between a star, a planet and a comet, but as we have found out in the
last decade or so, there are objects that bridge the gaps between them.
For example: Brown dwarfs, which are `failed stars´ and huge planets
ten times the size of Jupiter, which are still too small to be considered
brown dwarfs, but seem huge to be considered to be planets and
large numbers of them seem to be drifting alone in space. Are these
planets? Or are they `failed´ brown dwarfs?