Fred Lawrence Whipple - Obituary

Fred Lawrence Whipple, astronomer, born November 5 1906; died August
30 2004

http://www.guardian.co.uk/obituaries...295306,00.html
Small section of this long obituary

In 1949 Whipple published his proposal that comets are made of the
same materials as the outer planets, aggregates of ice and rock. This
was buttressed in 1951 by the Dutch astronomer Jan Hendrik Oort, who
pointed out that a substantial amount of material left over from the
formation of the planets is now dispersed in a huge belt of asteroids
which, although in solar orbit, is about a light year from the sun.
Gravitational perturbations would result in a few of these small
masses being shunted either out of solar orbit altogether, to become
wanderers in space, or into the solar system, where they could either
be captured to become periodic comets or perhaps to be seen only once
as they pass through on an extra-solar trajectory.

Whipple developed these ideas, eventually explaining slight variations
of cometary orbits through the propulsive forces of outgassing jets at
the surface of the nucleus (actually seen by spacecraft Giotto),
confirming the "tail" as a product of the solar wind and attributing
the short survival of near-sun comets to their icy structure. He also
promoted the development of meteor tracking systems, and investigated
and helped to develop the role of radio astronomy in planetary
studies. Supported by observations from around the world, including
Jodrell Bank, Whipple showed that the majority of meteors visible from
earth originate within the solar system and are produced predominantly
from the break-up of comets.