[AlphBravo (OP)]

Watching the telly in the beginning of the year, they had this show on Global Dimming, and apparently the basis for this is the reflective nature of pollutants in the upper atmosphere restricting the formation of rainfall droplets, but because of this the particles also reflect sunlight back into space, this got me thinking about reflective power, Asteroids would have to be many or huge, but then I realised that asteroids are not the only stellar objects and thought that if a comet came close to Earth would it not reflect light/heat onto the planet?
The tail being the major contributor to the heat, but would it heat up the planet significantly?

[itsjustme]

even the moon reflects light/heat back to earth but it has a verry small efficiency because it is not a perfect mirror.

[AlphBravo (OP)]

But surely the comet would take up a much bigger area relative to the planet?

When they are near the Sun and active, comets have several distinct parts:

    * nucleus: relatively solid and stable, mostly ice and gas with a small amount of dust and other solids;
    * coma: dense cloud of water, carbon dioxide and other neutral gases sublimed from the nucleus;
    * hydrogen cloud: huge (millions of km in diameter) but very sparse envelope of neutral hydrogen;
    * dust tail: up to 10 million km long composed of smoke-sized dust particles driven off the nucleus by escaping gases; this is the most prominent part of a comet to the unaided eye;
    * ion tail: as much as several hundred million km long composed of plasma and laced with rays and streamers caused by interactions with the solar wind.