At http://phys.org/print298024131.html … the quixotically named Don Quixote asteroid, the third largest Near Earth Object (or NEO), has changed status. It is now regarded as a comet. Such dead comets are thought to have shed all their water and carbon dioxide in the form of ice that gave them their coma and tail – it is thought that when approaching the vicinity of the Sun comets emit lots of dust and water ice as a result of heat. It is obvious that some kind of influence from the Sun brings this about as they do not normally display a prominent tail in the outer reaches of the solar system. Don Quixote does not quite conform to either asteroid or a dead comet. It behaves like an active comet. This emerged when images from 2009 were re-examined when it was in that part of its orbit closest to the Sun – and they found it displayed a coma and faint tail.
Images from 2004 determined its surface is composed of silicate dust – which is one kind of comet dust. It did not have a coma or tail at its furthest distance from the Sun (its orbit takes it way beyon Jupiter). It was only the Spitzer telescope that allowed them to see the coma and tail – not possible using optical telescopes on the surface. The discovery is exciting as far as the possibility of water ice on Don Quixote is concerned – but they don't mention the silicate dust. How much of this lands on the Earth? Likwise, when Don Quixote was a more active comet how did it affect people on Earth – coming as close as it does on a regular basis?
At http://phys.org/print298023070.html … the Sun grazing asteroid Phaethon has also switched from asteroid to comet – for the same reasons. It was observed to have a comet like tail of dust particles blown back by the solar wind as it orbits around the Sun. The tail of Phaethon, they say, doesn't arise through vapourisation of an icy nucleus but arises instead as a result of its rocks becoming so hot they crumble and turn to dust. Why don't the rocks melt? This was the explanation provided by British researcher David Jewitt and Jing Li his co-author. Jewitt gave a presentation at the 10th September European Planetary Science Congress in London.
Mostly, meteor showeres arise when the Earth orbits through streams of debris and dust released from comets in the 'inner solar system' – such as the December Geminids. However, the Geminids are not caused by a comet, it says, but by the asteroid Phaethon – which of course puzzled astronomers. Jewitt and Jing Li, in 2010, found Phaethon anomalously bright when closest to the Sun (using NASAs STEREO Sun observing spacecraft), and in further observations the comet like tail has been confirmed. In other words it ejects dust – and it is this dust that causes the Geminids when they collide with the orbit of the Earth. The problem the researchers have is that Phaethon regularly approaches the Sun which means it will reach temperatures over 700 degrees Celsius – too hot for ice to remain. Something else has to cause Phaethon to emit dust, it was decided, hence the theory about heat causing the rocky comet to crumble. There is a NASA video of Phaethon moving across the sky on the PhysOrg link.