“Sungrazer” is a title usually given to comets which come extraordinarily close to our Sun in the course of their long, looping orbits around the solar system. The standard line drawn to consider something a sungrazer is it passing closer than about 850,000 miles from the surface of the Sun. This distance is smaller than the diameter of the Sun, which clocks in at about 864,576 miles from edge to edge. On the scale of our solar system, this is very, very close to the Sun.
Now, comets, being largely ice, often do not survive coming this close to the Sun. This is a hot, windy, stressful environment for a comet, and so a huge fraction of their mass is lost to both evaporation and fragmentation as they come close to the Sun. Because of the tremendous rate at which the comet’s ice is lost to the solar wind, the comet will sometimes develop a huge tail, which can make them quite bright.
Can you have sungrazing asteroids? Sure -- but they’re less common than the sungrazing comets, as comets typically have more elongated looping orbits in the first place, where asteroids tends to follow more circular type orbits, keeping them further away from our star. They’re not as bright or dramatic as a sungrazing comet, as asteroids are largely rock and metal, and won’t vaporize into such a dramatic tail like a comet.
However, it’s extremely unlikely that one of these asteroids will then impact the Earth - first, it would have to survive its trip around the most hostile place in the solar system, and emerge on the other side at exactly the right time to, several weeks later, interact with the Earth. There’s a far simpler explanation for a meteorite to have a fusion crust.
A fusion crust on a meteorite is the result of being very near to a source of extreme heat, yes; but for meteorites, this source of heat was local. To make it to the surface of our planet, any incoming rock had to first traverse our atmosphere. Our atmosphere poses a major problem for incoming rocks - anything particularly small will simply vaporize as a shooting star, visible only in our night sky. If you have a larger rock coming in, you still vaporize the outer layers as the atmosphere resists the incoming rock, and prizing away any unstable pieces away from the core of the meteor.
As the meteorite slows, at some stage the pushback from the atmosphere becomes less severe, as the difference in speed between the rock and the atmosphere becomes less extreme. At that point, the surface of the rock, which had been constantly abraded and vaporized, can cool slightly, and generates what’s dubbed a fusion crust, a glassier, shinier surface than it would have had in space, before its atmospheric encounter.
All this said, meteorites are extremely rare to find, and statistically it’s far more likely that you’ve found a weird Earth rock than a meteorite, particularly if the inside of the rock is completely non-magnetic. Lots of rocks on Earth are particularly good at masquerading as meteorites for a variety of interesting geological reasons. These Earth rocks are fascinating too, but as their formation doesn’t involve a high speed collision with our atmosphere, talking to a geologist will get you a lot more information!
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