Early forming stars might have had many small potential planets. Some such asteroids merged often enough over many millennia to eventually grow big enough to ultimately become major planets. Some here have by now grown bigger than Earth to have masses 314.5 times bigger for Jupiter, 94.1 for Saturn, 14.4 for Uranus, 16.7 for Neptune. (Jupiter is .0955% of Sol's mass.)
Sol's planetary main asteroid belt, between Mars & Jupiter, presently has over 200 asteroids bigger than 100km. Spread over such a large volume, it would actually be unlikely for you to approach one closely without aiming for it. But a survey in infrared wavelengths indicated that our main asteroid belt has 700,000 to 1,700,000 of size 1km or more.
Solar gravity would keep even far away asteroids slowly orbiting Sol. There are lots in the Kuiper belt about 6 times Neptune's distance from Sol, having over 100,000 of big sizes over 100km, and lots more of smaller sizes. And also there's the much bigger Oort cloud of asteroids well outside there, to distances like a light-year from Sol.
Out beyond Neptune there remain millions of asteroids too far spread apart to have so far merged into planets. Occasionally one may get deflected by another big asteroid, and rarely may also get further deflected by gravitational attraction in toward distant massive Saturn and Jupiter. One deflected inside Jupiter's orbit and swinging in somewhat approaching Mars would start giving off a conspicuous comet tail from frost and ice getting vaporized from its surface due to the stronger warmer Solar light in this close.
Jupiter's orbit is at 5.22xEarth orbit radius. Any asteroid deflected inside there reaches the freezing/thawing point from Sol at ~2.7xEarth orbit radius [~ means approximately]. Mars orbits at 1.53xEarth orbit radius, (.293 of Jupiter's orbit), so an asteroid deflected in approaching Mars would start showing off a warning comet tail from its warmed and vaporized surface frost or ice.
We need to keep careful watch over any such deflected asteroid becoming an approaching comet. Lucky for us that such a sufficiently inward deflected asteroid gives off a conspicuous comet tail showing noticeably, trailing out away from Sol. One could possibly strike Earth, causing horrible destruction if big enough, such as what long ago killed 2/3 of Earth life, leading to the extinction of dinosaurs. But now we can send what can safely deflect one away enough if it may eventually seem significantly threatening.
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