how close has the closest asteroid to earth been?
355430
The Earth has been hit thousands of times.
382741
The sun is the closest star to Earth.
270315
Sirius is also one of the nearest stars to Earth At a distance of 2.6 pc or 8.57 light years\nfor more information go to http://www.crystalinks.com/sirius.html\nhttp://www.google.co.in/search?q=distance%2Bsirius&start=0&ie=utf-8&oe=utf-8&client=firefox-a&rls=org.mozilla:en-US:official
757753
744 million miles at the closest point
21269
It would take a very large asteroid to hit or pass near the Earth to shift the Earth's orbit suddenly.\n\nIf it passed near the Earth, that asteroid's gravity could pull the Earth and alter its orbit around ths sun, but it would have to be enormous to have a significant impact on the orbit in such a brief encounter - perhaps as big as some of the smallest planets.\n\nBut after billions of years, most of the large objects around the sun have settled into stable orbits.\n\nAnd small asteroids and comets pass by the Earth all the time without much change.
198141
The moon is closest at about 1/4 million miles, the sun is next, about 93 million miles, and the nearest star is Proxima Centauri, about 4 light years (3,800,000,000,000,000,000 miles)
198137
The average distance from the Moon to the Earth is 384,403 kilometers (238,857 miles). The Moon's diameter is 3,476 kilometers (2,160 miles).
376755
Nope, nothing that we have found will be coming very near us on that date.  Just to give an update on the object the Braxton_Paul mentioned, 2004MN4 (now designated 99942 Apophis), has pretty much been put into the clear for the close approach in 2029.  It will come close, but unless something funny happens to the orbit (collision, unexpected outgassing phase etc), it will miss us.\n\nWhat we are trying to get a fix on now is the next close pass by this bad-boy: 2036.  We can't rule out an impact probablitity for that encounter yet.
449066
Besides the sun, Proxima Centari is closest.  About four light years away.  (only visible from the southern hemisphere though).  Actually Alpha Centauri is a three-star system.  Alpha Centauri -C is the closest of the three to earth.
586535
A continuing effort to identify even near Earth asteroids which are relatively easy to track and have a greater possibility of collision is still incomplete, with only a fraction of their orbits plotted. Since the orbital periods of comets and other Oort cloud entities are in the tens, hundreds, or even thousands of years, many of these objects have never been seen in recorded history, much less their orbits calculated and possibility of collision with Earth evaluated. With this in mind, an event such as this might be possible at any point in the future, although since objects from the outer solar system tend to be rather reflective, chances are that they would be spotted and we would have some advance warning of a likely intersection with Earth orbit presenting a cause for concern. That said, the chance of it happening in any particular year is extremely remote, the likelihood of it happening sometime in the next several million years greater.
815533
Mercury, Venus and Mars are always closer than 574,380,430.5648 miles.  That number is close to the distance of Jupiter at it's farthest.
950664
I don't know the name, I think it just has a number.\n\nIt is about 1,000,000 km away and is a captured asteroid about 1km across.  It is irregular in shape.  It is in an unstable orbit and will eventually leave the Earth's orbit and enter solar orbit.
816136
The asteroid belt is between Mars and Jupiter and is a huge belt composed mainly from rocks and ice, it's about 6 AU (Astronomical Units) in size, 1 AU is the distance from the Sun to the Earth or 87,000,000 Kilometers..☺
769187
Near earth asteroids are those which pass near the earth .... What is near?? Well I guess 1,000,000 km is pretty near (even 1,000,000 miles!)\n\nMain belt asteroids are beyond the orbit of Mars so roughly 1 - 3 AU from us at nearest approach (1 AU = mean distance from earth to sun) so they are not "near" to us.
563193
From what I understand for a specific asteroid they can extrapolate its orbit from what they have observed so far while taking into account the uncertainity in their estimates (because of not enough time spent observing it for example) and see what probability it has of impacting the Earth.  What has happened with every potential Earth impactor we've found is that as the object was observed more closely and the uncertainities vanished the probability of Earth impact went to zero.\n\nIf OTOH you want to know the probability that an asteroid, not just a specific asteroid that is being observed will hit the Earth then you've got to make some assumptions.  First you have to assume how many asteroids of each size there are that cross the Earth's orbit and those assumptions are being changed as more observations come in.  Then you can calculate the probability of one of them hitting the Earth and how long on average between impacts (I'm not sure the actual steps of the calculation, you might have to check some journal articles on it but it'd probably be pretty tough going if you don't have a good grounding in maths and physics and if you haven't read a journal article before you probably won't want to).\n\nWith large asteroids the asteroid watchers seems to be pretty confident that all the big ones that we'd have plenty of warning if a big one were on its way so the small ones that might only wipe out a city or the comets that come from nowhere at much higher speed worries right now.
384258
About 92.9 million miles on average. This is also 147 million kilometers. The actual amount varies over time because the earth's orbit isn't a perfect circle. In astronomical terms, this is *very* close. If you made a scale model with the distance from the earth to the sun as 1 inch, then the distance to the nearest star (other than the sun) would be 4.3 *miles*.
605546
744 million miles from closest point