That is very close. By comparison, the Moon is 240,000 miles from the Earth. Impact of an object of this size with the Earth would cause massive damage. When originally announced, uncertainties in knowledge about the orbit made the margin of error large enough that an impact with the Earth could not be ruled out. This naturally caused much speculation, worry, and mis-information in public discussions about this discovery.
Astronomers calculate orbits based on multiple observations of an object's exact position.
The more observations that are made, the more accurate the orbit calculated. The initial estimate of the approach distance of 30,000 miles was based on a small number of observations, yielding an orbit with a relatively high error possibility. Requests were made for astronomers to search their archives for pre-discovery photos that might contain images of the asteroid, which would give positions over a longer time span. Additional positions were obtained, and a more accurate orbit was calculated, and on March 12, it was announced that closest approach to the Earth is now estimated at 600,000 miles, with zero probability of impact with the Earth. All rights reserved.