At 1:31, Narrator gives speed of comet as 108,000 MPH. At about 6:01, 'Sherwood' says that the observatory called (7 days ago), and the comet was: "a couple of hundred thousand miles" from the belt. By the time Sherwood says this, the comet is almost 18 MILLION miles past the belt.
The rockets sent to hit the meteor would not burn their engines for the entire trip from launch to impact. Rockets are very fuel-thirsty. The ones in the movie would burn for a few minutes to achieve escape velocity and get on an intercept path. They would coast the rest of the trip to the meteor.
Mission Control is able to communicate with Challenger 2 in real time, despite the fact that the Challenger 2 is outside the orbit of Mars, over a hundred million miles away. At that distance, transmissions would take several minutes to reach one another.
The meteoroid is headed for Earth at about 35,000 miles an hour, and it will hit Earth in a week. At 35,000 miles an hour, an object would take at least six months to travel from the asteroid belt to Earth orbit.
Due to loss of momentum, the Challenger II would not have been able to return to Mars after traveling past its orbit to explore/examine the asteroid belt. Mars is about 1.5 AU out, the asteroid belt begins about 2.3 AU out (1 AU = almost 93 million miles / 149.6 million km)
The splinter that hits New York City does not appear to be going very fast (no more than sixty miles an hour) when it hits; it glides lazily past the Statue of Liberty in one shot. An object traveling at that speed would do damage, but it wouldn't cause anywhere near the amount of devastation shown.
A small piece or rock or metal drifting through space is called a meteoroid. A meteoroid that enters Earth's atmosphere and burns up is called a meteor; the terms "meteor" and "shooting star" are one and the same. Most meteors are tiny, not much larger than a grain of sand. A meteor that does not burn up all the way and hits the Earth is called a meteorite. An object five miles wide would never be referred to as a "meteor", especially by astrophysicists. It would still be called an asteroid.
The sequence in Siberia supposedly takes place during a blizzard (flying snow is matted in over the scene). However, the Siberian man has no snow on his hair or clothes, does not have to fight any wind when walking, and leaves the flap to his hut open when he goes inside - and the smoke coming out of the hut's chimney rises vertically.
In the final scene, the flags of the U.S.S.R. and the U.S.A. are flown improperly. The U.S flag should properly been on the left and with the blue star field in the upper left. Film is not reversed as the foreign flag is not reversed.