ATLAS can provide one day's warning for a 30-kiloton "town killer," a week for a 5-megaton "city killer," and three weeks for a 100-megaton "county killer".http://www.fallingstar.com/home.php
I suggest that you sign up for alerts, directly from the astronomers.
Also, here is my really cool Asteroid Mass and Energy Calculator, as a download
Using the NASA NEO list you can see a 53 Meter object flew by Sept 15, 2015 at 8500 m/s and less than 3 lunar distances from the earth.
Using my Asteroid calculator above, it would have had the energy impact of 13 atomic bombs.
Elon Musk ain't that smart. If he wants to terraform Mars, you need to send comets, not atom bombs, duh!
For the sake of completeness, we include the NASA Near Earth Object Program