An asteroid named Apophis has been found in 2004.
Since, as it is done to all of them, its trajectory has been calculated.
Hence, astronomers have discovered that in 2029, it will be very close to Earth and will cross us between 36,000 and 40,000km (which is very small in the Solar system scale: this is ten times less than the distance between Earth and the Moon). Geostaionnary satellites are at 40,000km high, so this asteroid is willing to destroy some, which will perturb a lot the communications at this moment.
However, it will come back on April the 13th, 2036 and astronomers are planning that it will hit our planet.
It is between 250 and 270 meters long and it weights about 20 million tons. The energy created per the impact will be 1,5 gigatons (Hiroshima bomb had a 15 kilotons power, which is 100,000 times less).
But be rassured, chinese scientists have calculated that sending a spacecraft early enough to hit the asteroid could push it enough so that it won't hit our planet and will never come back.
Moreover, some astronomers say that probabilities are very close to zero.
Historical reminder: The two last asteroids that have been very close to Earth are '2011 CQ1' (size of a car) that came at 5,800km only away from Earth on February the 4th, 2011, and '2011 MD' (bus size) that came at 12,000km away on July the 4th, 2011.
The last one that hit Earth was '2008 TC3' and felt on October the 7th, 2008 at 02h46 UTC in the North-Est of Sudan, without any damage (satellite picture taken during the impact, here below, taken per Meteosat 8). It measured between 1 & 7 meters. Its parts have been found per Peter Jenniskens on February the 28th, 2009 in the Nubian desert.
Radar picture of Apophis then infrared picture of the impact of '2008 TC3' taken per Meteosat 8