The U.S. Army has found a long range rocket it really likes, and has ordered another 95 ARACMS rockets to replace the 495 it has used so far in Iraq. There are only about 2,000 ATACKMS rockets in inventory. These rockets use GPS guidance to hit targets up to 300 kilometers away with a 500 pound, high explosive, warhead. Sort of like the popular 500 pound JDAM smart bomb used by the air force.
When the U.S. Army first introduced it's long range ATACMS rocket twenty years ago, it designed fancy warheads that distributed lots of smaller bomblets. While these worked, there was always a problem with some of the bomblets not self-destructing, and later going off when civilians, or American troops, came along. Not a popular weapon. Then, when a version, with GPS guidance and a single, 500 pound high explosive (or "unitary") warhead was introduced, it proved very popular. These rockets cost about a million dollars each. A 500 pound JDAM costs about $25,000, although you can add a few thousand dollars more to cover the expense of operating the jet bomber that delivered it.
The ATACMS is a 610mm rocket that fits in the same size container that normally holdssix MLRS rockets. ATACMS is fired from the MLRS launcher. Although the army usually has enough bombers up there, carrying JDAMS, there are still times when a smart bomb is needed, and its quicker to send an ATACMS, than to wait for the JDAM carrying bomber to arrive. ATACMS can also get through in situations where there are anti-aircraft defenses over the target area, keeping bombers away.