Wormwood - Genesis 4:13

One of the Seven Trumpet Judgments from God during the last half of the tribulation period known as the time of His Great Wrath. During this time, the Antichrist - 666- Beast is ruling the earth.

"And the third angel sounded, and there fell a great star from heaven, burning as it were a lamp, and it fell upon the third part of the rivers, and upon the fountains of waters: And the name of the star is called Wormwood: and the third part of the waters became wormwood;  and many men died of the waters, because they were made bitter" (Revelation 8:10-11).

The term star can be symbolic for comet, meteor, asteroid, angel, or missile being some kind of weapons projectile. This judgment against our water can be caused by bio-chemical missiles, hailstone showers, projectiles from space, whatever it is, it's poison, toxin, and causes the water of 1/3 of the earth to become bitter. 

 Update: I found the term meteorite for wormwood

when it hits the earth, the water will become poison, bitter.

from this indication, it will hit the earth during the tribulation period.