Answer based on context:

Coming off their divisional home win over the Buccaneers, the Falcons flew to the Hubert H. Humphrey Metrodome for a Week 16 battle with the Minnesota Vikings.  Atlanta would get an early first quarter lead with running back Michael Turner getting a 1-yard touchdown run.  The Vikings would respond with quarterback Tarvaris Jackson completing a 21-yard touchdown pass to tight end Visanthe Shiancoe. In the second quarter, the Falcons would regain the lead as rookie quarterback Matt Ryan completed an 8-yard touchdown pass to running back Jerious Norwood, followed by kicker Jason Elam getting a 22-yard field goal. Atlanta would add onto their lead in the third quarter as Ryan tried to run into the endzone for a touchdown, fumbled the ball into the endzone, and offensive guard Justin Blalock would recover it in the endzone for a touchdown.  In the fourth quarter, Minnesota tried to come back as kicker Ryan Longwell nailed a 29-yard field goal, along with Jackson hooking up with Shiancoe again on a 17-yard touchdown pass.  Fortunately, the Falcon defense would stiffen for the win. With the win, not only did Atlanta improve to 10-5, but they also clinched a playoff berth for the first since 2004.

How many yards was the longest touchdown pass?
21