You're likely thinking about the Shoemaker-Levy 9 comet impact event that occurred in July 1994.

Here's the breakdown:

* Comet Shoemaker-Levy 9 was a fragmented comet that had been captured by Jupiter's gravity and was orbiting the planet.

* In 1992, the comet broke apart into numerous pieces due to tidal forces from Jupiter.

* Over the next two years, these fragments, ranging in size from a few hundred meters to several kilometers, were on a collision course with Jupiter.

* The impacts occurred over a period of six days in July 1994.

* The fragments impacted Jupiter's atmosphere at incredibly high speeds, estimated to be around 60 kilometers per second (37 miles per second).

The impacts created massive fireballs, plumes, and dark scars in Jupiter's atmosphere, providing a unique and spectacular opportunity to study the effects of such events.

So, the speeds of the dying comet fragments that hit Jupiter were incredibly fast, reaching tens of kilometers per second.