Boeing 777x

Wow, beautiful aircraft!
I noticed what appeared to be a “balloon” attached to the tail of the plane.
Both before take off and also upon landing.
Can someone say what the significance of it is and what it is?

Static cone, for an accurate static pressure measurement

After a month pause, I’ve seen the new 777X -9 test flights become pretty active this week. And now there’s two of them using both BFI and PAE.

I wish I had been on this test flight today…