[Yosemitest]

Possibly part of this Proton rocket and its Breeze M upper stage from Russia’s Baikonur Cosmodrome in Kazakhstan.

Or maybe it was off this heavy-lift Ariane 5 rocket launched on March 23 from Europe’s Guiana Space Center in French Guiana.
On second thought, since it was found March 21, 2012, it couldn't have been the Ariane 5.

[Yosemitest]

Adding date:

The Proton rocket and its Breeze M upper stage from Russia’s Baikonur Cosmodrome in Kazakhstanon was launched Feb. 15, 2012.

[Army Air Corps]

The diameter of a Proton is 4.1 metres, and this object looks like it has a diameter less than four metres.