Narrative:

We had been cleared from 14000 ft to 11000 ft. Copilot called traffic my 12 O'clock. It appeared to be closing fast. I turned hard right. As I turned I realized that it was still in my right quad forcing me to tighten my turn. As I was cleared to 11TH I started a descent, entered a brief scattered layer. As I was 1/4 of the way around I reasoned it was quicker to continue around and continue to olympia VOR. To digress, as the object passed by I saw it was an aluminum (shiny) mylar balloon. I've seen them before because of their size they give the illusion of a high rate of closure. I never seen plastic balloons that high. I suspect they pop before they can get that high. The mylar are much stronger. I hav heard that public utilities are moving to ban the mylar because they short out power lines. My suggestion is that they be made in a such away that they have a fuse plug that would release pressure if they got away. In my situation, there was no real hazard, but it is conceivable that there could be a scenario where a traffic conflict could arise avoiding these balloons. This is the first time I've mistaken one for another aircraft largely because first officer called it. I maneuvered then recognized it. I've seen 4 in about as many yrs.

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Original NASA ASRS Text

Title: NMAC BTWN LTT AND MYLAR BALLOON.

Narrative: WE HAD BEEN CLRED FROM 14000 FT TO 11000 FT. COPLT CALLED TFC MY 12 O'CLOCK. IT APPEARED TO BE CLOSING FAST. I TURNED HARD R. AS I TURNED I REALIZED THAT IT WAS STILL IN MY R QUAD FORCING ME TO TIGHTEN MY TURN. AS I WAS CLRED TO 11TH I STARTED A DSCNT, ENTERED A BRIEF SCATTERED LAYER. AS I WAS 1/4 OF THE WAY AROUND I REASONED IT WAS QUICKER TO CONTINUE AROUND AND CONTINUE TO OLYMPIA VOR. TO DIGRESS, AS THE OBJECT PASSED BY I SAW IT WAS AN ALUMINUM (SHINY) MYLAR BALLOON. I'VE SEEN THEM BEFORE BECAUSE OF THEIR SIZE THEY GIVE THE ILLUSION OF A HIGH RATE OF CLOSURE. I NEVER SEEN PLASTIC BALLOONS THAT HIGH. I SUSPECT THEY POP BEFORE THEY CAN GET THAT HIGH. THE MYLAR ARE MUCH STRONGER. I HAV HEARD THAT PUBLIC UTILITIES ARE MOVING TO BAN THE MYLAR BECAUSE THEY SHORT OUT PWR LINES. MY SUGGESTION IS THAT THEY BE MADE IN A SUCH AWAY THAT THEY HAVE A FUSE PLUG THAT WOULD RELEASE PRESSURE IF THEY GOT AWAY. IN MY SITUATION, THERE WAS NO REAL HAZARD, BUT IT IS CONCEIVABLE THAT THERE COULD BE A SCENARIO WHERE A TFC CONFLICT COULD ARISE AVOIDING THESE BALLOONS. THIS IS THE FIRST TIME I'VE MISTAKEN ONE FOR ANOTHER ACFT LARGELY BECAUSE FO CALLED IT. I MANEUVERED THEN RECOGNIZED IT. I'VE SEEN 4 IN ABOUT AS MANY YRS.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.