The UFO we saw from the badlands may have been identified. At the suggestion of my colleagues in Atmospheric Science, I contacted the Rapid City National Weather Service office to ask if what we saw could have been a weather balloon. Dr. Matt Bunkers responded:
This looks like a weather balloon to me. For reference, the black dots on this image,
represent locations from where upper-air balloons are launched. The closest site to your location would be UNR (i.e., Rapid City just above SDSMT).
These balloons are launched twice daily at or shortly after 1100 UTC and 2300 UTC. The balloons typically ascend to around 100,000 ft AGL before they burst and fall back to the surface with the aid of a parachute. A plot for our launch on October 1st, 5 pm MDT, is here:
Note that it is labelled as October 2nd, 0000 UTC. Although the overall winds are fairly light, there is a westerly component throughout most of the profile that would have taken the balloon east of the Black Hills. After looking at the archived data we have at our office the balloon stopped ascending around 652 pm MDT, which is 16 minutes before your picture time of 708 pm MDT. At this time the balloon was 27-28 miles east-southeast (~95°) of Rapid City. If the balloon didn’t completely burst, but instead was leaking, it would look like that what is seen in your picture. If you were looking toward the northwest, then my guess that is what you saw.
— Matt, Science & Operations Office, NWS Rapid City, SD
After I thank him for his help, he added:
The two lobes could be a deformed balloon; the parachute is orange and wouldn’t be that large. The balloon increases to the size of a small garage before it bursts. Sometimes the balloon doesn’t burst outright, but instead leaks. That might have been the case here.
It seems our mystery is solved!