The aircraft in question, tail # N17327, flew from SLC to RFD (Rockford) yesterday. Given that it filed for FL410, and that UA does not operate pax service to/from RFD, this was definitely a maintenance ferry flight. AAR at RFD has a maintenance contract with UA for their 737s.

As far as the space debris theory and ultimate risk - I got bored earlier and decided to brush the cobwebs off of my college physics. The FAA has previously assigned a risk of approximately one-trillion-to-one of a passenger casualty resulting from an impact with a space-originating object, and cocktail napkin math would seem to support this. First you have big sky theory, meaning the likelihood of two objects sharing the same point in space at the same point in time is low. Then you have the typical size of space debris and its residual kinetic energy at, say, FL360 as UA 1093 was. Most of this stuff is miniscule in size, and by the time it reaches the troposphere or lower stratosphere, has already decelerated from scary orbital/cosmic velocity to something more like a couple hundred miles per hour and nearing its terminal velocity. Don't think of a boulder traveling at several thousand miles per hour - think of a pebble traveling at the same speed or less as the multi-ton airliner it is impacting. The airliner wins. In this case (and assuming this was even space debris), the unlucky impact point of the windshield is probably the only reason the crew even knew about it before the plane landed.