This is the very bottom of the airspace used by commercial jets so it's not a problem. Below 10,000 feet you have possible uncontrolled aircraft operating VFR without communications equipment to talk to ATC. Above 10,000, you have to have a minimum set of equipment and be talking to ATC.
More importantly, if you RTFA, this spy balloon is being stationed at Aberdeen Proving Grounds in Maryland, which is already restricted airspace.
The FAA is amending 14 CFR part 73 by creating a new restricted area, designated R-4001C, within a part of existing restricted areas R-4001A and R-4001B at Aberdeen Proving Ground, MD. R-4001C is a rectangular area, approximately 4.5 nautical miles (NM) by 2 NM in size, that extends from the surface to 10,000 feet MSL. The time of designation for R-4001C is "continuous." Because the moored balloons contained in the area will be airborne 24 hours per day (except for periods when maintenance is required, or the winds exceed 60 knots), R-4001C is not a joint-use restricted area. R-4001A and R-4001B continue to be joint-use areas, meaning that they may be released, in whole or in part, to the FAA controlling agency when the airspace is not needed by the using agency. During times when the airspace is released to the controlling agency, air traffic may be cleared through R-4001A and/or R-4001B. In addition, an editorial change is made to the using agency name for R-4001A and R-4001B by adding "U.S. Army" at the beginning of the agency name for format standardization purposes.
TLDR: The airspace will be marked on aviation charts as restricted airspace for the duration of the balloon's deployment.