K. What kind of object do you think is capable of going from 28,000 feet to 50 in approx 0.78 seconds?
Well, as others have mentioned, a meteor.
But that's unlikely. What you are likely seeing is a radar artifact.
Radars don't watch things continuously. Military naval radars do a scan; they spin the antenna in azimuth to scan different sections of the sky. It's the big spinning thing you see on older ships, or the big radome on newer ships, or phased-array antennas on the newest ships. But they all work basically the same way - they divide the sky up into segments and scan them one at a time. The mechanically scanned ones do a full sweep about once a second.
That means that the radar CANNOT see it descend from 28,000 feet to 50 feet. On the first sweep it saw something at 28,000 feet, and on the next sweep it saw it at 50 feet. And those two scans would be about a second apart (or in this case, .78 seconds.) So it could easily have been two completely separate returns - but the operator interpreted it as the same return.
In addition, EM signal strength falls off at the ratio of 1/R2, which means if you are twice the distance away, you see 1/4 of the energy. Radar returns fall off at a ratio of 1/R4, which means at twice the distance you see one SIXTEENTH of the energy. So for example:
The radar gets a weak return from something 12 miles away at 28,000 feet. (This is 10 miles across the ocean and 5 miles up.) Then on the next pass it gets a return from 10 miles away at 50 feet. This return is going to be twice as strong as the return from 28,000 feet even if the objects are just as reflective. And if they are in fact at similar ranges (10 vs 12 miles) then the stronger return will tend to swamp the weaker return. So from the operator's perspective, that thing that was at 28,000 feet suddenly jumped to 50 feet. If the lower return then goes away (say, the ship that produced it is now in the trough of a wave) the 28,000 return will seem to reappear.
That does not mean that something went from 28,0000 feet to 50 feet and back to 28,000 feet in 2 seconds. It meant the operator saw two different returns.