February 20, 2008

Heaven Above's tracking of USA 193 fact sheet

Decaying spy satellite USA 193

There has been much discussion in the news media recently concerning this satellite and when and where is will come back to Earth. We would like to use this page to summarise what is known and not known.


USA 193 was launched on December 14th, 2006 from Vandenberg Air Force Base in California. The launch vehicle was a Delta II. Shortly after reaching orbit, ground controllers lost the ability to control the satellite, and have never regained it.


The exact design and purpose of USA 193 are, or course, closely guarded secrets, but specialists believe it is probably a high resolution radar satellite which was intended to produce images for the National Reconnaissance Office (NRO).

When and where will it hit the Earth ?

This is the question which interests most people, and unfortunately very difficult to answer. The satellite is being slowed down by friction with the tenuous upper atmosphere and losing height steadily, as can be seen in the plot below, which shows the orbital height over the last year. As it sinks further, the atmospheric density increases and so does the friction, making the descent faster and faster. Re-entry will happen when the height reaches about 100km.

USA 193 orbital height in km

Predicting exactly when and where it will re-enter the atmosphere and burn up is like trying to predict what the weather will be like in a months time - there are too many uncertainties. All that can be said now is that the re-entry will be some time around the end of March. As time passes, and the satellite gets lower, more accurate predictions of the time will be possible, but even a day before, it will be difficult to say exactly where in the orbit it will happen.

What are the risks ?

Fortunately, the risk to people on the ground is extremely low. The Earth is large and two-thirds of it is covered by ocean. Most of the satellite will burn up in the upper atmosphere, but a few large pieces are expected to survive the re-entry and fall to Earth. It would be very unlikely for any of them to hit someone.

Where is USA 193 now ?

You can see the current position in the ground track below.

USA-193 Ground Track

Can I see it before re-entry?

USA 193 is large and is now in a very low orbit, so it should be quite bright and easy to see with the unaided eye. Click here for predictions for your location.

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