Latest PHOBOS-GRUNT Reentry Prediction

Coverage started 2012 Jan 08

Updated 2012 Jan 17 06:30:00 UTC

Epilogue: The latest predicted reentry period (released 2012 Jan 15 15:30 UTC) on Space Track shows 2012 Jan 15 16:59–17:47 UTC, which, if correct, would mean that Phobos-Grunt has already reentered. The latest TLE released had an epoch of 2012 Jan 15 16:36 UTC, just prior to the predicted reentry period. Space Track does not yet show Phobos-Grunt as reentered, but this may be an administrative issue.

Based on this USSTRATCOM prediction, Phobos-Grunt would have reentered somewhere along the track shown in the figure below:

While this experiment in near–real-time updates of reentry predictions helped to illustrate how sensitive the predicted reentry location is to changes in the predicted reentry time, it appears that the predictions based on STK's Lifetime tool predicted a reentry too far out. I will be examining these results in more detail to see if I can determine why that would be and will likely go back and look at both the UARS and ROSAT reentries, as well. I will publish the results here on CelesTrak once that analysis is complete. Hopefully, that will allow us to produce better results for the next newsworthy reentry (yes, there will almost certainly be more, but hopefully not any time soon).

Update 2011 Jan 17: I've generated plots using the same technique as that used for Phobos-Grunt for both the UARS and ROSAT reentries, which seems to show similar behavior when compared to the actual reentry times reported by USSTRATCOM, although not underestimating the reentry times as much. Analysis continues. —TS Kelso


Using the latest TLE data for PHOBOS-GRUNT, together with the Lifetime tool in STK, the current estimated reentry date/time is now 2012 Jan 17 16:05:35 UTC.

The latest decay prediction from USSTRATCOM (available in the Current Decay Predictions (From TIP Messages) section of the Space Track web site) shows an estimated reentry date/time of N/A (no estimate is currently available). NOTE: It appears that USSTRATCOM will not be issuing any specific reentry predictions for this event, choosing instead to only issue a reentry time span—unlike any other reentry prediction currently or previously issued on Space Track.

This page was generated using a custom application that embeds STK technology to perform the calculations of all our reentry predictions and create the associated images and interactive simulations used to illustrate this reentry event. The application allows us to automatically update this page each time a new TLE is released throughout the day, permitting us to show how the situation is evolving in near–real time. This page contains detailed information on not only the current reentry prediction, but allows us to show how the predicted reentry times have changed with each TLE update and how that affects the associated reentry location.

Current Reentry Prediction

Figure 1 illustrates the latest estimate. It shows the position of PHOBOS-GRUNT at the STK reenty prediction time. The orbit is in green prior to the predicted reentry time and in orange after that time. There is a small orange sphere marking the predicted reentry time. Note that the sphere is only used to show where PHOBOS-GRUNT is predicted to be at the reentry time and does not indicate where any pieces might come down (if they do not burn up during reeentry, as expected).


Figure 1. Current Reentry Prediction

The upper-left portion of the image shows the current time along with the latitude, (east) longitude, and altitude of PHOBOS-GRUNT at that time. The lower-left portion of the image shows the USSTRATCOM predicted reentry date/time (if available) followed by the latest estimate using STK.

You may download an interactive simulation of the PHOBOS-GRUNT reentry scenario, which uses the free STK Viewer software to allow you to explore the latest reentry prediction in more detail.

PHOBOS-GRUNT Reentry Interactive Simulation

Historical Reentry Predictions

Using the TLE history starting 2012 Jan 1, we can also see how reentry predictions vary over time with each new TLE. Figure 2 shows a plot of the reentry time prediction for each TLE, illustrating how the prediction is trending and how much the predictions vary. The green dotted line is a ten-point simple moving average (SMA) of the predictions and the upper and lower black dotted lines are the associated ±3σ uncertainties (three times the standard deviation of the SMA, or a 98-percent confidence interval).


Figure 2. Historical Reentry Predictions Plot

The associated geographical distribution of these reentry prediction times is shown in Figures 3 (2D) and 4 (3D) and the interactive simulation below (which allows you to pan around the 3D globe and see the overall distribution). Each prediciton is shown by a small yellow sphere with an index corresponding to the order of the TLEs in Figure 2 (the current prediction is still shown in orange). The time span of the orbit shown in Figures 3 and 4 uses the ±3σ uncertainty value derived from the SMA.


Figure 3. 2D Historical Reentry Prediction


Figure 4. 3D Historical Reentry Prediction

As can be seen in Figures 2 and 3, even the relatively small fluctuations in predicted reentry times result in a somewhat random distribution of the associated reentry locations, illustrating why uncontrolled satellite reentry locations are so difficult to predict.

PHOBOS-GRUNT Historical Reentry Interactive Simulation

For more information on the PHOBOS-GRUNT mission, check out AGI's YouTube video:


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Dr. T.S. Kelso [TS.Kelso@celestrak.com]
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