RHESSI's Re-entry

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Nugget
Number: 447
1st Author: Pascal SAINT-HILAIRE, Albert SHIH, and Hugh HUDSON
2nd Author:
Published: April 17, 2023
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Contents

Introduction

Our inspiration for these Nuggets, the RHESSI hard X-ray and γ-ray observatory. went into orbit February 5, 2002, and observed the Sun successfully for many years. Many earlier Nuggets describe its achievements, and in fact data analyses continue to the present (for example, the recent Nugget No. 445) Such a re-entry is the final fate of any spacecraft in low Earth orbit and the delicate and precise machinery of RHESSI will become a colorful fireball in its last moments. We present our own calculations of the re-entry time here, but note that Aerospace, SATVIEW SPACETRACK and others also are watching. See our updating page of re-entry graphics for more details.

Orbit predictions

             19-Apr 07:00                  19 Apr 2023 23:00 UTC ± 3 HOURS   (Aerospace)
                                           20 Apr 2023 02:39 UTC             (Spacetrack)
                                           20 Apr 2023 13:29 UTC             (Satview, ex USstratcom)

Figure 1 shows the predicted altitude via estimates made on Friday, April 15. The actual re-entry forecast time for this epoch was 20 April (Thursday) 00:05 +- 30 hours, but this number will improve as we get closer to the end (see above). The final stages of re-entry are uncertain in detail because of unpredictable fluctuations in the drag force from Earth's upper atmosphere.

Figure 1: Upper, a map of satellite tracks for the whole Earth, in a Mercator projection. Lower, color-coded time-series plot showing the predicted orbital altitudes.

Observability

The re-entry will suddenly interrupt RHESSI's final orbit. The imprecision of the exact time means that it could happen anywhere along the predicted orbital path, and the conservation of momentum means that the resulting fireball will trace out that path. Figure 2 shows some of the orbits, spaced approximately at 90-minute intervals and therefore about 23-degree spacings in longitude as the Earth rotates. One can see the that RHESSI's orbit was inclined to match the latitude of its ground station (at the [Space Sciences Lab] in Berkeley, California.

Figure 2: Similar to Figure 1, except for the continental US.

As with any re-entry, we expect that visual observations of the fireball will be possible for a very lucky observer. Ideally one would have clear skies, a dark night, and the great luck of having the just the right moment along the orbital track. Figure 3 shows the local area around Berkeley, where some RHESSI-lovers may be watching.

Figure 3: More detail for the San Francisco Bay area now, in a Google Maps representation.

What will the observer see? Most elemental materials have distinctive flame spectrum colors, and germanium's is light blue. As a wild speculation, a lucky observer might see the last moments of the RHESSI detectors this way. This is very speculative, of course, as is the further thought that the tungsten grids (green flame spectrum), being massive, dense, and highly refractory, may even survive re-entry! Do not stand underneath....

Epilogue

Figure 4 shows the last few orbits; the final one seems to have been the pale red one (bottom at left, RHESSI moving to the E). According to definitive reporting, the end came near the Egypt/Sudan/Chad/Libya frontier area at 00:21 UT, 20 April 2023. There may be some singed and interestingly slotted tungsten disks waiting to be discovered somewhere out there in the Sahara desert.

Figure 3: The last moments, perhaps not too far from Pitcairn Island?

Goodbye, dear RHESSI. You are gone but these Nuggets will continue as long as your data yield interesting discoveries.

Facts about RHESSI's Re-entryRDF feed
RHESSI Nugget Date17 April 2023  +
RHESSI Nugget First AuthorPascal SAINT-HILAIRE, Albert SHIH, and Hugh HUDSON  +
RHESSI Nugget Index447  +
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