RHESSI's Re-entry

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Nugget
Number:	447
1st Author:	Pascal SAINT-HILAIRE
2nd Author:	and Hugh HUDSON
Published:	April 17, 2023
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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 SATVIEW and others also are watching.

Orbit predictions

             CURRENT PREDICTION 00:30 UT 20-APRIL-2023 (+- 30 HOURS), ORBIT ?????

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. The final stages of re-entry are uncertain in detail because of unpredictable fluctuations in the drag force from Earth's upper atmosphere.

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.

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.

What will the observer see? Most elements have distinctive flame spectra colors, and germanium 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 refractive, may even survive re-entry! Do not stand underneath....