Yohkoh's Troubles

Science Nugget: December 21, 2001

Introduction

On December 14 Yohkoh stopped taking scientific data, and its recovery remains problematic (see the ISAS statement issued December 18). This science nugget explains further what we know about the present condition of Yohkoh. In a nutshell, it is a situation very similar to that experienced by SOHO in 1998, from which a nice recovery could be made.

The accident itself

The immediate cause of the accident was the deep solar eclipse experienced by Yohkoh on December 14, as shown in Figure 1.

Figure 1. The trajectory of the Moon as seen from Yohkoh during the eclipses of December 14. The multiple looping behavior results from the fact that Yohkoh's orbital period, 96 minutes, is less than the time it takes the Moon's shadow to traverse the Earth. The circles show the Sun's location. Asterisks on the Moon's track show when Yohkoh was in sunlilght, and are spaced two minutes apart.

The central eclipse clearly was quite deep, nearly total; the probability for this happening is about the same as for totality at any given spot on the Earth. This was the first of the improbable factors leading to the predicament Yohkoh now finds itself in.

Yohkoh's soft X-ray telescope SXT captured images of the eclipse, which we show in movie form, with image spacing 32 sec and exposure time 1.34 sec. Just at the moment of near-totality, around 20:58 UT, responded to the loss of its "sun presence" signal by plunging into its attitude-control "safehold" mode, resulting in a slow drift (roughly 0.001 RPM) away from the nominal Sun-center pointing direction. This has happened on several previous occasions; the safehold mode functions as its name implies, simply to hold the spacecraft in a safe condition. The movie shows the event dramatically (note that individual frames of the movie are at spacings of half a minute, so the angular motion entering safehold was perhaps not so large as it seems). As the spacecraft slewed away from the Sun, commencing a roll motion that eventually reached about one RPM around its Y axis (solar EW), SXT continued to make images. The initial "kick" of motion inferred from the movie images would be about one degree per two minutes around the Y axis The partial-frame images included the novel exposure shown in Figure 2, showing the chance X-ray illumination of an SXT filter far (a few degrees) off axis.

Figure 2. A Yohkoh partial-frame image captured several degrees off-axis. Some random unknown path has illuminated the CCD through one of the X-ray analysis filters, and this gives a shadow pattern of the mesh needed to support the thin metal filter.

How it happened

Briefly, the deep eclipse upset the Yohkoh attitude control system during an early "invisible orbit," i.e. one for which uplinks could not be made from the Kagoshima Space Center. The satellite then drifted off its proper Sun-center pointing, as shown in the movie. The solar panels were not properly illuminated, and gradually the batteries drained to a relatively low state. Contact from KSC was re-established hours later (06:50 UT December 15), but attitude control was lost and the low-voltage state has persisted, even though the instruments were successfully switched off to minimize the load on the batteries.

A full analysis of the accident is not yet complete, but it is clear that several factors contributed. Each factor by itself would not have been dangerous, but taken together they have left Yohkoh in a state from which recovery will be difficult. One of the factors, for example, was the chance occurrence of a very deep eclipse - anybody who has traveled to a remote spot to see just such a phenomenon will realize how rare this is. Another factor was the fact that this eclipse happened during the Yohkoh "invisible orbits", as illustrated in Figure 3. Various technical factors were involved and are under investigation now. No single factor is likely to explain the situation completely; Yohkoh had to pass through several narrow windows of probability in order to get into its current state.

Figure 3. The Yohkoh location at the time it experienced a nearly-total eclipse. The spacecraft icon can be seen over the Pacific Ocean roughly halfway between Hawaii and the Galapagos. The orbit track swoops around the ellipse showing the telemetry range of the Kagoshima Space Center at the southwest tip of Japan. It took about nine hours for the orbit to precess around far enough for command uplinks.

The current state of Yohkoh

At present Yohkoh is rotating slowly (about one RPM) around its Y-axis. Figure 4 shows the orientation, by chance from the perspective of the +Y-axis. The batteries are mounted on the +Y face of the spacecraft, and during Y-axis rotation, initially, no sunlight touches this face and the components mounted there have reached temperatures of some -40 degrees.

Figure 4. A sketch of the spacecraft, viewed from the +Y direction (and a bit of +X in perpective). The Z axis (up) nominally points towards the Sun, and the Y-axis controls the heliograph east-west pointing. At present Yohkoh is rotating slowly around its Y-axis.

We are now waiting for warmth to return to the batteries, monitoring the conditions and studying ways and means of getting the batteries charged once again, and -- if successful at that, a big "if" indeed -- then re-orienting Yohkoh and establishing normal operations. We would dearly like to do this to support HESSI and to complete one full Hale cycle of solar magnetic variability.

In the interim, our Yohkoh science nuggets will not deal with current solar affairs so directly. We have a great data base to explore, even without a full Hale cycle.