lost and found (SOHO & recovery)

Hopefully, no one loses a billion-dollar spacecraft and then says "Oh well, that's that, it's a million miles away and there's nothing we can do about it!" The SOHO team was no exception.

SOHO needs to know where the Sun is, not only so its telescopes can observe the Sun, but also so its panels of solar cells point at the Sun and generate enough power to keep all the instruments running.

SOHO had disappeared! Had it blown up? Had it been hit by a meteor or some other space junk? If not, could we get it back under control?

My name is Jack and I was lucky enough to be working at the NASA Goddard Space Flight Center - the place where we control SOHO from - when the Million Mile Rescue (as I like to call it) was going on. It was very exciting, even though I was nervous we might never hear from SOHO again - I needed SOHO data for my work!

STAGE 1
Find SOHO
(25th June - 23rd July)

The first thing to do was to find out where precisely SOHO was. SOHO is so tiny, it was like looking for a tennis ball thousands of kilometres away. The SOHO recovery team had a brilliant idea, they thought they might be able to find SOHO if they could bounce a radio wave off it. They asked the scientists at the Arecibo Observatory in Puerto Rico to send a powerful radio signal in the direction that they thought SOHO might be. Good news! An echo from SOHO was picked up by the radio dishes of NASA's Deep Space Network. SOHO was found again!

Arecibo radio telescope

STAGE 2
Gain control of SOHO
(24th July - 3rd August)

From the last few seconds of information received from SOHO, the engineers worked out that SOHO was probably spinning rapidly. Even worse, its arrays of solar cells were not pointing towards to the Sun, and so its batteries had been drained of all their power and were dead. Unfortunately to recharge the batteries, the spacecraft needed to re-point at the Sun. To do this it needed power to fire the jets which change its position. The situation looked hopeless! The spacecraft engineers then realised that although the solar cells weren't pointing towards the Sun at that time, if they waited a few weeks they would be.

This is because of the way SOHO was both spinning on its axis and orbiting the Sun. After a long wait, their predictions were proved correct and power began to be generated by the solar cells.

Contact was eventually re-established with SOHO on 3rd August 1998, following six weeks of silence.

STAGE 3
Thaw out the fuel and charge the batteries
(4th August - 16th September)

Scientists worked round the clock to get SOHO back under proper control. First, they had to get SOHO's batteries charged up again. Next they had to thaw out the fuel, very slowly in case any pipes had burst - fortunately they hadn't. Next the on-board computers and computer programs had to be tested and fixed. Finally, SOHO could point accurately at the Sun again.

STAGE 4
Would SOHO's instruments still work?
(16th September - 5th October)

SOHO spacecraft fully operational again, but we had no way of knowing whether the scientific instruments would still work. Some instruments, like CDS got extremely hot, above 80 °C, enough to fry an egg. Others, like MDI, had been extremely cold, at least as low as -100 °C. It may have been even colder but the thermometer stopped working because it was so cold! Miraculously, most of the instruments still worked well. In fact, some worked even better than before! The holiday had done them good. Everyone celebrated.