Now you see it, now you don't
24th June, 1998.

At 23:16 GMT on 24 June 1998, Earth lost control of the SOHO spacecraft!

Every so often we need to make small corrections to SOHO's orbit. It was during one of these orbital manoeuvers that SOHO panicked - for some reason, it thought couldn't find the Sun and so tried to find it again. SOHO needs the Sun so it knows where it is in space, like looking for a landmark when you're in a busy town or out walking.

It was while it was moving around looking for the Sun that the Earth lost contact with the SOHO spacecraft.

Gone!
Disappeared!
Vanished!

Had SOHO blown up?

If not, could we get it back under control?

No-one knew the answers to these questions. There can be any number of reasons why you can't get in touch with a spacecraft, for instance -

it may be destroyed completely
it may be malfunctioning but just exactly where you left it
it may have its antennae pointing the wrong way; it's just like a radio station (do you know what a radio wave is?) that doesn't broadcast in your direction - you're going to find it very difficult to hear anything on your radio.
or you may just be looking in the wrong place!

What went wrong?

There were only two mistakes in the computer programs (do you know what a computer program is?) used to control SOHO.....a mistake in a computer program is called a bug.

	The first mistake was in a program that told SOHO how to control its movements when looking for the Sun.
	The second mistake was in another program that measured how SOHO moves in space. The program gave the wrong output.

SOHO couldn't control itself properly - the first programming mistake - and didn't know how it was moving exactly - the second programming mistake. That is how we lost it.

What now?

Gone!	Disappeared!	Vanished!
	*Had SOHO blown up?*
	*If not, could we get it back under control?*