Space probe or Satellite?

In my day job, I work on instrumentation for a telescope that will be launched into space early next year. Throughout the time I've been working on it I've always had a nagging thought at the back of my mind. Am I working on a satellite or a space probe?

I assume this isn't a question that too many people think about, even in the space community. The International Space Station, the Hubble Space Telescope and geosynchronous satellites are all, well, satellites of the Earth whereas New Horizons is a space probe on its way to Pluto and beyond. Venus Express, Mars Express and Cassini-Huygens were space probes which have now become satellites of Venus, Mars and Saturn. All very straightforward, so why don't I know which I work on?

My telescope, along with several other missions, will be heading to a special place in space named L2. L2 is one of five Lagrangian points - a point where the gravitational forces of the Earth and Sun balance each other - and is located 1.5 million km away from the Earth in the direction opposite to the Sun. As for Goldilocks, things have to be just right at a Lagrangian point. Just a bit further away from L2 and you would be left behind by the Earth in your orbit around the Sun. Just a little closer and you would orbit the Earth more than once per year. At L2 the Earth gives you just the right amount of a tug to keep you orbiting both the Earth and the Sun exactly once per year. The result is that you always appear to stay in the same place relative to the line joining the Earth and Sun.

L1 and L2 mark the extent of the Hill Sphere of the Earth. As the Bad Astronomer points out, objects within this sphere, such as the Moon, can be considered to be a satellite of Earth. As Planck will sit on the edge of that sphere, and thus on the very edge of interplanetary space, that makes it almost a space probe and only just a satellite.

Perhaps I'll just stick to calling it a spacecraft.

Posted in astro blog by Stuart on Monday 29th Sep 2008 (19:08 BST) | Permalink
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