The Explorer’s List


Neptune and Triton, seen by Voyager 2. Image credit: NASA, via APOD.

I’m interested in space exploration. (You’ve probably worked that out by now, right?) I thought it might be interesting to compile a list of places that we have (in some sense) ‘been to’. I’ve ranked the objects in the list by the degree to which they’ve been explored. So something visited by human beings outranks something orbited by a probe, which in turn outranks something just flown past. Note that the categories aren’t exclusive – objects that have had landers on them have also very likely been orbited or flown past too.

  • Explored by actual human beings:
  • The Moon
  • Landed on by remote probes:
  • Venus – the Soviet Venera missions
  • Mars – many (Viking, Pathfinder etc.)
  • Jupiter – the Galileo mission dropped a probe into Jupiter’s atmosphere.
  • Titan – the Huygens lander, returning our only surface photo of an outer-system body.
  • The asteroids Itokawa and Eros.
  • Fly-bys and orbits:
  • Mercury – Mariner 10 and now Messenger (note that Messenger has entered orbit around the planet)
  • Jupiter’s Galilean Moons, Io, Europa, Callisto and Ganymede – the Galileo probe again
  • Saturn – Poineer 10 and 11, Voyagers 1 and 2 and Cassini
  • (Also arguably various Saturnian moons, depending on what you count as a flyby – Cassini and also Voyagers 1 and 2 again)
  • Uranus – Voyager 2
  • Neptune – Voyager 2
  • Also various asteroids.
  • Scheduled flybys
  • Pluto/Charon – New Horizons, estimated for 2014
  • Vesta – Dawn mission, due 2011
  • Ceres – Dawn mission, due in 2015

And that’s it, as far as I know. Everything else is purely telescopic.

The list is interesting as much for its gaps as its items. It’s no surprise that the outer solar system isn’t too well explored. Uranus orbits twice as far out as Saturn, after all. However, on the face of it, it does seem rather surprising that Mercury has only been visited twice. There’s actually good reason for this – it’s because the planet is sat deep in the Sun’s gravity well, where manoeuvring is vastly more fuel-expensive, and the Sun is much more likely to snag an unfortunate probe off course. The upshot is, it’s actually cheaper to send a probe to Jupiter or Saturn then it is to send one inwards to Mercury, weirdly enough.

Also, this shows just how little we have explored even our own solar system. If it’s still possible for one human being to knock out a list like this, in the space of half an hour or so, well … draw your own conclusions! I’d like to see us add at least one more planet to the first category within my lifetime, but whether that will happen or not, who knows?

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