Question: How long does it take to travel around every planet from earth then come back to earth via the Pentagon.

Monique Henson answered on 17 Nov 2015:

It would depend how fast you were travelling. The fastest thing we’ve ever sent into space is the New Horizons probe that flew past Pluto earlier this year. That travels at around 50,000 km/h.

It would also depend on when you left Earth. All of the planets in the solar system are constantly moving. Sometimes they’re further apart – sometimes they’re closer together. For example, at their closest, Neptune and the Earth are around 4.3 billion kilometres apart. At their furthest, they’re around 4.7 billion kilometres apart. When I write it like that it doesn’t seem like a big difference, but it would take New Horizons more than 300 days to travel that extra difference.

Let’s just for a moment, assume we just want to travel to Neptune. If it’s at its closest to Earth, it would still take more than 9 years to get there (if I’ve done my very quick maths correctly) – that’s quite a long time. If you also want to fly by all of the other planets on the way, your route might get a bit complicated. It’s safe to say it would take a really long time.

It’s possible that we could make the journey time a bit shorter by travelling even faster. One way we could get a bit more speed is by getting a gravitational assist from planets as we pass by them. That’s what New Horizons did as it went past Jupiter. That’s why its path to Pluto (handily shown here) is so strange.