Almost ten years after it was launched, Nasa's New Horizons spacecraft this week performed a historic flyby of Pluto, skimming just 7,700 miles from the its surface.
The US can now claim to have explored every planet (or former planet, in Pluto's case) in the solar system. But doing so wasn't cheap by next year, the New Horizons mission will have cost Nasa around $700m, with most of that money spent on "spacecraft and instrument development, a launch vehicle, mission operations, data analysis, and education/public outreach".
$700m is "pretty close to the median for space missions" according to Forbes Tech. But many of Nasa's more famous exploratory missions cost a great deal more.
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The Viking Programme $4.4bn
The twin Viking space probes were launched in 1975. They provided scientists with the first ever high-resolution photos of Mars, as well as the first images taken from the surface of the planet itself. Nasa's archives coolly state the total cost of the Viking project as "roughly one billion dollars". That puts the inflation-adjusted cost of the mission at around $4.4bn in today's dollars.
The Viking mission returned incredibly useful findings. Mars was found to be a highly varied planet: half windswept plains, half cratered highlands. The probes also discovered extinct volcanoes, prehistoric river valleys and evidence of water on the planet's surface. In fact, it wasn't until just over ten years ago that the Viking mission's findings were improved upon.
Curiosity Rover $2.5bn
In 2012, another craft, the Curiosity rover, was sent to Mars to examine the planet's chemical and biological history. That mission is still going on, and has cost at least $2.5bn to date.
Cassini-Huygens $3.3bn
The two-part Cassini-Huygens spacecraft a joint venture between Nasa, the European Space Agency (ESA) and Agenzia Spaziale Italiana (ASI) launched in 1997, bound for Saturn. Seven years later, it successfully came into the planet's orbit.
The spacecraft's maiden voyage was fruitful. Cassini, the spacecraft's orbiter, was able to confirm the length of Saturn's rotational period, measure the planet's rings, and discover seven new moons between 2004 and 2009. ESA's Huygen's probe, meanwhile, collected data on the atmosphere and surface of Titan, Saturn's biggest moon.
During the 1990s, the Cassini-Huygens programme came under sustained criticism from environmentalists uneasy over plutonium being used on the craft. Criticism of the project's cost was rather less strong, which is somewhat surprising, considering. The mission cost $3.26bn, with the US contributing around 80% (or $2.6bn) of that total. ESA paid $500bn, ASI another $160m.
