After seeing the discussion of spacecraft within our solar system in the textbook, and the impact the mission type has on the cost, I was curious to see how these discussions were reflected in actual data on these missions. So, I copied each mission from the book into Excel, and then researched each. Thanks mostly to Wikipedia and the Google search “How much did [MISSION NAME] cost?”, I came up with this table. Feel free to skip over it – the rest of the blog is more interesting.
|Name||Launch Year||Cost (inf. adj.)||Launch Mass (probe)||Type||Planet Reached|
|MESSENGER||2004||$504 Million||1110 kg||Orbiter||Mercury|
|Magellan||1989||$1155 Million||1035 kg||Orbiter||Venus|
|Venus Express||2005||$128 Million||1270 kg||Orbiter||Venus|
|Spirit||2003||$1094 Million||1063 kg||Rover||Mars|
|Opportunity||2004||$1600 Million||1063 kg||Rover||Mars|
|Mars Express||2003||$472 Million||1120 kg||Rover||Mars|
|Mars Recon. Orbiter||2005||$928 Million||2180 kg||Orbiter||Mars|
|Phoenix||2007||$469 Million||670 kg||Lander||Mars|
|Curiosity||2011||$2798 Million||3839 kg||Rover||Mars|
|MAVEN||2013||$725 Million||2454 kg||Orbiter||Mars|
|Mars Insight||2018||$832 Million||694 kg||Lander||Mars|
|Voyager 1||1977||$1797 Million||825.5 kg||Flyby||Neptune|
|Voyager 2||1979||$1797 Million||825.5 kg||Flyby||Neptune|
|Galileo||1989||$2766 Million||2562 kg||Orbiter||Jupiter|
|Cassini||2004||$4336 Million||5712 kg||Orbiter||Saturn|
|Juno||2009||$1231 Million||3625 kg||Orbiter||Jupiter|
Then, with all the boring data out of the way, I could make (relatively) pretty graphs to see how each part of the mission related to cost:
Cost and Mass
The most intuitive finding of this data is that more massive probes are (quite literally) exponentially more expensive:
This makes sense. Here, “mass” refers to the mass of the final probe (including propellant for any maneuvers once it’s in orbit). So, a heavier probe would need a bigger rocket to bring it to space. But this is constrained by something called the rocket equation, which (in very rough terms) says that as you increase the final mass of what you’re bringing to space, the total mass of your rocket has to increase exponentially. So it makes sense that, the more massive your final probe already is, the greater the marginal cost of another kilogram.
Cost by Probe Type
The book suggested that flybys are less expensive than orbiters which are less expensive than landers (and, presumably, stationary landers are less expensive than rovers). In practice, this isn’t reflected in the actual costs of these missions.
Of course, none of this shows the inherent cost of each mission type. But it may indicate how much priority different projects are given. A rover will be driving all over a planet’s surface, so it makes sense to have a lot of gadgets to comb through all that data. And if the best you can do is fly by a planet, you only have one chance to collect a lot of data, so you need to make it worth it (perhaps with very expensive equipment). Landers, meanwhile, can’t see as far as an orbiter, so they can only do a handful of experiments.
Cost by Launch Year
The book didn’t discuss this directly, but I was curious if the price (adjusted for inflation) of missions within the solar system had come down. It has.
Excel’s trendline suggests that (after adjusting for inflation), the average cost of a mission within the solar system comes down by about $22 million every year. But looking at the data, a lot of this seems to be caused by the missions from 2000-2010. It’s hard to say how much of this “trend” is caused by changes in technology vs. changes in available funding vs. changes in mission designs.
Cost by Planet
We’ve sent missions to every planet in the solar system. I was curious which were most expensive.
Overall, missions to Saturn have been the most expensive on average. It’s not hard to come up with an explanation for why this could be. We’ve sent far more missions to Mars, and it’s cheaper to get back there. That means it may not be worth sticking everything you can on the ship and increasing the price. But with Saturn, it takes a lot just to get the rocket there. So if you have something going there, then everything to make the mission better – every extra hour of design, every special instrument to get more knowledge, every unique material to bring down the mass – becomes worth the cost. This doesn’t explain the trend in the opposite direction – that Mercury’s missions are cheaper than Venus’s, even though Venus is closer to Earth. That could be because we aren’t as interested in Mercury as we are Mars or Venus, but I really couldn’t say.