There are many space probes that flew past asteroids, some came close to it, some orbited and some landed on it and took samples but those asteroids belonged in the main belt. Now, Lucy will become one of the two space probes that is set to explore the Jupiter Trojans. It was planned in 2017 and is set to launch in October 2021. But why it took so long to plan a mission for Jupiter Trojans? Most of the previous asteroid missions were planned and executed in the early years of 21st century and even flybys were done by Galileo and Cassini. The success rate of asteroid flyby and exploration missions were quite high including asteroid mining missions considering how difficult those missions were. So, why exploration of Jupiter Trojans were not planned in those previous missions? Is it due to their complex orbits? Or due to financial reasons?
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$\begingroup$ My guess: finite money restricts number of missions, asteroids closer to Earth were more interesting because they've been studied more by ground-based systems and they may have similarity to asteroids that can hit Earth. Lucy will use conventional chemical propulsion so it wasn't waiting for electric propulsion to mature. Currently unanswered: Where can I read about Lucy's complete propulsion system? $\endgroup$– uhohAug 15, 2021 at 5:52
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5$\begingroup$ Relevant comment by @EricLipptert: space.stackexchange.com/questions/43326/#comment139672_43326, "The question "why has no one done X?" is a strange question; we are not required to give you reasons to NOT spend billions of dollars. Stuff gets done when there is a reason to do it, not when the reasons to not do it are all exhausted. There are infinitely many things you could have done this morning, and you don't expect people to ask you why you didn't do all of them." $\endgroup$– David HammenAug 15, 2021 at 8:55
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$\begingroup$ It was @EricLippert, not EricLipptert who made that comment. My fingers get shaky sometimes. $\endgroup$– David HammenAug 15, 2021 at 20:24
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2$\begingroup$ There have been no asteroid mining missions. There have been sample recovery missions, and asteroid redirection missions, but no targeted resource extraction missions. $\endgroup$– notovnySep 25 at 10:29
1 Answer
The first asteroid flyby missions were secondary missions: on its way to Jupiter, Galileo flew by a few asteroids that were close to its trajectory to Jupiter.
The first mission that changed its course to get close to an asteroid was Clementine, which was planned to do a flyby of 1620 Geographos. Geographos got very close to Earth.
The next missions were all near-Earth asteroids. It's much easier to get to a near-Earth asteroid than to get to Jupiter.
Ulysse Carion's "subway map" demonstrates this:
This image shows you need to change the spacecraft's speed by 9400 + 3210 + 3360 m/s is 15.9 km/s to get to Jupiter, while an Earth escape to get to a near Earth asteroid takes 9400 + 3210 is 12.6 km/s. This means you need a bigger rocket for a Jupiter trojan mission than for a NEA mission. The long transit time to Jupiter also makes the mission more expensive.
In general, the first mission to a new type of target is designed to minimize cost and risk, so you pick a nearby target that's easy to reach. Once the concept has been proven, bigger, more expensive missions are designed to reach targets farther away. We explored Mars before we went to Saturn, for instance.
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$\begingroup$ Any idea why the chart shows such a high value (27000) for reaching Venus' surface? Seems out of line with the numbers for the other terrestrial planets or Titan? $\endgroup$ Sep 27 at 16:39
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$\begingroup$ Yeah, that number seems incorrect. Venus escape speed is 10.3 km/s (nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.html) $\endgroup$– HobbesSep 27 at 18:50
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$\begingroup$ Stranger still: a completely different map also has 27 km/s: upload.wikimedia.org/wikipedia/commons/9/93/… $\endgroup$– HobbesSep 27 at 19:52
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$\begingroup$ I think that number is in km/h instead of m/s. Converting gives a value of about 8 km/s which sounds about right. $\endgroup$– HobbesSep 28 at 9:49
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$\begingroup$ It's an estimate of the delta-v needed to lift off from the Venusian surface to low Venusian orbit, if you were trying to calculate total delta v of that portion of a trip back to Earth, including atmospheric effects. Landing on Venus, the map assumes you'd be aerobraking and ignore that value. $\endgroup$– notovnySep 28 at 20:26
