When we discuss interplanetary or interstellar missions, it is very difficult to say what is feasible and what is not - the question needs to be qualified. Are we talking about robotic or crewed missions? Flyby or orbital? Small spacecraft or large spacecraft? Lightly or heavily instrumented? The assumptions you make at the start will affect everything from the vehicle configuration layout to the performance and eventually that most important issue of cost.
A “Reconnaissance” class probe such as the Voyager spacecraft has a mass between 0.2-0.8 tons and is intended for a flyby encounter with a limited science return. An “Exploration” class probe such as Galileo has a mass between 0.75-1.5 tons and would usually include an atmospheric re-entry probe with the main orbiter. A “Laboratory” class probe such as the Viking spacecraft would have a mass typically greater than 1.5 tons and include an orbiter and robotic lander.
The US space agency, NASA, categorised three major space mission programs with different vehicle architectures and mission capabilities. These were Discovery, New Frontiers and Flagship. The “Discovery Program” was a series of low cost (a few hundred million dollars) but highly focused scientific space missions with emphasis on a vision for “faster, better, cheaper”. These were privately led missions where the development time to launch was limited to 36 months. This program included missions such as NEAR Shoemaker to study an asteroid, Mars Pathfinder to explore the surface of Mars, Deep Impact, a comet impact experiment, and Stardust, which collected dust samples from the tail of a comet.
The “New Frontiers Program” was a series of medium cost highly focused scientific exploration missions of the solar system that were not to exceed $700 million. Proposals were open to the international community and included missions such as New Horizons mission to Pluto, arriving later this year, and a forthcoming mission to Jupiter, Juno. All planetary objects in the solar system were potential targets, with the exception of Mars, which was explored by other programs.
The “Flagship Program” is a series of NASA missions, the largest and most costly. They cost between $2 and $3 billion. These are complex missions with a particular focus on establishing the limits of habitable conditions within different environments. Such missions include atmospheric re-entry probes and landers. An example of such a mission is the proposed Europa Jupiter System Mission, which was due to explore the potential habitability of the moons in orbit around Jupiter. These missions are vital for understanding exoplanets because, even though we’ve mostly found gas giants and "super Earths", we assume that most solar systems have a wide variety of planets, just like ours. It’s not because we’ve found "hot Jupiter"’s that we think these missions will add to our knowledge, but because we assume these hot Jupiter’s are just the first traces of larger planetary systems.
What is needed is an extension to these programs, which we might call the “Boundary Program”. Such missions would be launched to explore the outer solar system, heliosphere, Dwarf Planets and the Kuiper belt. These would be interstellar precursor missions, pushing our technology forward. Such missions can probably be completed with costs between $3 and $5 billion, and would be a major stepping-stone in our advance out into the cosmos. This should be one of the future aims of national space agencies. To cut the costs, missions within the Boundary program could be split between NASA, ESA and the Chinese and Russian space agencies. More expensive, $5-$7 billion, missions under an “Outer Limits Program” could go even further. Into the Oort cloud, exploring the cold objects that are there and perhaps rich sources of deuterium as a potential fuel source, and other elements which are scarce in the inner solar system but which might be vital to practical interstellar mission.
So what would such missions look like? I suggest three candidates as part of a “Challenge Mission” that could be launched within the next few decades using propulsion technology that is already proven. These missions follow on from each other and build upon the lessons and technology of the previous missions.
Challenge Mission 1 (150-200AU)
Several spacecraft to be launched towards the outer part of our solar system with a flyby of Pluto and going beyond Voyager and Pioneer. They would confirm the measurements of these historical probes whilst yielding new discoveries in the solar heliosphere and into the Kuiper belt. This could be accomplished in a 15-20 year flight time with probes launched between the years 2020-2025.
Challenge Mission 2 (200-600AU)
An extension of Challenge Mission 1 allowing the technology to be pushed forward, utilizing engines with a high specific impulse. Approaching the gravitational lensing point of the Sun and thus permitting an unprecedented level of observational detail at both optical and radio wavelengths. This dedicated mission could be accomplished in around a 20-30 year flight time with probes launched between the years 2030 and 2035. Challenge Mission 3 (1,000 AU): A dedicated mission to the outer boundary of the gravitational lens point. It would require 30-50 year flight time with probes launched between the years 2035 and 2045.
These ambitious missions are within our grasp, and spacecraft designers would love to be involved with such bold programs. But what stops them? Is it money? Is it political will? Possibly a combination of the two. What we can say with certainty is that whenever we launch a mission out into deep space, we learn more science, more about what is out there than we thought, and that the act of doing so improves our capabilities to do more. This becomes a form of self-fulfilling prophesy. Perhaps we need a modern day Columbus to convince our political masters that the passage across the ocean of space will bring great rewards and, no doubt, many we could not even begin to predict.
Executive Director i4iS