Sedna Sample Return
Even if this is not feasible with current technology, it is a very interesting subject. Is it possible to send a Sedna Sample Return mission?
To get an answer, we must look to space missions that have already been launched.
Vehicle design: The spaceship must contain an orbiter and a lander. The lander will reach the surface of Sedna, will take a sample and will return. It must have enough propellant to land and to return. The orbiter must be used as a communication relay to Earth and must have enough propellant to return to Earth. The orbiter must also have an RTG, strong enough to function up to 100 years.
For this, we should look at ESA's Rosetta and Philae spacecrafts. Sedna has a low gravity, but probably no atmosphere. So, we need propellant both at landing and at launch, but not in too large an amount. We don't know Sedna's escape velocity, but Pluto's is 1.2, 10 times smaller then Earth's. Philae had 100 kg. Our lander will have probably 250 kg, with all its fuel tanks full of hydrazine.
The orbiter will have to survey and map Sedna, then it will deploy the lander, then it will wait, then capture the lander and return home. Rosetta had 1500 to 2000 kg, but New Horizons only 500 kg. We will need the lightest spaceship, to preserve hydrazine.
The lander can be battery powered. The orbiter will need an RTG. Plutonium used in an RTG has a half lifetime of over 80 years, but the thermocouples don't live that long. Basically, the energy produced by an RTG decreases by half in about 25 years. The mission will take probably 100 years, so energy production will decrease to less then 10%. This means that, without at least 100 kg of plutonium, a sample return mission is impossible. It could be possible to use a backup empty RTG, to reinsert plutonium in it. The probe, with lander and without its fuel, must not weight more then 1000 kg.
The vehicle will include a lander, an orbiter, but also a huge amount of fuel needed to decrease ship velocity, and also fuel needed to send the probe back to Earth. We might have to send into space a total of 5000 kg.
Launch. The only way to send into space such a large spaceship is with the use of some of the largest existing rockets. An Atlas V would hardly be enough.
Trajectory. Sending the probe directly into solar escape velocity is out of question. We have to use the advantages of Jupiter and possible Saturn flybys. If possible, a Mars or Earth flyby are also useful. This will also result in a longer time to reach Sedna. It will take at best 40 years.
Sedna Approach Phase. The probe will approach Sedna with high speed, probably 15 km/s. At that speed, it is impossible to be captured into orbit. We have to decrease speed somehow. Using conventional chemical engines is out of the question, because of the huge amount of fuel needed. The only solution will be a ion engine. As Dawn have shown us, with 400 kg of xenon, it managed to produce a total thrust close to 10 km/s. So, it is possible, if the RTG produces enough energy, to decrease speed from 15 km/s to 1 km/s in 12 years.
Bonus Ship. One extra mission that can benefit from a Sedna encounter is a fast flyby. We can add a 3rd spacecraft, with its own RTG and suite of science instruments, that will detach from the main ship before slowing down. The bonus ship will continue to move with the same speed and will soon reach the interstellar environment. Its mass can be of 300 kg, not much compared to the 5000 kg already needed.
Sedna Orbit Phase. Once in orbit, the probe will first enter a high-altitude orbit, to search for moons and characterize Sedna system. Just as Dawn did at Vesta and later at Ceres, our probe will enter closer orbits, mapping the dwarf planet and in the final phase, looking for a place for the lander. Finally, it will deploy the lander who will have a few days to touchdown, to explore the surface, to drill, to take samples and to return. To return to the orbiter, the lander will need to perform high-accuracy maneuvers. The orbit phase will probably take 5 years to complete.
At the end of this phase, the spacecraft will need to detach all unnecessary weight. This might include much of its scientific payload, empty propellant tanks, the lander (except for the tank containing samples) and used RTG.
It is also possible to make the whole spaceship land on Sedna, without the need of a special lander. However, this will result in more fuel needed.
Return phase. Sedna has a low gravity. As shown above, the escape velocity for Pluto is a bit above 1 km/s. For Sedna, it is probably 0.7. The orbital speed is only 1.07 km/s. so, a spaceship will only need a 2 km/s thrust to detach Sedna's surface and to get to a 0 km/s speed relative to the Sun. At that point, solar gravity will do all the work and will take the ship on a trajectory towards the Sun. The resulting orbit will not be a straight line, but something looking more like the path of a long-term comet.
As the probe approaches, its speed will increase gradually. It will move very slowly up to the orbit of Neptune. To speed-up the process, almost all the remaining xenon should be used by the ion engines to increase speed. If not, it could take 400 years to reach Earth. Nobody wishes to wait that long. So, we must accelerate the ship to at least 10 km/s.
As the probe passes the orbit of Neptune, its speed increases fast. At Earth's orbit, the spaceship can move with 30 km/s faster then Earth. It can become impossible to land on Earth with such a huge speed. To slow down, the best option is to use a Jupiter flyby, and possibly other flybys.
Earth Landing. The final phase of the mission is an Earth landing. At that time, after almost 100 years, the RTG will produce only a very small amount of power. Many devices will not be working. Maybe, it will be a good idea to have onboard a small solar panel, to provide the power needed for the final phase of the mission. The spacecraft will conduct its final trajectory correction maneuvers, then it will detach the tank containing samples from Sedna. Only the samples will return, softly landing with the help of a parachute. The spacecraft will enter a solar orbit or will burn in the atmosphere.
Mission Conclusion: After 100 years of travel and with great costs, the mission will return to Earth a sample from Sedna. Probably there will be samples from the surface, materials drilled from a few meters deep and an atmospheric sample (if Sedna has any). Also, the probe can bring dust samples acquired along the journey.
There is one major problem about this mission. On Sedna, many solid rocks can be in fact gasses on Earth. We know from Pluto, that its crust is made almost entirely of substances that on Earth should be gasses or liquids. Rocks brought from Sedna should melt or even evaporate. In the lab, scientists will have big problems analyzing the samples. The long needed time and degradation of the samples, together with high mission costs, are the reasons why a sample return mission to Sedna is unfeasible.
P.S.: No spacecraft has flown for that long and nobody has ever designed complex electronic devices able to operate for 100 years. A mission to Sedna requires that all devices will still be functioning after many years. Also, for the return phase, we must make sure that navigation computers, engines, the RTG and reaction wheels are working well until the end of the mission. A sample return mission will be at high risk because we have not enough time to test all systems and sub-systems.
In addition, there are many unknown things. Nobody knows what the mass and gravity of Sedna is. We must design the ship to the highest estimated mass, to make sure we have enough propellant. And, as we don't know the exact mass, there is no way to know how much fuel will remain for the return phase. If we have more fuel available for the return phase, we can get our samples faster, but we will have to re-calculate the return route and Jupiter flyby to decrease speed.
Also, since we don't know the mass, we don't know how to enter orbit. All these not known parameters means that many parts of the mission will have to be calculated fast and only when the ship will get to a certain point. There will not be much time for estimations and backup plans. Powerful computers on Earth should decide what to do within hours.
Communication delay is big, because of distance. There will not be much time for analyzing problems. It can be a very good thing to program the ship to automatically find the best trajectory.