I'm trying to come up with a scenario involving a mining base built on a planet that may only be approached from the shadowed side; the star (or other body) allows only that narrow lane of approach. The base is in a fortified bunker to protect it when it faces the star.
The question is: What phenomenon would realistically limit approach without melting the planet or otherwise making the mining base impossible?
Have the planet orbit very close to a Pulsar or some other exotic star.
A neutron star is very small (few Km in diameter), for this reason your planet would have a very large shadow, thus making the planetary approach more feasible. The planetary mass would provide shielding from most of the star solar wind and radiation (at least in the shadowed area).
You can go more extreme and use another star like object to create a stronger justification for travelling in the shadow. Like a quark star, an antimatter star or a strangelet star. Your planet would be perfect for housing a scientific research station for studing the unusual star!
If the planet was pretty close to the star that could do it, such that ships outside of the umbra/penumbra would be hit by the full force of the solar radiation and thermal energy.
That is kind of a problem though, because the umbra is kind of a small area.
You would have to get pretty close to the planet before you could get into it's shadow.
Factors that determine how big the umbra is are the distance of the planet to the star, the size of the star, the size of the planet.
You might have to have another way to shield the ship until you get into the shadow, such as a solar parasol ship, like a giant umbrella that reflects the energy that it can, and with huge amounts of cooling to keep it from being consumed, that could ferry other ships to the planet.
If you take Mercury as an example, the surface temperature of the planet is somewhere around 700 K, and its black body temperature is about 400 K. The planet is far more of a black body than your hypothetical spacecraft would probably be, but needless to say you would not want to expose your ship to those kinds of temperatures for long.
As AndyD273 was kind enough to demonstrate, there are several varieties of shadow behind a planet, only one of which is full enclosure, but all three would allow some protection from solar radiation.
What you could do is say that the mining base is on the dark side of the planet, and approaching ships try to approach from within the shadows to minimize heating problems. This might also make for interesting drama if the ship has to make a break for it straight into the direct sunlight for some reason!
I was thinking about this, and it occurred to me that the only kind of substances one might be willing to mine in an extreme environment would be if they were extremely rare minerals made of stable transuranic elements or unlikely isotopes made stable by currently undiscovered supersymmetric particles. These kind of elements might only be made in highly energetic events, like the merging of two neutron stars, or possibly some kind of event with a black hole. The planet itself might be accreted from byproducts of the primary, making it not only necessary to approach the planet in its shadow, but to be very, very careful where you step.
I think it would be best to have a single primary, because you need the planet to be tidally locked.
It might be interesting to have a primary body that's almost, but not quite a black hole. It would be a condition that might make highly unlikely things be a little more likely.
It sounds like it might be a fun read. It also seems like you'll need to do some serious research to make it a serious science fiction story..
Having read these answers, consider a binary star system. One star's radiation is manageable, but two stars is overload. Therefore, you may only approach the planet when one star occludes the other. Of course, to get to the planet, you would have to travel in the plane of orbit when the stars and planet line up. This will necessarily be in the planet's shadow as you get close, but it won't be the planet itself which protects your ships.
maybe its a pulsar and I would say the phenomena IS melting the planet. its just taking a while (planets are small compared to stars but they are still big). In the interim its worth picking up the 'rare mineral' being created. You are travelling to another star so have FTL. The only safe place to 'emerge' is in the planet's shadow (as deep in the umbra as possible) and you can only land / take off / mine while the underground base is in the shadow
If the planet is near enough to its primary, it will get tidally locked and your base will either be always in the daylight or always in the shadow.
Also, if the planet is not locked and rotates, and the surface is so lethal, it's unclear how the base could get built in the first place.
You could try and solve these issues by positing that the planet is a recent addition to the solar system (it would need to have been captured by a very rare three-body interaction with one of the existing planets).
This yields some interesting consequences:
the planet can optionally be on a unstable or very elongated orbit, which implies long and harsh winters, impossibly hot summers, and even a limited life expectancy before it drops in the star or is further destabilized by another encounter with one of the planets and gets smashed, consumed by the star, or ejected from the solar system altogether. This can justify a time window of anywhere from a few days to several centuries before the planet is no longer mineable.
the planet's surface can have an interesting composition. Let's imagine a gaseous planet that got its atmosphere blown off by a nova (or supernova), becoming an ejected cinder covered by condensing volatiles from the outer rim of the planetary nebula.
You would get a very large snowball with a possibly very valuable core, which rotates not too rapidly around a star, melting on one side and resolidifying on the other. This means that the surface can be reached at any time, but landing is only possible during the night. The base would be some sort of submersible that resurfaces in the evening before night freeze, and resubmerges just before dawn, and has been simply "dropped" on the planet.
Whatever the planet's original star's death has deposited on the solid surface, under several hundred meters of ice, has better be worth the trouble.
Our cinder might have begun its existence as a loose aggregate of rocks inside the equivalent of a Kuiper Belt Object (KBO) around a massive star massing some 25 Sols. The star expansion during the carbon-neon burning stage would have stripped our KBO and left the core exposed. Then, in the last week of its life the star would have burned silicon, increasing its temperature to two billion Kelvin and irradiating the KBO with an enormous neutrino flux, pushing it outwards a little in its orbit and initiating all kinds of weird nuclear reactions. In the final explosion (core temperatures in excess of 100 billion K), the neutron and neutrino flux and the inflow of exotic materials could have allowed a runaway nucleosinthesys process to reach the fabled 'island of stability' creating a long-lived heavy transuranic (henceforth unobtainium), imbued with all sorts of useful properties. The synthesis of such a substance, while possible, would be ruinously expensive.
The particular radioactive spectrum of the cinder, revealing it to be a close witness of a supernova explosion, would then explain why someone went to the trouble of dropping an unrecoverable mining base on an almost-inaccessible planet.
If you have some kind of stellar drive involved in the plot, you could have it depend on unobtainium quantum flux capacitors. At that point, whoever owns Cinder and its (relatively) cheap unobtaiunium would automatically own galactic transportation and economy.
Since radiation intensity falls off with the square of the distance, it's entirely possible that your story's spacecraft have heat/radiation management systems sufficient to deal with the star's radiation so long as they're at least a certain distance out, but not sufficient to deal with the influx at the planet's current orbit. Under those circumstances, building on AndyD273's answer, staying in the penumbra would be increasingly desirable as one approached the planet (and the star it was orbiting).
Eventually, per the premises of the question, the penumbra is no longer sufficient, and the approaching craft must stay within the umbra to avoid overheating - or, if you're feeling particularly dramatic, must get to the umbra before the heat buildup becomes lethal.
The base itself, meanwhile, may have access to nifty heat-management techniques unavailable to spacecraft (at least during its approach), such as "atmosphere" or "oceans". For all we know the planet itself might well be habitable, just a little close to its star for your spacecraft. Especially if your spacecraft were owned by stingy poor economical captains unwilling to pay for the considerable extra expense of cooling systems that are only really useful when approaching a certain handful of inhabited planets, anyway.
The mining base is of questionable legality or must stay hidden for other reasons. The prevailing technology level in the system is sufficiently low that approaching in shadow is an effective means of keeping the powers that be from noticing.
