This is the second most common hazard, and the least threatening. Asteroid and debris fields are large clouds of immobile rocks. Asteroids cannot harm you even if you crash into them, although if you hit one you may be disoriented by the sudden jolt. Asteroid fields often contain other hazards, though, so watch out.
Asteroid fields always contain small free-floating rocks which appear and disappear all around you. In mineable zones, these have a chance of dropping naturally-occurring Commodities when you blast them. Depending on the field, these can range from Scrap Metal to Diamonds.
Warning. Sensor efficiency reduced.
Nebulae are simply large clouds of gas. In most of them, your sensor range is reduced to only a couple of kilometers, as noted by the message. As with Asteroids (with which they are often intermixed), these tend to contain other hazards.
Nebulae are frequently lit up by lightning-like flashes. This is a good thing, since the flashes silhouette objects that are distant enough to be hidden in the fog. Useful if, say, you just beat up some pirates but lost sight of your Trade Lane in doing so.
Danger. Mines detected.
Minefields are basically artificial Asteroids, with the notable difference that these asteroids blow up at you. You cannot clear minefields, because the mines can and will explode repeatedly. Some mines, such as the Mollys' field in the Dublin system, are fairly weak and can be survived or even shrugged off with a strong enough shield. Others, such as the ones in Zone 21, are extremely powerful and will murder you no matter what. It's generally wise to drop out of cruise speed in minefields, as this gives your ship a better chance of avoiding the mines' detonation zones.
NPC patrol routes will often pass through minefields, with the ships flying heedlessly to their destruction. If the opportunity presents itself, it can be entertaining to lure enemies into minefields and then come to a stop and watch the fun.
Danger. Volatile gases detected.
These are basically naturally-occurring Minefields. Like mines, they occur in discrete patches that explode if you go near them.
Danger. Radiation damage detected.
This is easily the most annoying space hazard. Radiation fields will slowly but surely inflict damage on your hull and equipment, requiring nanobot use and expensive repairs. Your shield will not protect you. The strength of the radiation varies between locations, even within a single field. In some places, the damage is barely noticeable; in others, such as the Omega 41 and Omega 11 systems, it can destroy you shockingly fast.
Dark Matter Nebulae are all very radioactive. One nebula, the Giftnebel ("poison fog") in the Dresden system, will chemically corrode your hull, but this is really just a radiation hazard dressed up differently. Despite what you'd think, carrying MOX does not inflict radiation damage.
Danger. Entering sun corona.
These are spherical regions of space surrounding all planets. If you disregard the automated warning and fly into an atmosphere, you will be instantly destroyed. To land on a planet you must use the docking ring.
The sun corona is basically a very large atmosphere surrounding the star. It is large enough that there is a significant chance for a mission target to spawn inside it. Depending on how deep it is, this may prove to be either frustrating or hilarious.
Given a chance, your ship will automatically avoid atmospheres. If you ask it to dock with a planet, it will go to the docking ring; if you give it a waypoint on the other side of a planet or sun it will go around.
I'm engaging Freelancer Alpha One-dash-One!
This is the most common, most interesting and often most dangerous hazard. Hostile ships are found everywhere in Sirius, limited only by your list of enemies. They range from the laughably pathetic Xenos of Liberty, all the way up to the fearsome Nomads. You can fight or you can flee, although some enemies will just zap you with a Cruise Disruptor if you try to flee. Depending on your play style, you might be dreading the sight of hostiles, or you might actively seek them out as a source of loot and Weapons. For that matter, hostiles are the reason you want weapons in the first place.