Minerals crashing in the sun, emigrating cult, and frozen man

I'm looking for a book, that I read 10 years ago. But the book is of course way older. It contained 3 stories, which have a common main charater. I'll try to remember of as many details as possible. The book was in German, but hopefully it was a translation and somebody here knows it.

First story: Minerals crashing into sun

The first story started with mining minerals on Pluto. Only two persons work on Pluto. Their job is to monitor the machines, which are mining. One transport ship arrived (with two man), which collect the minerals and take them to earth (or mars, which was inhabited I think).

During this transport there's a radioactive accident, the two people on board flee with a rescue ship. When they finally managed to contact earth, the mining cooperation is angry (millions of money is lost), and the mining cooperation tries the rescue the minerals. And this is the job of the main character.

As it turns out the ship with the minerals are on the way directly to the sun and they have to make a risky stuff to prevent this from happening. Not sure, if they had success.

Second story: Emigrating cult

There's a cult that tries to emigrate to another planet. The planet is about 2000 (?) years away. So they build a gigantic ship, with lots of nuclear power plants, fields for planting, and so on. A few thousand (?) people started this mission. After they started the earth government calculated, that such a mission is far to risky and try to stop them. The main character and a second one fly after the ship with a little one, and convince the cult leaders, that they also want to join the mission. On board they try to persuade the people to turn around.

Third story: Man from Neptun (?)

Humans are the only race in these storys. There are no aliens or so. Therefore there was quite a big excitement and fear on earth, when a unidentified spaceship turned up at the radars. As it later turned out, it was a spaceship from earth, but it was very old and didn't had the common identifying radio signals. The ship was lost on a mission to neptun (?).

The rest is only very vague in my memory and I think I'm mixing a few other stories. I think there's one (or more) person frozen on board and the main character has to go on board and he finds them.

Other things

The stories play in our universe a few hundred years in the future.

The main character has a girlfriend and even marries her. I think for one of the three missions he has to cancel their honeymoon.

There were a few super computers on earth. The main characters used them a few times to compute different stuff, for instance for computing the routes for the spaceships. During the second story he asked the computer to tell him everything important about the cult in 10 minutes and the computer prepared a 10 minutes documentation film for him.