From MangaArt:

This is another one-volume series by Reiko Shimizu. There are 3 stories in the book, the futuristic "Noah's Space Ship", sentimental "Robot and Bride", and "Love Symphony".

Noah's Space Ship - Junior was the head pilot of the space ship. His life was endangered while fixing the ship's engine. The crew had to temporarily delete part of Junior's memory in order to save him. Junior became the 14 year old he once was. He longed for Earth, the ranch, and Mei, his little pony. He couldn't understand how he could have left all that behind and hopped on a space ship...

Robot and Bride - This is the series in which Jack made his first appearance. He was trained into a hit man and he was instructed to assassinate a political figure by getting close to the man's daughter...

Romance Symphony - A high school kid has a crush on the young and beautiful Home Economics teacher. Meanwhile, he was arranged to date and possibly marry someone...