A book about a group of people travelling to another star on a one way trip. They are boosted out of the solar system by re-purposed military mirrors. They have a robotic helper that looks like a Christmas tree. There are sequels that detail their exploration of the star system.
This is Robert Forward's "Flight of the Dragonfly", which was later expanded and retitled "Rocheworld", published 1985.
Dr. Forward actually invented the interstellar mirror and laser system in his AAAS paper AFAL TR-87-070, Advanced Space Propulsion Study - Antiproton and Beamed Power Propulsion, Appendix A - Beamed Power Propulsion To The Stars.
According to the introduction to the novel, the idea of a Christmas bush robot was "jointly conceived" by Forward and Austrian roboticist Has Moravec.
The "hands" of the Christmas Bush have capabilities that go way beyond that of the human hand. The Christmas Bush can stick a "hand" inside a delicate piece of equipment, and using its lasers as a light source and its detectors as eyes, rearrange the parts inside for a near instantaneous repair. The Christmas Bush also has the ability to detach portions of itself to make smaller motiles. These can walk up the walls and along the ceilings with the tiny cilia holding onto microscopic cracks in the surface. The smaller twigs on the Christmas Bush are capable of very rapid motion. In free fall, these rapidly beating twigs allow the motile to propel itself through the air. The speed of motion of the smaller cilia is rapid enough that the motiles can generate sound and thus can talk directly with the humans. Each astronaut in the crew has a small subtree or "imp" that stays with him or her to act as the communication link to the main computer. Most of the crew have the tiny imp ride on their shoulder, although some of the women prefer to keep theirs in their hairdo. In addition to acting as the communication link to the computer, the imps also act as health monitors and personal servants. They are the ideal solution to the perennial problem of spacesuits ... scratching an itchy nose. The imps go into the spacesuit with the humans, and more than one human life was saved by an imp detecting and repairing a suit failure or patching a leak. In fact, there are two computer motiles with each suited human. The personal one that stays with the human, and the spacesuit motile that stays with the suit. This motile is usually outside in the life-support backpack, but can worm its way inside through the air supply hose