How to move in space with just solar panels or without them?

Question

What are the technologies known to humankind, to do propulsion in space, with a spacecraft having solar panels, when:

1. It is under ample solar flux?
2. It is interstellar under negligible solar flux?

Provided it launched from a base with limited fuel just enough for the launch and no bases on the way.

Answer 1

Partial answer based on the constraints the question appears to have. Some solutions would not be considered practical by today's standards, but this doesn't appear to be asking for only practical solutions, as "no bases on the way" sounds like interstellar travel. You can't buy these at Spacecraft-R-Us.

1. It is under ample solar flux

• Use the solar panels as solar sails. Absorbing or reflecting photons from the Sun produces a small but useful force on the solar panels
• Use them to accelerate charged particles, either a plasma produced from consuming parts of the spacecraft, or interplanetary or interstellar material like protons or micrometeorites

2. It is interstellar under negligible solar flux

• Use battery or on-board nuclear power to grind them up and gasify them, then use them as propellant for an ion engine.

Answer 2

In theory, the photon drive exists, and has infinite ISP but low energy efficiency. Any electrical power can be used to provide thrust. But in practice, solar panels make better solar sails than photon drive power sources.

Answer 3

As long as your panels produce electric energy - as under solar flux - you can use that energy to produce thrust, e.g. either using an ion engine or (as long as you are within a strong magnetic field like that of Earth or of Jupiter) an electrodynamic tether. If solar flux is too low to produce electricity they became dead mass (and could be consumed as propellant as suggested here). You could try to use them as solar sails, but they are far less efficient as a dedicated solar sail (made of lighter material and spread out wide as skinny membrane).