In general it is allowed to volunteer in the US when you are there on a B1 or B2 visa, or on visa waiver program, and for most other visas that do not permit paid employment. However there are some restrictions, and it is best to check them out.
The most significant restriction is that you can only volunteer to do things that are normally done on a voluntary basis, i.e. without pay. So for example you could volunteer at a hospital as a 'candy striper' because that is a volunteer position, and you would be working alongside other unpaid volunteers. You could not do the work of a nurse even if you volunteered to do it unpaid because a nurse is normally a paid position.
You also can't do "anything that would benefit a commercial enterprise", and there are restrictions on things like unpaid internships. However assuming that the Planetary Society of Pasadena is a not-for-profit you should be fine.
Here are some references: Volunteers for Peace, Duke University, Nolo, US Fish and Wildlife service