We propose to build a lightweight, low cost, long lifetime cryostat and magnetic refrigerator for cooling detectors to 0.1 K. The passively cooled cryostat will be compact, highly reliable and will have a lifetime greater than 100 days. The passive cooling bypasses the use of active cryocoolers and their associated vibration, reliability, and power dissipation problems. The magnetic refrigerator will have lower power consumption (0.2 W nominal), and it will be remotely recyclable. This system makes use of several innovative design features that include: a two stage adiabatic demagnetization refrigerator (ADR) that operates from a 4 K liquid helium stage rather than a 1.5 K pumped helium stage, a 17 K solid neon (SNe) reservoir that is used as the secondary coolant, and very efficient heat exchangers and two vapor cooled shields. In Phase I we will build a prototype LHe-SNe ADR that has a lifetime of at least 10 days, and in Phase II we will build a LHe-SNe ADR with a lifetime of at least 100 days that can be used in NASA's 100 Day Ultra Long Duration Balloon program. These unique design features are readily transferable to cryostats used in more conventional applications.