Gamma-ray bursts (GRBs) provide extremely luminous background light sources that can be used to study the high redshift universe out to z ∼ 12. Identification of high-z GRBs has been difficult to date because no good high-z indicators have been found in the prompt or afterglow emission of GRBs, so ground-based spectroscopic observations are required. JANUS is an Explorer mission that incorporates a GRB locator and a near-IR telescope with low resolution spectroscopic capability so that it can measure the redshifts of GRBs immediately after their discovery. It is expected to discover 50 GRBs with z > 5 as well as hundreds of high redshift quasars. JANUS will facilitate study of the reionization phase, star formation, and galaxy formation in the very early universe. Here we discuss the mission design and status.