Spider is a balloon-borne experiment designed to search for primordial gravitational waves imprinted on the cosmic microwave background (CMB).

Each telescope is coupled to a polarisation-sensitive transition-edge bolometer (TES) array cooled to 300 mK.

Spider was the first instrument to successfully demonstrate time-domain multiplexed TES detectors in a space-like environment.

The data from this flight produced high signal-to-noise images of the intensity and linear polarization of the Cosmic Microwave Background, with noise levels 3—5 times lower than the Planck spacecraft in the same region of the sky, resulting in precise measurements of the CMB and Galactic foreground radiation, as well as a robust limit on the cosmological tensor-to-scalar ratio.

Further flights planned for successive seasons enable upgrades and changes to the modular telescope, increased frequency coverage and depth.