Inspired by the initial COBE results of an extremely isotropic and homogeneous background, a series of ground-based and balloon-based experiments quantified CMB anisotropies on smaller angular scales over the next decade. The primary goal of those experiments was to measure the angular scale of the first acoustic peak, for which COBE did not have sufficient resolution. The measurements were able to rule out cosmic strings as the leading theory of cosmic structure formation, and suggested cosmic inflation was the right theory. During the 1990s, the first peak was measured with increasing sensitivity and by 2000 the BOOMERanG experiment reported that the highest power fluctuations occur at scales of approximately one degree. Together with other cosmological data, these results implied that the geometry of the Universe is flat. A number of ground-based interferometers provided measurements of the fluctuations with higher accuracy over the next three years, including the Very Small Array, Degree Angular Scale Interferometer (DASI) and the Cosmic Background Imager (CBI). DASI made the first detection of the polarization of the CMB and the CBI provided the first E-mode spectrum with compelling evidence that it is out of phase with the T-mode spectrum. In June 2001, NASA launched a second CMB space mission, WMAP, to make much more precise measurements of the large-scale anisotropies over the full sky. The first results from this mission, disclosed in 2003, were detailed measurements of the angular power spectrum to below degree scales, tightly constraining various cosmological parameters. The results are broadly consistent with those expected from cosmic inflation as well as various other competing theories, and are available in detail at NASA's data center for Cosmic Microwave Background (CMB) (see links below). Although WMAP provided very accurate measurements of the large angular-scale fluctuations in the CMB (structures about as large in the sky as the Moon), it did not have the angular resolution to measure the smaller scale fluctuations which had been observed using previous ground-based interferometers. A third space mission, Planck, was launched in May 2009. Planck employs both HEMT radiometers and bolometer technology and measures the CMB anisotropies at a higher resolution than WMAP. Unlike the previous two space missions, Planck is a collaboration between NASA and the European Space Agency (ESA). Its detectors got a trial run at the Antarctic Viper telescope as ACBAR (Arcminute Cosmology Bolometer Array Receiver) experiment – which has produced the most precise measurements at small angular scales to date – and at the Archeops balloon telescope.