We report the first results of the LISA Pathfinder in-flight experiment. The results demonstrate that two free-falling reference test masses, such as those needed for a space-based gravitational wave observatory like LISA, can be put in free fall with a relative acceleration noise with a square root of the power spectral density of $5.2\pm 0.1\mathrm{fm}{\mathrm{s}}^{-2}/\sqrt{\mathrm{Hz}}$, or $(0.54\pm 0.01)\times {10}^{-15}$g$/\sqrt{\mathrm{Hz}}$, with g the standard gravity, for frequencies between 0.7 and 20 mHz. This value is lower than the LISA Pathfinder requirement by more than a factor 5 and within a factor 1.25 of the requirement for the LISA mission, and is compatible with Brownian noise from viscous damping due to the residual gas surrounding the test masses. Above 60 mHz the acceleration noise is dominated by interferometer displacement readout noise at a level of $(34.8\pm 0.3)\mathrm{fm}/\sqrt{\mathrm{Hz}}$, about 2 orders of magnitude better than requirements. At $f\le 0.5\mathrm{mHz}$ we observe a low-frequency tail that stays below $12\mathrm{fm}{\mathrm{s}}^{-2}/\sqrt{\mathrm{Hz}}$ down to 0.1 mHz. This performance would allow for a space-based gravitational wave observatory with a sensitivity close to what was originally foreseen for LISA.

In the months since the publication of the first results, the noise performance of LISA Pathfinder has improved because of reduced Brownian noise due to the continued decrease in pressure around the test masses, from a better correction of noninertial effects, and from a better calibration of the electrostatic force actuation. In addition, the availability of numerous long noise measurement runs, during which no perturbation is purposely applied to the test masses, has allowed the measurement of noise with good statistics down to $20\mu \mathrm{Hz}$. The Letter presents the measured differential acceleration noise figure, which is at $(1.74\pm 0.05)\mathrm{fm}{\mathrm{s}}^{-2}/\sqrt{\mathrm{Hz}}$ above 2 mHz and $(6\pm 1)\times 10\mathrm{fm}{\mathrm{s}}^{-2}/\sqrt{\mathrm{Hz}}$ at $20\mu \mathrm{Hz}$, and discusses the physical sources for the measured noise. This performance provides an experimental benchmark demonstrating the ability to realize the low-frequency science potential of the LISA mission, recently selected by the European Space Agency.

The Laser Ranging Interferometer (LRI) instrument on the Gravity Recovery and Climate Experiment (GRACE) Follow-On mission has provided the first laser interferometric range measurements between remote spacecraft, separated by approximately 220 km. Autonomous controls that lock the laser frequency to a cavity reference and establish the 5 degrees of freedom two-way laser link between remote spacecraft succeeded on the first attempt. Active beam pointing based on differential wave front sensing compensates spacecraft attitude fluctuations. The LRI has operated continuously without breaks in phase tracking for more than 50 days, and has shown biased range measurements similar to the primary ranging instrument based on microwaves, but with much less noise at a level of $1\mathrm{nm}/\sqrt{\mathrm{Hz}}$ at Fourier frequencies above 100 mHz.

TAMA300, an interferometric gravitational-wave detector with 300-m baseline length, has been developed and operated with sufficient sensitivity to detect gravitational-wave events within our galaxy and sufficient stability for observations; the interferometer was operated for over 10 hours stably and continuously. With a strain-equivalent noise level of $h\sim 5\times {10}^{-21\mathrm{}}/\sqrt{\mathrm{Hz}}$, a signal-to-noise ratio of 30 is expected for gravitational waves generated by a coalescence of ${1.4M}_{\odot }-{1.4M}_{\odot }$ binary neutron stars at 10 kpc distance. We evaluated the stability of the detector sensitivity with a 2-week data-taking run, collecting 160 hours of data to be analyzed in the search for gravitational waves.