  Weak lensing by large-scale structure provides a unique method to directly
measure matter fluctuations in the universe, and has recently been detected
from the ground. Here, we report the first detection of this `cosmic shear'
based on space-based images. The detection was derived from the Hubble Space
Telescope (HST) Survey Strip (or Groth Strip), a 4' by 42' set of 28 contiguous
WFPC2 pointings with I<27. The small size of the HST Point-Spread Function
(PSF) affords both a lower statistical noise, and a much weaker sensitivity to
systematic effects, a crucial limiting factor of cosmic shear measurements. Our
method and treatment of systematic effects were discussed in an earlier paper
(Rhodes, Refregier & Groth 2000). We measure an rms shear of 1.8% on the WFPC2
chip scale (1.27'), in agreement with the predictions of cluster-normalized CDM
models. Using a Maximum Likelihood (ML) analysis, we show that our detection is
significant at the 99.5% confidence level (CL), and measure the normalization
of the matter power spectrum to be sigma8*Omega_m^(0.48) = 0.51 (+0.14,-0.17),
in a LambdaCDM universe. These 68% CL errors include (Gaussian) cosmic
variance, systematic effects and the uncertainty in the redshift distribution
of the background galaxies. Our result is consistent with earlier lensing
measurements from the ground, and with the normalization derived from cluster
abundance. We discuss how our measurement can be improved with the analysis of
a large number of independent WFPC2 fields.
