  HD 179070, aka Kepler-21, is a V = 8.25 F6IV star and the brightest exoplanet
host discovered by Kepler. An early detailed analysis by Howell et al. (2012)
of the first thirteen months (Q0 - Q5) of Kepler light curves revealed transits
of a planetary companion, Kepler-21b, with a radius of about 1.60 +/- 0.04
R_earth and an orbital period of about 2.7857 days. However, they could not
determine the mass of the planet from the initial radial velocity observations
with Keck-HIRES, and were only able to impose a 2-sigma upper limit of 10
M_earth. Here we present results from the analysis of 82 new radial velocity
observations of this system obtained with HARPS-N, together with the existing
14 HIRES data points. We detect the Doppler signal of Kepler-21b with a radial
velocity semi-amplitude K = 2.00 +/- 0.65 m/s, which corresponds to a planetary
mass of 5.1 +/- 1.7 M_earth. We also measure an improved radius for the planet
of 1.639 (+0.019, -0.015) R_earth, in agreement with the radius reported by
Howell et al. (2012). We conclude that Kepler-21b, with a density of 6.4 +/-
2.1 g/cm^3, belongs to the population of terrestrial planets with iron,
magnesium silicate interiors, which have lost the majority of their envelope
volatiles via stellar winds or gravitational escape. The radial velocity
analysis presented in this paper serves as example of the type of analysis that
will be necessary to confirm the masses of TESS small planet candidates.
