  The momentum ray transform $I^k$ integrates a rank $m$ symmetric tensor field
$f$ over lines with the weight $t^k$:
  $
  (I^k\!f)(x,\xi)=\int_{-\infty}^\infty t^k\langle f(x+t\xi),\xi^m\rangle\,dt.
  $
  In particular, the ray transform $I=I^0$ was studied by several authors since
it had many tomographic applications. We present an algorithm for recovering
$f$ from the data $(I^0\!f,I^1\!f,\dots, I^m\!f)$. In the cases of $m=1$ and
$m=2$, we derive the Reshetnyak formula that expresses
$\|f\|_{H^s_t({\mathbb{R}}^n)}$ through some norm of $(I^0\!f,I^1\!f,\dots,
I^m\!f)$. The $H^{s}_{t}$-norm is a modification of the Sobolev norm weighted
differently at high and low frequencies. Using the Reshetnyak formula, we
obtain a stability estimate.
