Answer
$1.58\times10^{16} \ kg$
Work Step by Step
We use the equation for the force of gravity and Newton's second law to find:
$ma=\frac{GMm}{r^2}$
$ a= \frac{GM}{r^2}$
Thus, if we plot $ \frac{G}{r^2}$ versus a, it will be a straight line of slope M. Doing this, we find that the asteroid's mass is $1.58\times10^{16} \ kg$.
