A little less than four years from now, a killer asteroid will narrowly fly past planet Earth. This will be a celestial event visible around the world—for a few weeks, Apophis will shine among the brightest objects in the night sky.

The near miss by the large Apophis asteroid in April 2029 offers NASA a golden—and exceedingly rare—opportunity to observe such an object like this up close. Critically, the interaction between Apophis and Earth's gravitational pull will offer scientists an unprecedented chance to study the interior of an asteroid.

This is fascinating for planetary science, but it also has serious implications for planetary defense. In the future, were such an asteroid on course to strike Earth, an effective plan to deflect it would depend on knowing what the interior looks like.

"This is a remarkable opportunity," said Bobby Braun, who leads space exploration for the Johns Hopkins Applied Physics Laboratory, in an interview. "From a probability standpoint, there’s not going to be another chance to study a killer asteroid like this for thousands of years. Sooner or later, we’re going to need this knowledge."