Ancient Impacts and Today’s Planetary Defense

Lori Glaze

Twenty-five years ago, in July of 1994, the world witnessed for the first time a collision between two bodies in the solar system: the planet Jupiter and fragments of the shattered comet Shoemaker-Levy 9. The spectacular celestial event, witnessed by telescopes on the ground and in space, was also a wakeup call for humanity — we are, like our prehistorical ancestors millions of years in the past, vulnerable to asteroid impacts.

Fortunately, space is large and Earth is a fraction the size of Jupiter, so it’s extremely rare that our planet and an asteroid would cross paths at the same exact time in their orbits around the Sun. But it’s better to be prepared, so in the late 1990s, Congress directed NASA to start a concerted effort to track and monitor the asteroids and comets in our neighborhood. Those small bodies that come within 50 million kilometers of our planet were deemed “Near-Earth Objects” (NEOs) and have been vigilantly tracked ever since. In 2016, NASA created the Planetary Defense Coordination Office, which is hosted in the Planetary Science Division.

We’re not just studying NEOs from Earth. One of the Planetary Science Division’s missions, OSIRIS-REx, is studying a NEO called Bennu up close. Bennu has a 1 in 2700 chance of colliding with Earth in the late 2100s, and now is our chance to become deeply knowledgeable about the asteroid and its orbit and composition. In 2020, OSIRIS-REx will become the first NASA mission to collect a regolith sample of an asteroid, and in 2023 it’ll deliver that sample to Earth for humans to study.

We’re also celebrating several planetary defense milestones in 2019. This year marks not only the 25th anniversary of the Shoemaker-Levy 9 impact with Jupiter, but also the 40th anniversary of the establishment of the Infrared Telescope Facility, one of NASA’s asteroid observing telescopes. Located at the 13,800-foot summit of Maunakea on the Big Island of Hawaii, the IRTF was originally established to assist in the Voyager missions, but has helped planetary scientists with so much more. Scientists have used the telescope to map the region of Galileo entry probe on Jupiter, assist Cassini observations with coincident data, and measure winds on Titan ahead of the Huygens insertion, and use it in the present day to characterize numerous comets, asteroids, and other small bodies. The astrophysics mission-turned-asteroid hunter NEOWISE (Near Earth Object Wide-field Infrared Survey Explorer) is in its fifth year as a planetary defense mission, helping us discover NEOs that are too dark for our groundbased telescopes to see.

In two years, NASA will launch its very first spacecraft to demonstrate the “kinetic impactor” technique for deflecting an asteroid. The Double Asteroid Redirection Test, or DART, will fly to the binary asteroid system Didymos and we’ll deliberately crash the spacecraft into the football-stadium-sized moon. Already, an international network of scientists are observing Didymos to calculate the orbit of its moon. After the mission’s dramatic climax, we’ll once again measure the moon’s orbit to see how much it was changed.

We only have one Earth, and it’s our duty to defend it. With our fellow planetary scientists studying impacts in the deep past, we can better understand what can happen in the present and into the future.

— Lori S. Glaze, Director, NASA’s Planetary Science Division, April 2019