In 1994, Comet Shoemaker-Levy 9 (SL-9) collided spectacularly with Jupiter, providing a first ringside view of an extraterrestrial collision with a planet. Once captured in orbit by Jupiter, SL-9 passed within Jupiter’s Roche limit, and the planet’s tidal forces acted to rip the comet apart into multiple pieces ranging up to 2 kilometers in diameter. These fragments finally collided with Jupiter between July 16 and July 22, 1994, at a very high velocity, discharging an extreme amount of energy on impact, equivalent to six million megatons of TNT. In fact, had those fragments hit Earth, the resulting damage would have been catastrophic. Since the SL-9 event, groundbased amateur astronomers began to look for impacts in Jupiter’s atmosphere and have observed several over the years. Perhaps Jupiter, in a sense, protects the Earth. The giant planet must attract small bodies — whether asteroids or comets — as they pass close by. For this reason, it is thought that the giant planets in our solar system, and Jupiter in particular, shields the inner planets from numerous potential bombardments by comets and asteroids. However, this theory is still rather controversial.

Jupiter is further out than the asteroid belt, and its massive gravity is also pushing asteroids around. Through gravitational resonances, Jupiter either pulls asteroids in the asteroid belt out of the solar system or throws them inward where they could become potentially hazardous objects. Some theories argue that Jupiter and its immense gravity have helped it attract small bodies, keeping them away from Earth and thus fostering a habitable environment on our planet. Some potentially hazardous objects in the solar system are known to be long-period comets. As they travel toward the Sun, they receive gravity assists that distort their trajectory. In the case of Jupiter, they receive more than a small gravity assist; they get “sling-shotted” into different directions (or, if they get too close to the giant planet, they get completely absorbed). In that sense, instead of continuing in a trajectory toward the Sun and possibly on a collision course with Earth, they collide with Jupiter and disappear.

We do know that this theory is not completely unchallenged, even though not much research has been focused on verifying or completely denying this idea. Simulations of the impact flux on Earth using a test of the asteroid population were constructed to study the variations of the impact rate on Earth as a function of a Jupiter-like object. The resulting conclusion showed that the shielding rating in a solar system containing a giant planet is comparable to one without.

Regardless of which of these theories is correct, one conclusion remains realistic and reasonable to draw: SL-9 had an direct effect here on Earth, and more specifically, on U.S. space policy. At the time, the idea of a giant space rock (whether comet or asteroid) slamming into Earth seemed much too remote a possibility to worry about. Even though science had attributed the extinction of the dinosaurs to near-Earth objects (NEOs) colliding on Earth, it was only when the SL-9 event at Jupiter occurred that the idea of planetary defense began to really take hold.

Indeed, the collision of SL-9 with Jupiter emphasized that impacts are still currently possible and some NEOs could be potentially hazardous to the Earth. NASA had been studying various aspects of NEOs since the 1970s, but it wasn’t until 1998, when Congress directed NASA to conduct a program to discover at least 90% of 1-kilometer-diameter or larger NEOs within 10 years, that the real work began. NASA immediately established a NEO program in response, and in 2010 fulfilled this Congressional mandate.

Then, in 2005, Congress again directed NASA to survey 90% of the potentially hazardous objects measuring at least 140 meters in diameter by the end of 2020. In addition, this legislation directed NASA to submit an analysis of alternatives that NASA could employ to divert an object on a likely collision course with Earth. The 2005 authorization act also amended the National Aeronautics and Space Act of 1958 to state that “the general welfare and security of the United States require that the unique competence of [NASA] be directed to detecting, tracking, cataloguing, and characterizing near-Earth asteroids and comets in order to provide warning and mitigation of the potential hazard of such near-Earth objects to the Earth.”

This brings us to the recent establishment last year of the NASA’s Planetary Defense Coordination Office (PDCO). The PDCO is managed within the Planetary Science Division of the Science Mission Directorate at NASA Headquarters in Washington, DC. The PDCO is responsible for:

• Ensuring the early detection of potentially hazardous objects (PHOs) — asteroids and comets whose orbits are predicted to bring them within 5 million miles of Earth, and which are of a size large enough to reach Earth’s surface —i.e., greater than ~30 to 50 meters;
• Tracking and characterizing PHOs and issuing warnings about potential impacts;
• Providing timely and accurate communications about PHOs; and
• Performing a lead coordination role in U.S. Government planning for response to an actual impact threat.

In addition to finding, tracking, and characterizing PHOs, NASA’s planetary defense goals include developing techniques for deflecting or redirecting PHOs, if possible, that are determined to be on an impact course with Earth. In the event that deflection or redirection is not possible, the PDCO is responsible for providing expert input to the Federal Emergency Management Agency for emergency response operations should a PHO be on course to actually impact Earth.

The PDCO relies on data from projects supported by NASA’s Near-Earth Object Observations (NEOO) Program. The PDCO also coordinates NEO observation efforts conducted at groundbased observatories sponsored by the National Science Foundation and space situational awareness facilities of the United States Air Force.

The NEO Observations Program supports NEO surveys that contribute to a sustained and productive campaign to find and track NEOs, collecting data of sufficient precision to allow accurate predictions of the future trajectories of discovered objects. The program also supports efforts to characterize a representative sample of NEOs by measuring their sizes, shapes, and compositions. In addition, the program devotes a limited amount of funding to research into NEO impact mitigation and deflection strategies and techniques.

So where are we today? The NASA-funded surveys have found nearly 17,000 NEOs from an estimated population of about 60,000. These surveys are currently finding NEOs at a rate of about 1800 per year. Roughly half of the known catalog of NEOs — over 7000 — are objects larger than 140 meters in size. The estimated population of NEOs of this size is about 25,000. Current surveys are finding NEOs of this size at a rate of about 500 per year.

When we think back to the SL-9 event, we have made remarkable progress in finding, cataloging, characterizing, and understanding the NEO population, but we have a ways to go. We recognize we are not in danger of extinction from an NEO anytime soon, but we didn’t really know that prior to SL-9. Thanks to you, Jupiter, who woke us up!

— James L. Green, Director, NASA’s Planetary Science Division, April 2018