Vision and Voyages for Planetary Science in the - Solar System ...

destroy areas of land equivalent to moderate sized states. Larger impacts, like the Cretaceous/Tertiary
impactor 65 million years ago, can dramatically affect life over the entire surface of Earth. Although the
impactor hazard to life on Earth is not negligible, it is less than might otherwise be the case without the
giant planets. Around one million asteroids are believed to be larger than 1 kilometer and there are likely
far more comets of this size and larger. All of these objects represent potential Earth impactors.
The solar system’s giant planets, particularly Jupiter, exert a major influence on the orbits of such
objects. Asteroids or comets on elliptical orbits that would bring them to the inner solar system must
cross the orbit of Jupiter. Close encounters with Jupiter can dramatically alter a potentially dangerous
body’s orbit, possibly sending it out of the solar system. While in some cases Jupiter might cause an
otherwise harmless object to take a dangerous turn, some n-body simulations suggest that in other cases
Jupiter protects Earth. 66
The number and timing of jovian impacts provides insight into the rate at which Jupiter deflects
small bodies. Each impact delivers species to Jupiter’s stratosphere that would not be produced by
internal jovian processes; a better inventory of jovian stratospheric composition along with improved
atmospheric models and numerical models of asteroid and comet orbits would constrain impact history.
More importantly, these events serve as laboratories for the physics of large airbursts. We now
have an open, unclassified source of Earth bolide observational data, but these are for relatively small
events. Those who have studied the subject of Earth impacts estimate intervals of hundreds of years
between events the size of the Tunguska impact in Siberia in 1908. Yet as of this writing, astronomers
have observed four such impact events on Jupiter in the past 16 years (counting the demise of
D/Shoemaker-Levy 9 in 1994 as “one” event). By understanding the physics of large airbursts, better
estimates their threat to Earth can be made.
Important Questions
Some important questions concerning the giant planets’ role in creating a habitable Earth via
large impacts include the following:
• What is the current impact rate on Jupiter?
• To what extent can Jupiter’s current atmospheric composition be utilized as a record of the
impact history?
• What are the characteristics of bolides and large airbursts on Jupiter, and how do they
compare with known bolides and airbursts on Earth?
Future Directions for Investigations and Measurements
The best approach to determining the rate and characteristics of jovian atmospheric impacts is
continuous observation of Jupiter. Today, such work relies on a small number of highly motivated
amateur observers; these unfunded volunteer observers, however, cannot cover Jupiter at all times. Small,
automated, planetary monitoring telescopes could provide a comprehensive survey of future impacts on
Jupiter, perhaps as a National Science Foundation project. The Large Synoptic Survey Telescope (LSST)
or other survey telescopes may also provide observational constraints on moving objects that could be on
Jupiter impact trajectories. 67
Determine the Role of Surface Modification via Smaller Impacts
A number of important external processes govern the size, structure and dynamics of the giant
planets, their ring systems and their satellites (in addition to the obvious role of solar illumination). Many
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