1. 9.4 Cosmic Collisions: small bodies vs. the planets
Our Goals for Learning
• Have we ever witnessed a major impact?
• Did an impact kill the dinosaurs?
• Is the impact threat a real danger or just media hype?
• How do other planets affect impact rates and life on Earth?

2. Have we ever witnessed a major impact?
Smithsonian (SITES) and the Space Telescope Science Institute produced a marvelous minidocumentary of the Shoemaker Levy coverage and real scientist commentary.
Download “Comet Crash”
QT movie (21 MB), broadcast quality mpeg-2 file (124 MB)

3. Comet SL9 caused a string of violent impacts on Jupiter in 1994, reminding us that catastrophic collisions still happen.
Tidal forces tore it apart during previous encounter with Jupiter

4. Chain of craters on Callisto, of another comet torn apart by tidal forces from Jupiter.

6. Impact plume rises high above Jupiter’s surface

11. Did an impact kill the dinosaurs?

12. Mass Extinctions
Large dips in total species diversity in the fossil record.
The most recent was 65 million years ago, ending the reign of the dinosaurs.
Was it caused by an impact?
How would it have happened?

13. No dinosaur fossils in these rock layers
Thin layer containing iridium from impactor
Dinosaur fossils in lower rock layers

14. Iridium - evidence of an impact
Iridium is very rare in Earth surface rocks but often found in meteorites.
Luis and Walter Alvarez found a worldwide layer containing iridium, laid down 65 million years ago.

15. Comet or asteroid about 10km in diameter approaches Earth

20. show that a large impact occurred
An iridium-rich sediment layer and an impact crater on the Mexican coast
show that a large impact occurred
at the time the dinosaurs died out, 65 million years ago.
Map of variable gravity strength; white lines show the outlines of land masses
and the Mexican regions.

21. The Impact Threat: Real danger or media hype?

22. Facts Asteroids and comets have hit the Earth.
A major impact is only a matter of time: not IF but WHEN.
Major impact are very rare.
Extinction level events ~ millions of years.
Major damage ~ tens-hundreds of years.

23. Tunguska, Siberia: June 30, 1908
Several atomic bombs worth of energy.
Tunguska, Siberia: June 30, 1908
The ~40 meter object disintegrated and exploded in the atmosphere

24. Meteor Crater, Arizona: 50,000 years ago (50 meter object)
Crater is 1 km in diameter.
Impact was 20 megatons
Privately owned National Landmark.
Meteor Crater, Arizona: 50,000 years ago (50 meter object)

25. An object 1km in size would produce a mile-high tidal wave
This slide is placeholder in case you would like to download and show the movie noted below (notes field).
A simulation movie is available for public use at
The description is at:
The impact scene in the Hollywood movie from 1998’s Deep Impact is actually pretty good.
An object 1km in size would produce a mile-high tidal wave

26. Chance of death by asteroid: About one in 100,000
From 7-April-2004 congressional testimony of Astronaut Ed Lu, B612 Foundation: 10% chance during our lifetime of a 70 m asteroid, impacting with energy of 10 megatons or 700 Hiroshima bombs. (Senate Subcommittee on Science, Technology, and Space.) Note that 50 meters was the size of the object that created Meteor Crater.
Impacts will certainly occur in the future, and while the chance of a major impact in our lifetimes is small, the effects could be devastating.

27. The asteroid with our name on it
We haven’t seen it yet.
Deflection is more probable with years of advance warning.
Control is critical: breaking a big asteroid into a bunch of little asteroids is unlikely to help.
We get less advance warning of a killer comet…

28. What are we doing about it?
Stay tuned to

29. How do other planets affect impact rates and life on Earth?

30. Jovian planets determine where asteroids and comets are today
They might protect Earth from most of bombardment
Fig 9.20

31. Was Jupiter necessary for life on Earth?
Impacts can extinguish life.
But were they necessary for ‘life as we know it’?

32. What have we learned? • Have we ever witnessed a major impact?
In 1994, we observed the impacts of comet Shoemaker–Levy 9 on Jupiter. The comet had fragmented into a string of individual nuclei, so there was a string of impacts that left Jupiter’s atmosphere scarred for months

33. What have we learned? • Did an impact kill the dinosaurs?
We are not certain whether an impact was the sole cause, but a major impact clearly coincided with the mass extinction in which the dinosaurs died out, about 65 million years ago. Sediments from the time show clear evidence of an impact, and an impact crater of the right age has been found near the coast of Mexico.

34. What have we learned?
• Is the impact threat a real danger or just media hype?
Impacts certainly pose a threat, though the probability of a major impact in our lifetimes is fairly low.

35. What have we learned?
• How do other planets affect impact rates and life on Earth?
Impacts of asteroids and comets are always linked in at least some way to the gravitational influences of Jupiter and the other jovian planets. These gravitational influences have shaped the asteroid belt, the Kuiper belt, and the Oort cloud, and sometimes still help determine when an object is flung our way.