I spent most of my childhood in Vancouver, British Columbia. On those rare winter days when we did get snow it was wet and slushy. Consequently, getting hit by a snowball in Vancouver was a much more unpleasant experience than in other parts of Canada. I quickly learned the value of building a strong snow fort before getting involved in a snowball fight.
Our planet also benefits from having strong shields between it and potential colliders. Those shields are Jupiter, Saturn, Uranus, and Neptune. Because of its size and proximity to Earth, Jupiter is by far the most important of Earth’s protectors.
As Figure 1 illustrates, more than 1,300 planet Earths would fit inside of Jupiter. Though not as dense as our planet, Jupiter still weighs in at 318 times the mass of Earth. Jupiter’s volume and mass are such that asteroids and comets headed for Earth are almost always either deflected away (the much more common outcome) or absorbed into the volume that makes up Jupiter.
Figure 1: Earth-Jupiter Size Comparison
Even a single storm eddy, known as the Great Red Spot, on Jupiter is larger in size than planet Earth. Image credit: NASA.
The first planetary impact event observed by astronomers occurred in 1994, when comet Shoemaker-Levy 9 crashed into Jupiter. Figure 2 shows the aftermath of the collision event. At the time, astronomers believed such impact events would prove rare—less than one such event per astronomer lifetime (astronomers manifest notoriously long life spans). However, on July 19, 2009, amateur astronomers observed another collision event on Jupiter, and in the June 1, 2010 issue of the Astrophysical Journal Letters two teams of professional astronomers published the results of their analysis of the event.1
Figure 2: Aftermath of the Collision of Comet Shoemaker-Levy 9 with Jupiter
Just before the collision, Jupiter’s gravity broke up comet Shoemaker-Levy 9 into at least 21 distinct pieces. Collisions with Jupiter occurred between July 16 and 22 in 1994. The purple spots in the image above mark a few of the collision sites. The largest collision spots were as big as Earth’s diameter. At the impact moments, fireball temperatures reached 24,000° Centigrade (43,000° Fahrenheit). Image credit: NASA/Hubble Space Telescope Comet Team.
The collision itself was not observed because it occurred on Jupiter’s far side. Within a few hours, however, Jupiter had rotated around enough so that a number of amateur astronomers could observe and report “an anomalous feature.” Subsequent infrared observations confirmed that the feature was exogenic (the result of an external collider). Figure 3 shows a Hubble Space Telescope image taken of the debris from the collision impact four days after the collision event.
Figure 3: Impact Scar from the July 19, 2009 Collision Event
This Hubble Space Telescope image was taken on July 23, 2009, with the newly installed Wide Field Camera 3. The dark smudge toward the bottom right of Jupiter is the debris from the asteroid collision. Image credit: NASA/ESA/STScI/Space Science Institute, Boulder, Colorado/Jupiter Impact Team.
Astronomers observed just one debris spot, indicating that the collider did not break up before the collision event. A research team led by Heidi Hammel of the Space Science Institute in Boulder, Colorado, noted that the July 2009 impact site differed from the 1994 Shoemaker-Levy 9 sites in both ultraviolet morphology and contrast lifetime. These three distinguishing features point to an asteroid, rather than a comet, as the culprit for the collision event.
Another research team, led by A. Sánchez-Lavega, estimated the size of the collider as 0.5–1.0 kilometers. They also put the collision rates both for this type of impact and the Shoemaker-Levy 9 event at about five to ten times more frequent than what astronomers had previously determined.
That the July 19, 2009 Jupiter collider was an asteroid rather than a comet and that Jupiter colliders are five to ten times more frequent than previous estimates reveals how dangerous a place the solar system can be. One of the 21 fragments of Shoemaker-Levy 9 generated by itself the equivalent of a six trillion ton TNT energy release upon impact. (This is about 600 times greater than the energy that would be released if the world’s total nuclear arsenal were detonated all at once.) Yet, the solar system is probably a much safer environment than other planetary systems thanks to a 1:2 orbital resonance between Jupiter and Saturn (a situation where Jupiter orbits the Sun exactly twice for every single orbit of Saturn). This event occurred 3.9 billion years ago and caused the Kuiper Belt, the solar system’s most dangerous belt of asteroids and comets, to be reduced to about one percent of its previous size.
The July 16, 1994 and July 19, 2009 collision events on Jupiter demonstrate just how crucial a role the planet plays in protecting life on Earth. Without Jupiter’s gravitational shield our planet would be pummeled by frequent life-exterminating events.
Yet Jupiter by itself is not an adequate shield. The best protection is achieved via a specific arrangement of several gas giant planets. The most massive gas giant must be nearest to the life support planet and the second most massive gas giant the next nearest, followed by smaller, more distant gas giants. Together Jupiter, Saturn, Uranus, and Neptune provide Earth with this ideal shield.
Some might argue that a gas giant planet more massive and closer than Jupiter would better protect Earth. While such a planet would provide Earth with superior protection from collision events, its gravity would be strong enough to seriously disrupt Earth’s orbit. A Jupiter more or less massive would pose a greater risk to life on Earth, as would a Jupiter more or less distant. The same delicate balances hold true for Saturn, Uranus, and Neptune.
The Jupiter collision events remind all of us how thankful we need to be for God providing Earth with such wonderfully designed planetary partners. These celestial bodies also illustrate that the more we learn about our solar system, the more reasons we discover to believe in Christ as Creator, Lord, and Savior.
1. A. Sánchez-Lavega et al., “The Impact of a Large Object on Jupiter in 2009 July,”Astrophysical Journal Letters 715 (June 1, 2010): L155–L159; H. B. Hammel et al., “Jupiter After the 2009 Impact: Hubble Space Telescope Imaging of the Impact-Generated Debris and Its Temporal Evolution,” Astrophysical Journal Letters 715 (June 1, 2009): L150–L154.