One reason Earth is habitable is because the gravity of Jupiter helps protect us from long-period comets, in particular, which enter the solar system from the outer reaches of the Oort Cloud. Jupiter’s gravity is thought to sling most of these fast-moving dirty snow balls out of the solar system before they can get close to Earth. As a result, long-period comets are thought to strike Earth only on very long timescales of tens of millions of years. Without Jupiterand its enormous gravity well being relatively nearby, long-period comets would collide with our planet much more frequently with calamitous results.
In recent decades, for instance, astronomers have been able to see signs of comets that have crashed into Jupiter, like the fragmented Comet Shoemaker-Levy 9 in 1994.
Comet Shoemaker-Levy 9/Jupiter collision - Stock Image ...sciencephoto.com
In 2009, astronomers, including amateur astronomer Anthony Wesley, observed a dark gash in one side of the giant planet, likely caused by a comet.
In 2009, amateur astronomer Anthony Wesley noticed a dark mark on Jupiter. It turned out to be a scar from an impact with some object, presumably a comet.
The late Brian G. Marsden*, former director of the the International Astronomical Union’s Central Bureau for Astronomical Telegrams, related to Dennis Overbye of the New York Times
in 2009, that shortly after the dark gash appeared on Jupiter. It’s rare for a comet to come within 1 astronomical unit of Earth (that is, one Earth-sun distance, 92 million miles, or about 150 million kilometers). But, in the year 1770, a Comet Lexell streaked past Earth at a distance of only a million miles. Dr Marsden explained to Overbye that: "… the comet had come streaking in from the outer solar system three years earlier and passed close to Jupiter, which diverted it into a new orbit and straight toward Earth. The comet made two passes around the sun and in 1779 again passed very close to Jupiter, which then threw it back out of the solar system. It was as if Jupiter aimed at us and missed."
The other reason is the protection the Moon offers us from the impact of meteors and space debris.
Today, we know that the Earth is bombarded by small meteoroids at a higher frequency than previously thought. The number of meteor impacts on our planet has tripled during the last 290 million years to the point that 33 tons of space debris fall to Earth every day. Due to their size, most of these rocks burn completely in the Earth’s atmosphere. However, without the presence of the Moon, the impact rate could be much higher, making the Earth a fairly hostile place.
The Moon is almost 3,500 kilometers (2,175 mi) in diameter—about 27 percent of the Earth’s diameter. Thanks to its large size, the Moon has served as a protection for Earth in times of cosmic adversity.
During the planet’s early days, the Moon attracted to itself a large part of the interplanetary debris and asteroids wandering in this part of our solar system. Had our natural companion not existed, the neighborhood of Earth would have been like a minefield - too dangerous for the development of life.
Even today, the Moon is still like a small shield that protects us from meteor impacts. Studies show that the gravity of the Moon helps to prevent more asteroid collisions with the Earth than it causes.
And how much is that?
Well, just between 2005 and 2013, NASA's Lunar Reconnaissance Orbiter detected more than 300 impacts on the lunar surface. This means that in the absence of the Moon, hundreds of such bodies could have ended up impacting us on the Earth.
Bright impact flash made by a foot-wide rock that struck the moon on March 17, 2013, St. Patrick's Day. The moon was a crescent in the evening sky at the time. The impact occurred in the dark, earthlit part of the moon away from the sun-lit crescent. Click photo to see video about the event. Credit: NASA
So, in this sense, it is hard to imagine Earth without its nightly romantic companion, our silvery Mr Moon.
* Brian G. Marsden (1937-2010) specialized in celestial mechanics and astrometry, collecting data on the positions of asteroids and comets and computing their orbits, often from minimal observational information and providing their future positions on International Astronomical Union (IAU) circulars. In addition to serving asMinor Planet Center ( MPC) director since 1978, he served as the director of the Central Bureau for Astronomical Telegrams (CBAT) from 1968 to 1999. He was president of IAU Commission 6 (2000–2003) and Commission 20 (1976–1979).
Marsden helped recover once lost asteroids and lost comets. Some asteroid and comet discoveries of previous decades were “lost” because not enough observational data had been obtained at the time to determine a reliable enough orbit to know where to look for re-observation at future dates. Occasionally, a newly discovered object turns out to be a rediscovery of a previously lost object, which can be determined by calculating its orbit backwards into the past and matching calculated positions with the previously recorded positions of the lost object. In the case of comets this is especially tricky because of nongravitational forces that can affect their orbits (one of which is emission of jets of gas from the comet nucleus), but Marsden has specialized in calculating such nongravitational forces. Notably, he successfully predicted the 1992 return of the once-lost Comet Swift-Tuttle.
In May 1993, Marsden concluded that the trajectory of Comet Shoemaker-Levy 9 would put it onto a course to collide with Jupiter in July 1994, marking the first ever time that a cometary-planetary impact was successfully predicted in advance.
In 1998, he calculated that an asteroid, (35396) 1997 XF11 had a small probability of striking the Earth in 2028. Marsden chose to issue a press release, which Robert Roy Britt called a false alarm.
“… astronomers created a media storm by announcing that an asteroid could collide with Earth in 2028, only to revise the estimates hours later.”—Gretchen Vogel, Science, 20 March 1998.
Other asteroid researchers demonstrated within hours that the computation was in error. Marsden himself admitted the announcement was a strategy which needed “rethinking”, and NASA asked astronomers not to sound a public alarm like that again but to communicate with each other. He took some criticism for publicizing this prediction right when movie companies were publicizing films like Deep Impact (see also Science by press conference). However, Marsden justified his actions with the argument that the problem of detecting asteroids needs more attention: “Much as the incident was bad for my reputation, we needed a scare like that to bring attention to this problem.” (Scientific American magazine, 2003). Follow-up work determined that an impact would be unlikely.
He once proposed that Pluto should be cross-listed as both a planet and a minor planet and assigned the asteroid number 10000; however, this proposal was not accepted. A similar proposal was, however, finally accepted in 2006 when Pluto was designated minor planet 134340 and also declared a dwarf planet.
Marsden campaigned to reclassify Pluto as one of the newly discovered and rapidly growing class of Trans-Neptunian objects, the discovery of which was made possible by CCD-array detectors and dedicated surveys or incidental discoveries of these objects with relatively large telescopes.
Partly at his urging, the International Astronomical Union voted at a meeting in Prague in 2006 to designate Pluto and three asteroids “dwarf planets.”, which are objects that have not dynamically cleared their orbits of other debris (except, e.g., for collections of objects stably librating dynamically at the “Lagrange-points”, the libration points L4 and L5 of large, classical planets, as in the case of the Jovian “Trojan” asteroids).