Incredible...One more Big impact on Jupiter! In mid-September, another asteroid plunged into
the thick atmosphere of the giant of the Solar System, disintegrating in a flash of light.
Since 1994, the year of the first incredible collision, that of the comet Shoemaker-Levy 9,
this is the eighth event that testifies to the ability of Jupiter to attract comets
and small asteroids. A phenomenon numerically in continuous ascent thanks to the always better
techniques of digital shooting... A phenomenon that deserves to be
told in all its historical path.
"Roll intro
It is September 13. In Brazil, the sun has just set. For many astronomy enthusiasts, it is the
beginning of an observing night like many others. Jupiter is on the eastern horizon, about 25° high.
José Luis Pereira, an astrophotographer from São Caetano do Sul, just south of São Paulo,
begins to capture the planet in high resolution with his 275 mm diameter reflector telescope.
As he has been doing since 2004, when he began a methodical monitoring activity of the gas giant.
The evening offers the meridian passage of the great red spot,
but the seeing is not constant. After a few good images, the conditions worsen and the
shots become muddy and poor in detail, so that the temptation comes to disassemble everything
and go to bed waiting for better nights. So the acquisition of footage is interrupted
and the mouse moves the cursor to the button that would turn off the computer, but for some reason,
Jose hesitates ... and it is by chance, intuition, or simple dedication, he decides not to click,
but to give himself another half hour of waiting. Luck is on his side and the weather conditions
miraculously begin to improve after a while: new shots begin.
The red spot is now close to the trailing edge, making the planet less spectacular, but patience,
we must be content. Here, however, the miracle happens!
To his great surprise, during the filming of the first video, Pereira noticed a bright dot
lighting upon the planet's disk, but he didn't pay much attention to it, partly because he was
pressured by the weather conditions that were starting to deteriorate again and partly because
he thought it might be some kind of artifact linked to the recording parameters adopted,
Then he closes everything, and entrusts the analysis of the footage to software called
DeTeCt, an open source for astronomy that checks each frame of the film and
identifies any light anomalies distinguishing them from artifacts due to the acquisition
or electronics of the instrumentation. The software will take hours to do its job, and so
Jose launches the program and goes to sleep. The next morning, when he wakes up, Jose almost
doesn't remember what happened, but it is a screen of the program that makes him remember:
an alert is trying to attract his attention on a flashing text that says: "probable detection
of an impact on Jupiter. Frames 122-153". Very excited, Jose launches an alert via internet,
so that other observers can confirm the discovery. The news bounced all over the world and thousands
of visitors crashed his site, forcing him to deactivate it... but it was also the beginning
of success that would be confirmed shortly after by the official confirmation. Other observers,
in fact, rushed to check if the bright spot appeared in their footage of the same night,
and other footage emerged where the impact was clearly visible. In short, Jose can rest assured,
it is not a spurious signal: once again Jupiter was definitely hit by an object of cosmic nature!
An asteroid? A comet? This is not known. To know it, we should be able to observe the shape
and distribution of the scars of the impact, or the dark spots that form in the agitated Jovian
atmosphere as a result of the pulverization of the object. Residues that at the moment have not been
found, which suggests that the impactor body, probably a small asteroid, should not be very
large ... at most about fifty meters in diameter. At the moment therefore the community of amateur
astronomers is committed not only to view images and videos of those moments
to find traces of the glow, but is also invited to search for signs left by the alleged impact,
which may have appeared, or appear in these nights, to confirm the nature of the glow.
OK guys, but wait a minute... What's with all the excitement over a "pebble" slamming into Jupiter?
Well, it all probably has to do with the fact that the discovery of these events is almost
all thanks to the observational network formed by a few thousand enthusiastic amateur astronomers.
Then there are important issues involved, such as estimates of the frequency of impacts in the
inner solar system. Which, of course, affects the estimate of the risk to our planet in this regard.
When estimating the probability of an asteroid impacting the Earth, one must not only consider
the mere geometric cross-section of the planet, but also the gravitational cross-section. In the
case of planet Jupiter, the gravitational field is very intense - due to the mass of 317 times
that of Earth - consequently, its gravitational cross-section is much larger than that of Earth.
For this reason, Jupiter is hit much more often than our planet and collisions occur at a minimum
speed of 60 km/s, the escape velocity of Jupiter. In addition to the mass factor, the relative
proximity to the inner solar system allows Jupiter to influence the distribution of minor bodies.
For a long time, it was believed that these characteristics led the gas giant to eject from
the system or to attract most of the stray objects in its vicinity and, consequently,
to determine a reduction in the number of potentially dangerous objects for the Earth.
Subsequent dynamical studies have shown that in reality Jupiter's function as an "inner solar
system shield" has often been overestimated. The situation is in fact much more complex:
the presence of Jupiter, in fact, tends to reduce the frequency of impact on Earth of
objects coming from the Oort Cloud (long-period comets), while it increases it in the case of
asteroids and short-period comets, which are deflected towards Earth's orbit!
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For this reason, Jupiter is the planet in the solar system characterized by the highest
frequency of impacts, which justifies its name of "sweeper" or "cosmic vacuum cleaner" of the solar
system. Studies of 2009 suggest a frequency of one impact every 50-350 years, for an object of
0.5-1 km in diameter; impacts with smaller objects would occur with greater frequency,
as is natural and as evidenced by the chronology of events that we are about to tell you.
1994 was a memorable year for planetary astronomy. For the first time, in fact,
when comet Shoemaker-Levy 9 was captured by Jupiter and dismembered in at least 21
fragments that fell on the planet starting from July 16 of that year, it was possible to witness
live the collision between two celestial bodies. Unfortunately, the geometry of the collision
was not conducive to direct observations from Earth, and impacts could be observed only by the
Galileo probe. The only thing left for us earthlings to do was to follow the
evolution of the scars left in the gaseous atmosphere of the planet, with perturbations
so evident and contrasted that even very modest optical instruments allowed the observation.
It seemed an exceptional case, so that to find something similar we had to go back to December
1690, when Gian Domenico Cassini left to posterity a drawing of Jupiter's disk on which appeared
spots very similar to those left by the impacts of July 1994, and this belief on the rarity of
these events lasted in practice until our days. Discovered on March 25, 1993, by astronomers
Eugene and Carolyn Shoemaker and by amateur David Levy examining some photographic plates
of Jupiter's surroundings, the comet immediately aroused the interest of the scientific community:
it had never happened that a comet was discovered orbiting a planet and not directly around the Sun.
Captured by Jupiter presumably between the second half of the sixties and the early seventies,
it traveled in two years a very long orbit, which took it from a minimum distance of 35000 km from
Jupiter to a maximum of 25 million kilometers. The comet made several transits near the gas giant,
during which it suffered the intense tidal forces responsible for the final fragmentation of a
nucleus of at least two kilometers in diameter. In 1993 Shoemaker-Levy 9 appeared as a long line of
bright dots, bathed in the luminescence of their tails and on a collision course with Jupiter.
The studies conducted on the orbit of the comet shortly after its discovery led to the conclusion
that it would fall on the planet by July 1994. It was then started an extensive observational
campaign that involved many instruments for the recording of the event; among these,
the Hubble Space Telescope, the ROSAT satellite, and the Galileo probe, which was en route for
a rendezvous with the planet planned for 1995. The impacts occurred on the side of the planet
opposite the Earth, but the Galileo probe was able to observe them directly from a distance
of 240 million kilometers. The rapid rotation of Jupiter then made observable from Earth the
signs of the entry of individual fragments in the upper atmosphere of the planet.
The first impact occurred at 20:13 UT on July 16, 1994, when fragment A of the nucleus hit
the southern hemisphere of the planet at a speed of 60 km/s. The instruments
onboard of Galileo probe detected a fireball that reached the temperature of 24000°C.
Over the next six days, 21 distinct impacts were observed, with the largest coming on July 18 at
07:33 UT when fragment G struck Jupiter. This impact created a giant dark spot over
12,000 km across and was estimated to have released an energy equivalent to 6,000,000
megatons of TNT (600 times the world's nuclear arsenal). Two impacts 12 hours apart on July 19
created impact marks of similar size to that caused by fragment G, and impacts continued
until July 22, when fragment W struck the planet. So great was the disruption caused in the Jovian
atmosphere, that the scars of the event remained observable from Earth for several months.
The direct observation of impact events on Jupiter has led to the growing awareness, even in public
opinion, that the impact of a comet or asteroid on our planet would have potentially devastating
consequences. Therefore, the possibility of such a fall has become something concrete, from which
one must, as far as possible, guard against. The general public was very impressed by the
event, and the media devoted a great deal of attention to Shoemaker-Levy 9.
The role played by non-professional astronomers in identifying the signs of impact is also
significant, thanks to a reduction in the cost of advanced observation instruments.
In 2009 and 2010, in fact, other impacts were recorded: the first on July 19, 2009 (just 15
years after that of Shoemaker-Levy 9), when the Australian amateur Anthony Wesley noticed
from his observatory in New Wales a conspicuous dark spot that lasted for several days, probably
the residue from the fall of a small asteroid. The second occurred a year later, on June 3,
2010, when the same Wesley and the Philippine Christopher Go
independently photographed a small flash in the Southern Equatorial Band.
The light emission lasted only a couple of seconds and did not leave obvious signs in the atmosphere,
so that the fall was interpreted as that of an object of only 8-13 meters in diameter.
The third was even less obvious and was recorded on August 20 of the same year
by the Japanese amateur Masayuki Tachikawa in the Northern Equatorial Band.
On 10 September 2012 at 11:35 UT amateur astronomer Dan Petersen detected from Racine,
Wisconsin, a fireball on Jupiter that lasted 1 or 2 seconds. The fireball was created by a
meteoroid less than 10 meters in diameter, quite similar to the flash observed on 20 August 2010.
On March 17, 2016, an impact fireball observed on Jupiter's limb was recorded by Gerrit Kernbauer,
Moedling, Austria. This report was later confirmed by an independent observation by
amateur John McKeon. The size of impact object was estimated to be between 7 and 19 meters.
On May 26, 2017, amateur astronomer Sauveur Pedranghelu observed another flash
from Corsica (France). The event was announced the next day, and was quickly confirmed by two
German observers. The impactor had an estimated size of 4 to 10 meters.[
In practice, after the impressive cometary collision of 1994, seven more
impacts were recorded over 27 years, which forced astronomers to revise their estimates.
In a study published in 1988, the Japanese Nakamura and Kurahashi estimated that a
comet with a diameter greater than 1 km could impact Jupiter every 500-1000 years,
but obviously the statistics were revised in the light of the Shoemaker-Levy 9
event, so that soon were considered values between 50 and 350 years for an object of 0.5-1 km.
The evaluations fell even more after the impact of 2009, when always for an object of 0.5-1
km it came to assume a frequency of 10 years, and 1 year for a meteoroid of about 10 m in diameter.
And obviously, they are further lowering in this period, after the two flashes of 2010.
In the end, it is understood that to get to a casuistry able to provide reliable data on
the real rate of impact of meteoroids and comets on Jupiter, it will be necessary to monitor the
planet constantly, and in this type of research could be the amateurs to do the lion's share.
After all, it is clear that the increase of the findings in recent years, is
not due to a random and abnormal increase of impacts, but to the increasing number of
amateurs engaged in planetary observation and the increasing availability of astronomical
instrumentation of absolute excellence. The light emission that accompanies the entry of
a meteoroid into the Jovian atmosphere lasts very few seconds (1-2 s) and is therefore necessary for
continuous monitoring of the planet's surface at high frequency for their detection. Professional
astronomers agree that telescopes with a diameter between 15 and 20 cm are the
ideal tools for their detection if equipped with webcams or other video recording tools.
So, what are we waiting for? Let's all go and observe Jupiter!