Sandoval Signpost

An Independent Monthly Newspaper Serving the Community since 1988

  Night Sky

Night Sky

A Hubble Space Telescope image of the Jupiter impact scar taken on July 23, 2009.

September 2009 Night Sky

—Charlie Christmann, Signpost

Jupiter Gets Another Black Eye

It happened again. In July, something smacked Jupiter. You might remember the Jupiter-Comet impact of July 1994. Over the course of six days, at least twenty-one fragments with diameters estimated at up to a mile-and-a-half wide from Comet P/Shoemaker-Levy 9 hit the planet.

The carnage all started when Shoemaker-Levy was torn into pieces as a result of a close approach to Jupiter two years earlier. That impact delivered more energy to Jupiter than the largest nuclear warhead ever built by humans. The results of those collisions were many Earth-sized black blotches on Jupiter that lasted for months. Scientists observed these spots during and after the collision, trying to learn more about the chemistry of the Jovian planet and the comet.

It is thought that Shoemaker-Levy had not been orbiting the sun like a normal comet, but actually orbiting Jupiter. Tracing back the comet’s orbit using computer models, it is believed that the comet was probably once a sun-orbiter, but had been captured sometime in the late 1960s or early 1970s when it ventured too close to the giant planet.

The comet’s orbit was highly elliptical, causing it to pass close to Jupiter about every two years. The orbit slowly decayed, and the comet slowly moved closer to the planet. In July 1992, the comet passed only twenty-five thousand miles above the cloud tops. At this distance, the comet was well inside the closest moon to Jupiter (Metis), and the Roche limit. Inside the Roche limit, anything held together only by gravity is torn apart. This is how Shoemaker-Levy fragmented. And on the next orbit past Jupiter two years later, wham!

Jupiter is known to have captured sun-orbiting comets. Two are currently known: 82P/Gehrels and 111P/Helin-Roman-Crockett. Their orbits are also unstable and will most likely be gobbled up by Jupiter in the future.

In July 2009, something else hit Jupiter. The impact aftermath was noticed by Anthony Wesley, an amateur astronomer in Australia. After he convinced himself he had actually seen a collusion spot and not just another storm on the planet, he announced his findings. Surprisingly, no one else was looking at Jupiter that evening. It didn’t take long for all telescopes to zoom in on the event. The astronomical community is now trying to determine just what hit the planet and probably exploded in the upper atmosphere.

The problem is that no one saw the comet or asteroid coming. While it is thought that comets or asteroids larger than several hundred feet in diameter collide with Jupiter about every ninety to five hundred years, seeing them before the collision is difficult. This is a problem for all of the planets and moons in our solar system. Earth is not immune.

NASA has a project running to locate all of the potentially hazardous rocks that orbit dangerously close to Earth. As of August 20, 1,067 have been identified. More are being discovered all the time. None are currently known to be on a collision course. The next known danger will be April 13, 2029 when 99942 Apophis will pass inside the orbit of many man-made satellites. Another close approach will be on April 13, 2036. If the 2029 fly-by hits a very narrow tunnel in space, it is predicted to impact Earth in 2036. But, the probability of the asteroid hitting this tiny tunnel is extremely slim. In the meantime, NASA is trying to locate as many of these objects as possible. If we find one on a collision course, hopefully we will have time to do something to nudge it out of harm‘s way.

Solar Update

I have been tracking the progress of the current solar sunspot cycle. This is one of the deepest, least active, longest solar minimums this century. As of August 20, there have been 693 days where the sun has been blank—no spots. “This is the quietest sun we’ve seen in almost a century,” agrees sunspot expert David Hathaway of the Marshall Space Flight Center.

Here are some statistics for the sun as of August 20:

Spotless days—current stretch: forty days
2009 total: 182 days (79%)
Since 2004: 693 days
Typical solar minimum: 485 days

According to NASA, the sun set some records in 2008:

A fifty-year low in solar wind pressure: Measurements by the Ulysses spacecraft reveal a twenty percent drop in solar wind pressure since the mid-1990s—the lowest point since such measurements began in the 1960s. The solar wind helps keep galactic cosmic rays out of the inner solar system. With the solar wind flagging, more cosmic rays are permitted to enter, resulting in increased health hazards for astronauts. Weaker solar wind also means fewer geomagnetic storms and auroras on Earth.

A twelve-year low in solar irradiance: Careful measurements by several NASA spacecraft show that the sun’s brightness has dropped by 0.02% at visible wavelengths and six percent at extreme UV wavelengths since the solar minimum of 1996.

A fifty-five-year low in solar radio emissions: After World War II, astronomers began keeping records of the sun’s brightness at radio wavelengths. Records of 10.7 cm flux extend back all the way to the early 1950s. Radio telescopes are now recording the dimmest “radio sun” since 1955. Some researchers believe that the lessening of radio emissions is an indication of weakness in the sun’s global magnetic field. No one is certain, however, because the source of these long-monitored radio emissions is not fully understood.

Thank You

I just wanted to say thank you to the editors and readers of this article. September 2009 starts my eighth year writing the Night Sky for the Sandoval Signpost.

The Planets and the Moon

Venus is a bright morning “star” in the east before sunrise. A Venus-Moon conjunction occurs on the 16th. Look about thirty minutes before sunrise.

Find Mars in the east in the early morning. Look for the Moon to join Mars on the 13th. Look an hour or more before sunrise.

Jupiter is shining bright in the east after sunset. Look for a Moon-Jupiter conjunction on the 2nd and the 29th.

The Moon is full on the 4th and new on the 18th.

The Autumn Equinox occurs at 3:18 p.m. on the 22nd. Say goodbye to summer and hello to fall.






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