NASA is planning a few fireworks of its own for July fourth. Early in the morning a probe named Deep Impact will smash into Comet Temple 1 at a speed of 23,000 miles per hour. The impact, equivalent to about 5 tons of TNT, is expected to carve out a crater the size of a football field and between 7 and 14 stories deep.

The probe, launched on Jan. 12, 2005, has traveled for 173 days and over 268 million miles for its kamikaze dive into the comet. The purpose of the impact is to determine the composition of the interior of the comet thought to contain pristine material dating back to the formation of the solar system. The mother spacecraft that released the probe will analyze the gases and dust thrown up by the impact with a series of four instruments and relay the information back to the Jet Propulsion Laboratory (JPL) some 82 million miles away.

The impact will be monitored by 30 telescopes around the world and by space telescopes such as the Hubble Space Telescope and the Chandra X-ray Telescope. Rick Grammier, project manager for Deep Impact at JPL, said that trying to hit a comet traveling at tens of thousands of miles per hour with an impactor fired from a spacecraft is, “like a bullet trying to hit a second bullet with a third bullet.” It will take place around 2:30 a.m. on Independence Day.

Some fears have been raised as to the possibility that the impact could throw the comet off course and pose a potential hazard to Earth. NASA says the comet will never come within 50 million miles of the Earth and it will barely notice the impact. Dr. Don Yeomans, a mission specialist with JPL, quipped that “It is the equivalent of a collision between a 747 and a mosquito.” Deep Impact is the size of a wine cask and weighs 820 pounds while Comet Temple 1 is 9 miles long, about half the size of Manhattan Island, and weighs about one billion tons.

Focus on the Planets

Mercury may be found low in the southwest shortly after sunset as July opens. The easiest way to spot Mercury is its close proximity to its inner planet neighbor Venus. Mercury will disappear from view about mid-month.

Venus shines brightly on the western horizon as twilight darkens the sky. Venus sets about an hour and a half after sunset.

Mars is prominent in the east-southeast during the predawn hours where it continues to gain in size and brilliance all month.

Jupiter is high in the southwest at dusk well to the upper left of Venus. Jupiter sets earlier each night and, by month’s end, is setting around 11 p.m.

Saturn may be glimpsed to the lower right of Mercury and Venus on July 1 but then becomes lost to view until last August when it reappears in the predawn sky.

Uranus is in the east in the constellation of Aquarius where its blue-green disk should be easily identifiable with a good pair of binoculars and the finder’s chart located in the June issue of Sky & Telescope.

Neptune, a mere 930 million miles further from us than Uranus, is a daunting challenge to spot among the stars of Capricornus. The same finder’s chart you use to spot Uranus will help to locate Neptune’s blue-gray disk.

Pluto is a lost cause to all but the larger telescopes and most experienced planet hunters.

July events

1 Sunrise, 4:53 a.m.; sunset, 8:25 p.m. Look to the west-northwest about 40 minutes after sunset where Mercury and Venus are close together and you may be able to spot Saturn to the pair’s lower right.

2 According to the Astronomical Calendar, today at 1 p.m., Daylight-Saving Time, marks the mid-point of the year.

5 The Earth is at aphelion, or greatest distance from the Sun, today. We are about 3.1 million miles more distant from the Sun than at our closest approach last January.

6 New moon, 8:04 a.m.

8 The moon is at apogee, or greatest distance from Earth, today. Check out the west-northwest horizon after sunset where, reading from top to bottom, the thin crescent moon, Venus, and Mercury form a tight column.

14 Moon in first quarter, 11:21 a.m. The star to the immediate right of the moon is Spica with Jupiter far to the moon’s right.

15 Anytime around this date is a good time to check out Mars well up on the southeastern horizon during the predawn hours, The faint collection of stars below Mars belong to the constellation of Cetus the Whale.

20 The Sun enters the constellation of Cancer the Crab on the ecliptic.

21 Full moon, 7:02 a.m. The full moon of July is called the Thunder Moon or Hay Moon. The moon is at perigee, or closest approach to the Earth, today and this fact, coupled with the full moon will give rise to abnormally high tides.

22 The bright star just below Venus on the western horizon tonight is Regulus. The Sun enters the astrological sign of Leo but, astronomically, has just entered Cancer. The Sickle of Leo, by the way, is just to the upper right of Venus.

31 Sunrise, 5:19 a.m.; sunset, 8:03 p.m.

Clair Wood taught physics and chemistry for more than a decade at Eastern Maine Technical College in Bangor.