On this Fourth of July, the fireworks are frozen.

At 1:52 a.m. Eastern time July 4, the 40-cubic-inch impact device from the spacecraft Deep Impact will ram into Comet Tempel 1 -- a comet half the size of Manhattan traveling at ultrahigh speed. From that controlled crash, the Deep Impact craft will gather data from the comet's flying debris and return information to Earth so scientists can glean what makes the comet's nucleus and provide insight into the solar system's early years.

The mission's scientific team, led by Michael F. A'Hearn, a University of Maryland astronomer, will use onboard cameras and an infrared spectrometer to record the comet's shower of debris. Catch NASA's live coverage of the returning images on NASA-TV, which is available on the Internet at www.nasa.gov.

The scientists and Jet Propulsion Laboratory's operations managers are attempting quite a feat. Consider that the comet -- believed to be a dirty snowball -- moves at 6.3 miles per second, a traveler at that speed could fly from New York to Los Angeles in 61/2 minutes. The impact device, deployed from the main craft and replete with an onboard automatic navigation system, will fly into the comet's path. The crater left by the impact could range from the size of a small house and could be about 150 feet deep.

The Deep Impact spacecraft will then have 13 minutes to collect a blizzard of data.

Could this mission knock the comet out of its orbit? No, says Don Yeomans, a Jet Propulsion Laboratory scientist. He likens the crash to a Boeing 767 aircraft running into a mosquito.

For mission details, go to www.nasa.gov/deepimpact.

When we last left the planets, Venus, Mercury and Saturn had formed into a love triangle. By the end of June, the trio was breaking up.

Now look to the west-northwest at dusk to find Venus and Mercury loitering together low on the horizon, as the jilted Saturn slinks lower into the western horizon each evening over the next week. By mid-July, the fleet Mercury moves toward the horizon, leaving Venus alone. The effervescent Venus looks like a distant jetliner with its landing lights on as it skims the western horizon.

Jupiter, a true king of planets, reigns over the night sky at negative second magnitude (very bright). Find it high in the south-southwest at dusk. As July continues, the gaseous giant saunters farther west. By the end of the month, Jupiter starts the night in west-southwest.

Mars ascends the eastern horizon before 2 a.m. now, as it is visible at zero magnitude. It rises a half-hour earlier by mid-month, and it rises before 1 a.m. at month's end. Beyond the sand and shells, early rising beachcombers can enjoy the magnificent red planet high in the east-southeast at dawn.

Down-to-Earth Events

July 5 -- Plethora of planets: Hubble Space Telescope expert Ray Villard examines "The Scorecard on Extrasolar Planets" at the Space Science Telescope Institute's auditorium, Johns Hopkins University campus, Baltimore. 8 p.m. Information: 410-338-4700; hubblesite.org/about_us/public-talks.shtml.

July 5 -- Astronomer Sylvain Veilleux discusses "Supermassive Black Holes" at the astronomy open house at the University of Maryland Observatory, College Park. Gaze at the night sky through a telescope, weather permitting. 9 p.m. Information: 301-405-6555; www.astro.umd.edu.

July 9 -- Astronomer Sean O'Brien of the National Air and Space Museum provides a real sky tour at Sky Meadows State Park, near Paris, Va. 8:30 to 11 p.m. Parking $4. Information: 540-592-3556; www.dcr.state.va.us/parks/skymeado.htm.

July 16 -- The National Park Service and the National Capital Astronomers host "Exploring the Sky" at Rock Creek Park, near the Nature Center, in the field south of Military and Glover roads NW. 9 p.m. Information: 202-895-6070; capitalastronomers.org.

July 20 -- Astronomer Melissa Hayes-Gehrke lectures on "Stellar Activity in Sun-Like Stars," at the astronomy open house at the University of Maryland Observatory, College Park. Weather permitting, sky gazing through a telescope afterward. 9 p.m. Information: 301-405-6555; www.astro.umd.edu.

July 22 -- Mars Day at the National Air and Space Museum, the Mall. Learn the geologic history of the red planet. 10 a.m. to 3 p.m. Information: 202-633-1000; www.nasm.si.edu.

Blaine Friedlander can be reached at bfriedlander@earthlink.net.