Finally! On July 14, after 9 and a half years of interplanetary travel, NASA’s New Horizons probe will fly past the ex-planet Pluto. New Horizons isn’t the first probe to cross this region of the solar system, but its mission is unique: to map Pluto and its main moon, Charon. This July, while all astronomers have their eyes fixed on the outer confines of our solar system, Saturn, Jupiter and Venus, will offer a dazzling celestial show nonetheless.
In 2006, a new class of objects was defined by astronomers — dwarf planets — which are in the spotlight this month. Dwarf planets are roughly spherical and orbit the Sun, but they have a weak gravitational influence. To date relatively few dwarf planets have been discovered: Among them are Ceres, Pluto, Eris, Haumea and Makemake. Ceres is located in the asteroid belt between Mars and Jupiter, while the other four are in the Kuiper belt region, beyond Neptune.
The creation of this new class of objects in our solar system generated a spirited debate: The definition of "dwarf planet" was somewhat open to interpretation; and Pluto had lost its status as a full-fledged planet.
Fortunately, space exploration ignored these new definitions. The Dawn space probe arrived at Ceres last March and has been orbiting the dwarf planet in order to study its surface in detail. Its one-year mission will provide a better understanding of the origin of asteroids and how they formed; and it will establish a connection to certain meteorites found on Earth.
The New Horizons probe will fly past Pluto and its family of moons on July 14. The probe's movements and instruments have all been pre-programmed, so by the time of its closest approach, 12 500 km from Pluto's surface, it will gather the maximum amount of scientific data possible: detailed surface images, atmospheric analysis, etc. Careful planning for this phase of the mission was essential since it will only last about 16 hours! And it can't be repeated: Because of the probe's high launch speed, and subsequent acceleration by Jupiter's gravity, slowing down to orbit Pluto would be impossible.
The probe will concentrate on storing data as it flies past Pluto and will signal Earth only after the flyby. It will let us know if its mission was successful, and then the wait for the first images will ensue. The mission team will spend nearly a year analyzing the data, a task requiring great patience.
Venus and Jupiter are dazzling
Perhaps as a counterpoint to Pluto, July begins with an exceptionally close grouping of Venus and Jupiter, in the west at sunset. Unfortunately, as the month progresses, the two planets move apart from each other and become harder to see in the glow of twilight. The lunar crescent will appear just below Venus early on the evening of July 18, adding a touch of magic to the scene. For observers in New Guinea and French Polynesia, Venus will be occulted by the Moon.
Meanwhile, Saturn remains the real star of the sky this July. Situated in Libra, Saturn is visible throughout the night, and its magnificent rings are readily available for all to see in a telescope. The planet is currently inclined 24 degrees toward Earth, providing an excellent occasion to observe the darker regions around the planet's north pole; under excellent seeing conditions, seasoned astronomers might even spot Saturn's enigmatic polar hexagon. The gibbous moon will frame Saturn on the nights of July 25 and 26, which will help to identify the ringed planet.
For early risers, especially those who don't back away from a challenge, Mercury is visible during the first few days of July; look for the tiny planet very close to the north-northeast horizon, thirty minutes before sunrise. Binoculars and an unobstructed view are mandatory.
In closing, a note of interest: On July 6, Earth will be at aphelion, the farthest point in its orbit from the Sun: the Earth–Sun distance will then be 152 093 478 km. By comparison, six months earlier, when Earth was closest to the Sun (perihelion), that distance was only 147 096 204 km.
Clear skies… and good luck New Horizons!