SKY REPORTER: Passing Pluto main content.

SKY REPORTER: Passing Pluto

by Steve Beyer on


New Horizons encounter with Pluto
New Horizons encounter with Pluto, an artist’s representation.
Credit: Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

On Tuesday, July 14, NASA’s New Horizons spacecraft will fly past Pluto, providing our best views yet of that much-discussed little orb so loved by many youngsters. The name Pluto brings several things to mind including Mickey Mouse’s dog, a childhood favorite since Walt Disney introduced him in 1930. Months earlier, twenty-four-year-old Clyde Tombaugh, working at Arizona’s Lowell Observatory, discovered a solar system object revealed as a sequence of faint points of light hopping across otherwise identical photographs of stars in the constellation Gemini. Soon his new object was dubbed "Pluto" and hailed as the solar system’s ninth planet. New Horizons carries an ounce of Dr. Tombaugh’s ashes.

Pluto held the planet moniker for seventy-six years, but by then its unique status had been compromised. It was measured at just two-thirds the Moon’s diameter and, using the venerable 48-inch Schmidt-Samuel Oschin telescope at Palomar Observatory, astronomers discovered Sedna and Eris, little worlds about three times Pluto’s distance from the Sun and very similar to Pluto in size and icy composition. The recently opened Rose Center for Earth and Space at the American Museum of Natural History already had presented models of just eight planets suspended from its ceiling—Pluto was famously missing. Official shift of Pluto to the new category “dwarf planet” came during the International Astronomical Union’s 2006 summer meeting.

Although more than a few astronomers, including Alan Stern, principal investigator of the New Horizons project, object to Pluto’s relegation, it seems to be a fait accompli. Perhaps in a conciliatory move, Pluto and other relatively spherical objects orbiting the Sun beyond Neptune are now also called “Plutoids.” However, during twenty years of its eccentric, 248-year orbit, Pluto is actually closer to the Sun than Neptune.      

The New Horizons spacecraft, launched in January 2006 from Cape Canaveral atop an Atlas V rocket, was designed for NASA by the Southwest Research Institute and Johns Hopkins University’s Applied Physics Laboratory. Moving away from Earth at a record initial speed over 36,000 miles per hour, it crossed the orbit of Mars that April, then exercised some of its instruments while receiving a gravity-assisted speed boost from Jupiter during a February 2007 fly-by. Saturn’s orbit was crossed the next year. The orbit of Uranus was passed in 2011 and Neptune’s was left behind last August. New Horizons’ circuits are powered by a radioisotope thermoelectric generator containing plutonium-238 oxide pellets, an essential source out by Pluto, where sunlight is 1,630 times weaker than it is at Earth. A 6' 7"-diameter dish antenna serves for primary communications with home.

At the heart of the New Horizons mission are instruments to study Pluto’s geology, surface composition, pressure, and temperature of its thin atmosphere, as well as aspects of the interplanetary medium. These devices include: The Long Range Reconnaissance Imager (LORRI), a long focal length Ritchey–Chretien telescope system with an 8.2-inch aperture designed for high resolution at visible wavelengths. A Plasma and high-energy particle spectrometer suite (PAM) aboard New Horizons comprises a Solar Wind At Pluto (SWAP) electrostatic measuring instrument and Pluto Energetic Particle Spectrometer Science Instrument (PEPSSI). They are used to study solar wind components reaching Pluto.

New Horizons ' Pluto Exploration Remote Sensing Investigation (PERSI) has two main components: Alice, an ultraviolet imaging spectrometer and Ralph, a 2.4-inch aperture telescope with visual and infrared imaging capability. They will observe Pluto’s thin atmosphere, and may have New York connections. The instruments are said to have been named after Alice and Ralph Kramden, that immortal city bus driver and his wife.  

A team of students from the University of Colorado at Boulder devised an interplanetary dust detector for New Horizons simply called Student Dust Counter (SDC). It operates while travelling to Pluto and will continue to function beyond in the Kuiper Belt.

On the evening of July 1 you might look for the Full Moon. It will then be about five degrees of arc (the width of three fingers viewed at arm’s length) to the northwest of Pluto. This year the little world is located in the direction of the starry teaspoon asterism in Sagittarius. But even with a rather large telescope, moonlight would make it impossible to discern that faint distant object. If you miss the Moon then, it will be back near the direction of Pluto July 29, when its bright gibbous phase will be five degrees northeast of theplanet dwarf planet/plutoid.

By Tuesday night July 14, scientific missions from Earth will have reached all the big names on the Solar System’s leader board—Mercury all the way to Pluto. That’s a thrilling accomplishment that took just half a century. But, I’ll also be a bit wistful at the end of this great chapter in an age of discovery that humanity can experience firsthand only once.


The Moon

Lunar Phases, July 2015
Full Moon July 1
Last Quarter July 8
New Moon July 15
First Quarter July 24
Full Moon July 31




Mercury and Mars are now in the pre-dawn sky, but the red planet is much too close to the Sun for casual observation. Mercury may be glimpsed with some difficulty early this month but in later weeks both planets will be lost against the glow of morning twilight.

Venus and Jupiter are just half a degree apart during the evening of Wednesday, July 1. As the days pass their separation gradually increases, reversing last month’s captivating scene. This is also the night of July’s first Full Moon, sometimes known in tradition as the Full Thunder Moon.

Saturn is also an "evening star." It now shines at magnitude zero, about the same vividness as Arcturus and Vega, brightest actual stars visible these summer evenings. Saturn’s characteristic yellowish glow, like Venus and Jupiter’s white, shows constant light, not the twinkling of a star.

At 3:41 p.m. Monday July 6, Earth is at its furthest distance from the Sun for this year, aphelion—94,506,507.39 miles, center to center. That’s about 1.7% further than our average solar distance.

On Tuesday the 14, New Horizons encounters Pluto. That evening Venus appears two degrees of arc from first magnitude Regulus, brightest star in the constellation Leo.

On Saturday July 18 at 9 pm, the crescent Moon is just one degree from Venus and four degrees of arc in a line from Regulus. The thin crescent Moon is then six degrees from Jupiter.

On the nights of July 22 and 23, a wide crescent Moon is near Spica, the brightest star in Virgo.

The Moon is in the vicinity of Saturn on Friday and Saturday, July 24 and 25.

On Friday night, July 31, we may see the second Full Moon of this month—a “Blue Moon.”

Planets for July 15th
Mercury Rises 4:49 a.m. Gemini
Venus Sets 10:02 p.m. Leo
Mars Rises 4:53 a.m. Gemini
Jupiter Sets 10:03 p.m. Leo
Saturn Sets 2:10 a.m. Libra
Uranus Rises 12:13 a.m. Pisces
Neptune Rises 10:34 a.m. Aquarius