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July 18, 2011

The Roman pantheon of gods and goddesses was huge. That's probably one reason why astronomers continued to tap into that name pool beyond the names of the classical planets, Mercury, Venus, Mars, Jupiter and Saturn. Even the River Nymphs were called into service, for the naming of Naiad, the innermost satellite of Neptune.

Vesta was the Roman goddess of hearth and home. She is identified with the Greek goddess Hestia. The Vestal Virgins were the priestess order of Vesta, and they were charged with keeping her sacred fire ever burning. The concept of an eternal flame exists even today.

Vesta is the name of a rather large asteroid in the asteroid belt between the orbits of Mars and Jupiter. Second only in mass to the asteroid (or is it dwarf planet? It's hard to keep track, nowadays.), Ceres, it contains about nine percent of the total mass of the asteroid belt, and it's about 500 kilometers in diameter. About five percent of meteorites falling to Earth originate from Vesta.[1]

Vesta star chart

Location of Vesta in mid-July, 2011, via Kstars

There have been so many spacecraft launched, both manned and unmanned, that it's easy to forget how difficult such a task is, and how excited we were in the 1950s and 1960s when we were taking our first steps into the final frontier. As a child, I kept a scrap book of newspaper and magazine articles about rockets and spacecraft. In those days, every launch was front-page news, and the popular weekly magazines, such as Life, regularly published lengthy articles about our future in space. The Disney enterprise devoted many of their Sunday broadcasts to space tutorials.

NASA is attempting to revive interest in its spacecraft by calling for a celebration of a Vesta Fiesta to celebrate the arrival of the Dawn spacecraft at Vesta.[1-8] In my area, the United Astronomy Clubs of New Jersey has scheduled its Vesta Fiesta for August 5-7, 2011.

Dawn spacecraft leaving Earth

An artist's rendition of the Dawn spacecraft leaving Earth on its way to Vesta.

NASA/JPL-Caltech/UCLA/McREL Image)

The Dawn spacecraft is remarkable for having huge solar panels that are a total of sixteen meters in length. It's the largest interplanetary spacecraft ever launched by NASA. All that photovoltaic energy is needed to power Dawn's three ion thruster engines. These ion engines have very low thrust, but they are very efficient. In terms most automotive buffs will understand, Dawn's "zero to sixty" using ion thrust is about four days. The ion engines, however, can operate continually, piling on the acceleration.[4]

Dawn is NASA's first spacecraft with ion propulsion. Ion propulsion is highly efficient, since it provides an order of magnitude greater thrust per weight of fuel than chemical propellants. The source of the ions is an onboard supply of 425 kilograms of xenon. Xenon is used, since it's a cryogen that liquefies at 165 K, and it's a heavy ion with atomic weight about 131.[6]

Dawn's journey to Vesta was 1.7 billion miles, requiring two orbits around the Sun over the course of almost four years. As it arrives at Vesta, its distance from Earth is about 117 million miles (188 million kilometers).[1] It will spend a year in orbit about Vesta, studying mineral composition and surface features. Dawn will then travel 96,000 miles to Ceres, where it should orbit in February 2015.

Vesta and Ceres are quite different from each other. Vesta has a density (3.42 g/cc) that's quite close to that of basalt, and it has an iron core. This makes it very similar to the other terrestrial planets, Earth, Mars and Mercury. Vesta is a protoplanet; namely, a planetary body that just didn't reach the size of the other terrestrial planets. Ceres, which is about 950 kilometers in diameter, is low density (2.08 g/cc), and it likely contains water ice and a rocky core.[6]

Vesta imaged by Dawn on July 9, 2011.

An image of the asteroid, Vesta, obtained on July 9, 2011, by the Dawn spacecraft.

Vesta was 41,000 km distant (26,000 miles) at the time, and the image resolution is about two-and-a-half miles.


Dawn will be in an initial orbits of about 9,900 miles (16,000 kilometers), but there are plans to approach to within 200 kilometers.[1] The first color images should be acquired almost immediately. At the closest planned approach, the image resolution will be twenty meters per pixel.[2]

Today, July 18, 2011, is John Glenn's ninetieth birthday. Glenn was the first American to orbit the Earth, on February 20, 1962. This was a bit behind Yuri Gagarin, who was the first human in outer space, and the first to orbit the Earth, on April 12, 1961. Thus started the space race. Happy birthday, John!


  1. Jonathan Amos, "Dawn probe set to orbit Asteroid Vesta," BBC News, July 14, 2011
  2. Jonathan Amos, "Asteroid Vesta comes into focus," BBC News, July 13, 2011
  3. Jia-Rui Cook and Priscilla Vega, "NASA Spacecraft to Enter Asteroid's Orbit on July 15," JPL Press Release, July 14, 2011.
  4. Suzanne Presto, "US Spacecraft to Rendezvous with Asteroid," Voice of America, July 14, 2011.
  5. Brian Vastag, "Destination asteroid: NASA probe arrives at ancient 'mini moon'," Washington Post, July 15, 2011.
  6. Alok Jha, "Dawn spacecraft keeps her date with asteroid Vesta 117m miles away," Guardian (UK), July 15, 2011
  7. Jonathan Amos, "Dawn probe orbits asteroid Vesta," BBC News, July 17, 2011.
  8. Dawn site at JPL.                                        

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