Why We Explore

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Voyages to the Asteroids
Why We Explore

Editor's Note: This is the 18th in a series of essays on exploration by NASA's Chief Historian, Steven J. Dick.

Daring though voyages to comets have been, with comet material often pelting and even damaging passing spacecraft, voyages to asteroids have gone one step further, achieving an actual landing, or maybe two landings. Comets and asteroids are relatives in the astronomical zoo: both are composed of ice and dust left over from the formation of the solar system 4.6 billion years ago. Asteroids are inner solar system leftovers, mostly formed between the orbits of Mars and Jupiter, while comets formed in colder reaches of the outer solar system. Both types of objects can be perturbed inward to become “Near Earth Objects,” and as such they can be dangerous if they come too near for comfort. Comets in general have more ‘volatiles,’ materials that may be ejected from the surface as they near the Sun, while asteroids appear to be inactive objects, and thus safer to approach.

Composite view of three asteroids that have been imaged at close range by spacecraft. The image of Mathilde (left) was taken by the NEAR spacecraft on June 27, 1997. Images of the asteroids Gaspra (middle) and Ida (right) were taken by the Galileo spacecraft in 1991 and 1993, respectively.
Composite view of three asteroids that have been imaged at close range by spacecraft. The image of Mathilde (left) was taken by the NEAR spacecraft on June 27, 1997. Images of the asteroids Gaspra (middle) and Ida (right) were taken by the Galileo spacecraft in 1991 and 1993, respectively. All three objects are presented at the same scale. The relative brightness has been made similar for easy viewing; Mathilde is actually much darker than either Ida or Gaspra. Credit: Near Earth Object Program, JPL.

Though asteroids are not the grandstanders that comets can be when they show off their tails as they approach Earth, they are no less important for understanding the primordial material from which the planets were formed. Thus, when the Galileo spacecraft made its journey to Jupiter, mission planners were excited to be able to visit two asteroids along the way – the first visits to this class of object.

Galileo’s voyage was another one of those incredible journeys that we have now become accustomed to.
After gravity assists from Venus and Earth, Galileo passed near the asteroid belt, and on October 29, 1991 approached within 1000 miles of 951 Gaspra (asteroids are numbered according to their discovery dates, so the lower the number, the earlier the discovery; 1 Ceres was discovered on the first day of the 19th century, January 1, 1801). No one quite knew what to expect of Gaspra, but as the images were beamed back from deep space they showed an irregular object with dimensions 12 x 7 x 7 miles, with numerous small craters, and rotating every 7 hours. It is composed of metal-rich silicates and perhaps blocks of pure metal.

Making use of another gravity assist during which it passed only 185 miles from Earth, on August 28, 1993 Galileo approached 243 Ida on its way to Jupiter. Ida turned out to be similarly elongated but considerably larger, measuring 35 x 15 x 13 miles. It is more heavily cratered than Gaspra, indicating it may be older. The big surprise from this Galileo flyby (within 1,500 miles) was the discovery of the first confirmed asteroid moon, named Dactyl, only about a mile across in its largest dimension. Numerous other moons of asteroids have since been discovered. Both Ida and its moon appear to be made of silicates. So detailed were the views of Ida that we can now view a topographic map of the tiny world.

These color images of Eros was acquired by NEAR on February 12, 2000, at a range of 1800 kilometers (1100 miles) during the final approach imaging sequence prior to orbit insertion.
These color images of Eros was acquired by NEAR on February 12, 2000, at a range of 1800 kilometers (1100 miles) during the final approach imaging sequence prior to orbit insertion. A five and one-half hour long sequence of images covering visible and infrared wavelengths was taken at that time, to provide a global overview of the color and spectral properties of the asteroid. The images show approximately the color that Eros would appear to the unaided human eye.

Having observed Venus, the Earth-Moon system twice, and two asteroids, Galileo went on to its successful mission at Jupiter. After 14 years in space, including 8 years orbiting Jupiter, the mission was terminated. Despite problems with deployment of its high-gain antenna that lowered the rate at which data was returned to Earth, Galileo enters the annals of exploration as one of the greatest of all voyages of discovery.

So successful were these first visits to asteroids, and so many the remaining questions, that the next voyage to asteroids was a dedicated mission known as the Near Earth Asteroid Mission (NEAR), renamed NEAR-Shoemaker after launch in honor of planetary geologist Eugene Shoemaker. Again, a single spacecraft would visit two asteroids, and this time one of them would serve as its final resting place.

On June 27, 1997 NEAR flew within 753 miles of asteroid 253 Mathilde, which proved to be rounder in shape and relatively large at 41 miles in diameter. The asteroid proved to be rich in carbon, darker than charcoal, and sported a huge crater. NEAR then flew back to Earth for a gravity assist, one that put it on a trajectory for 433 Eros. Eros, which sometimes comes closer than Venus, is famous in the history of astronomy because it comes so close to the Earth that it was used early in the 20th century to determine the scale of the solar system, with greater accuracy than the rare but famous transits of Venus. NEAR reached Eros on December 23, 1998, and on Valentine’s day, 2000 was inserted into orbit around the asteroid named after the Greek god of love and lust.

After orbiting and mapping Eros for a year, on February 12, 2001 NEAR actually landed on the asteroid, capturing 69 images during its descent. NEAR returned a treasure trove of information about Eros, which turned out to be a 21-mile-long rock with a vast number of large craters between 1500 and 3000 feet in diameter. Scientists concluded that its surface features were shaped by impacts rather than internal activity. A variety of geological features were observed.

The bare facts of the NEAR mission described above conceal a great deal of drama unusual even for space missions. The spacecraft was supposed to go into Eros orbit in late 1998, but the main engine burn to achieve this on December 20 did not go as planned. Worse than that, signals from the spacecraft ceased, and it was basically lost for a day. During that time the spacecraft was spewing fuel and went off course, but scientists managed flyby images in the midst of its difficulties. They then had the spacecraft follow Eros for 14 months until conditions were right for the Valentine’s day rendezvous.

Two other asteroid rendezvous attempts did not end as successfully as NEAR. An attempt by the New Millennium Deep Space 1 spacecraft to image asteroid 9969 Braille on July 28, 1999 was of only limited success due to camera problems. In late 2005 the Japanese Hayabusa (peregrine falcon) spacecraft achieved rendezvous with the tiny near-Earth asteroid 25143 Itokawa, a 600 meter-sized, potato-shaped asteroid. The spacecraft is believed to have briefly touched down on the surface of the asteroid, but the sample return remains in jeopardy due to technical problems. The true success of the mission will not be known until a re-entry capsule returns to Earth in 2010.

Skeptics may well ask why we should explore these chunks of rock Scientists have long known that they are of more than academic interest. In the future, they may be the source of rich minerals, and the site of mining operations. More immediately, the close approaches of some of them threaten the Earth. It therefore behooves us to know as much as possible about the size and structure of these roaming celestial neighbors. As the Near Earth Object team at the Jet Propulsion Laboratory has stated, "whether looking for the richest source of raw materials or trying to nudge an Earth threatening object out of harm’s way, it makes a big difference whether we're dealing with a 50-meter sized fluff ball or a one-mile slab of solid iron." Such hits have happened in the past, and we do not know if the next hit will be in a few years or many years. Awareness of Earth's space environment, as well as monitoring its more highly publicized surface environment, is one of the most compelling reasons why we explore.

Further Reading

Bell, Jim and Jacqueline Mitton, eds. Asteroid Rendezvous: NEAR Shoemaker’s Adventures at Eros (New York: Cambridge University Press, 2002).

Hayabusa Website. http://www.isas.ac.jp/e/enterp/missions/hayabusa/index.shtml

McCurdy, Howard, Low-Cost Innovation in Spaceflight: The Near Earth Asteroid Rendezvous (NEAR) Shoemaker Mission, NASA SP-2005-4536, Monographs in Aerospace History Number 36 (2005). http://history.nasa.gov/monograph36.pdf

NEAR Website, http://near.jhuapl.edu/

Near Earth Object Website: http://neo.jpl.nasa.gov/neo/

Steele, Duncan. Rogue Asteroids and Doomsday Comets (John Wiley & Sons, New York, 1995)

Steven J. Dick
NASA Chief Historian