To learn more about the physical properties of asteroids we first have to reach them with spacecraft.
| Defending the Earth: |
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TO NUKE OR TO NUDGE? Blowing up asteroids may work in the movies, but in reality it could lead to bigger problems. However, nuclear warheads could be used to alter the course of an errant asteroid or comet. [more] |
SAILING WITH SUNLIGHT: From solar-powered cannons to steering an asteroid away from Earth with giant solar sails, the alternatives to blowing up space objects are cutting edge. [more] |
The
NEAR mission is the first attempt to rendezvous with an asteroid. A rendezvous involves carefully maneuvering the spacecraft so that it follows nearly the same orbital path as the asteroid. The spacecraft (pictured below) slowly approaches the object, then adjusts its speed so that the spacecraft and asteroid follow the same path around the sun. In the case of NEAR, a further maneuver will put the spacecraft into orbit around the asteroid.
Previous spacecraft missions to asteroids and comets have involved quick flybys with no attempt to match the speed of the object. These missions were important steps in our exploration of these bodies, but improved technology was needed to achieve a rendezvous.
One recent space mission was designed to test new technology. In July last year the
Deep Space 1 spacecraft passed within 10 miles (16 kilometers) of Asteroid Braille. This mission successfully tested two important new technologies -- auto-navigation and the ion drive.
Auto-navigation means that the robot spacecraft worked out its own location in space and plotted the course to the target object.
The ion drive (pictured below) is an advanced form of propulsion where particles coming out of the exhaust are electrically charged (ions) that are accelerated by electrical means to very high speeds. Solar cells or a nuclear generator could provide electrical power.
(Credit: Johns Hopkins University)
Deep Space 1 used an advanced solar collector to generate a stunning 2,500 watts of power. By using a steady, reliable power source, the ion drive can gradually accelerate the spacecraft to interplanetary speeds. Within 12 months Deep Space 1 will have consumed all of its 180 pounds (80 kilograms) of xenon propellant, having achieved a speed of 9,000 m.p.h. (4 kilometers per second).
Dr Marc Rayman from the Deep Space 1 mission team explained that the Braille flyby was a bonus for the primary mission, which was mainly designed to test new technology. The experience gained at Braille will help them plan an encounter with Comet Borrelly -- the main target of the extended mission. He added that the failure, last November, of the spacecraft's "star tracker" navigation aid meant that they had to drop plans to reach a second comet, but otherwise, the failure would not seriously hamper the extended mission.
(Credit: NASA/JPL)
Slingshots from planets
Many recent interplanetary space missions have involved a gravity-assist Earth flyby. This slingshot technique (illustrated below) can produce substantial reductions in the size of the rocket needed to reach a planet or asteroid. For example, in January 1998 the NEAR spacecraft whizzed within 340 miles (550 kilometers) of Earth's surface.
(Credit: Johns Hopkins University)
This planned encounter changed the course of the spacecraft so that it would reach Eros one year later. (Unfortunately a technical bug had prevented it from going into orbit and the mission scientists had to wait an additional year for the next opportunity, which is on February 14, 2000.)
Of course, an Earth flyby would be very difficult to sell to the world's population if the spacecraft was carrying nuclear weapons intended to deflect an asteroid. Adding further to the difficulties, the best time to nudge an asteroid is when it is closest to the sun, but this can make the mission much more challenging.
Alan Harris, senior research scientist with the Jet Propulsion Laboratory in California, points out a mission to rendezvous with the asteroid 1999 AN 10 -- which is in an orbit that is typical of a "potentially hazardous asteroid" -- would involve a space mission that is formidable with current rocket technology. They run out of fuel well before the necessary speeds are achieved.
Maybe we should be dusting off the blueprints for the giant Saturn 5 rockets that were used for the Apollo moon landings -- just in case we need to quickly intercept an asteroid or comet on a collision course with Earth. This may not be that easy -- in his book "Mining the Sky", planetary scientist John Lewis reports that he went looking for the Saturn 5 blueprints a few years ago and concluded, incredibly, they had been "lost".
Harris cautions that even the mighty Saturn 5 could only deliver a few pounds/kilograms of payload to land on, or orbit, an asteroid such as 1999 AN 10. He adds, "Ion drive is probably the most feasible way out of this quandary."
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