Attack from outer space

Between Mars and Jupiter there is a vast region full of small rocky bodies: the asteroid belt. The largest of these rocky worlds is Ceres, with a diameter of 950 kilometers (590 miles), but most of them are much smaller. Scientists estimate that there are several millions of them with diameters over a kilometer. The asteroid belt seems to be a failed planet, a junkyard full of scrap left over from the creation of the Solar System. The original dust particles sometimes merged into relatively big planetoids, but the resulting large chunks apparently never managed to combine to create a proper planet. This was mainly because they were continually being stirred up by the gravity of giant Jupiter.

Not all asteroids orbit in the belt. Some of them follow elliptical orbits that can bring them dangerously close to the Earth. As there are far more asteroids than comets coming close to the Earth, "planetary defense" should primarily focus on them. If we know well in advance that an asteroid is going to hit us, we may be able to land a probe with a large propulsion system on it and push it into a slightly different orbit. If applied on time, even a minute change in direction or speed can make it miss our planet.

However, before we attempt to do something like that, we need to know what the surface of such an asteroid looks like and what it is made of. The surface may, for instance, be too soft and unstable to withstand a lot of rocket thrust, or the uneven distribution of its mass may make it start to tumble if we push at the wrong place.

We should also be able to accurately predict the orbits of potentially dangerous asteroids. Former astronaut Russell Schweickart therefore advocates landing a radio beacon on 2004 MN4, an asteroid about 320 meters (1,050 feet) across that currently has a chance of 1 in 10,000 of hitting the Earth in 2036. With such a beacon attached, we could track this potential killer precisely and calculate its exact orbit to find out if we are on a collision course. If we are, we may be able to do something about it, but only if we find out in time. That is why Schweickart would like NASA to send a radio beacon probe as soon as possible.

However, scientists believe that ground-based observations of 2004 MN4 will be just as effective in fine-tuning the impact risk assessments. Until 2012 the asteroid is too close to the Sun for observation, but in 2013 tracking by telescopes on Earth should enable them to map its orbit more accurately. In 2029 it makes a close approach to Earth, and then we should be able to make the very refined trajectory determinations that will tell us whether we are in any real danger.

In the meantime, we have already learned a lot about asteroids. On its way to Jupiter, NASA's Galileo crossed the asteroid belt and took the opportunity to make the first close-up images of two large ones. Gaspra turned out to be a potato-shaped rock about 18 by 11 by 9 kilometers (11

FIGURE 7.24 NASA's Near Earth Asteroid Rendezvous spacecraft. [NASA]

1.5-m antenna -■ Gallium arsenide

I / solar panels

FIGURE 7.24 NASA's Near Earth Asteroid Rendezvous spacecraft. [NASA]

by 7 by 6 miles) in length. The second, Ida, was much larger. This asteroid - 55 by 24 by 20 kilometers (34 by 15 by 12 miles) in diameter - even turned out to have a little moon of its own, which was named Dactyl.

The first real asteroid mission was NASA's NEAR (Near Earth Asteroid Rendezvous). It became the first spacecraft ever to go into orbit around an asteroid. The prime target was Eros, an irregularly shaped asteroid about 13 by 13 by 33 kilometers (8 by 8 by 21 miles) in size.

The small, octagonal prism-shaped NEAR explorer was equipped with an X-ray/gamma-ray spectrometer and a near-infrared imaging spectrograph to find out what Eros is made of, a multispectral camera for detailed images, a laser altimeter to map elevations and depths on the surface, and a magnetometer to measure Eros's magnetic field. A radio science experiment using the NEAR radio-tracking system's Doppler shift would give some idea of the gravitational field of the elongated asteroid.

NEAR, later renamed NEAR Shoemaker in honor of the famous deceased asteroid and meteorite scientist, Eugene Shoemaker, was launched in February 1996, flew within 1,200 kilometers (750 miles) of another asteroid called Mathilde in June 1997, swept past Earth in January the following year, and then finally got into orbit around Eros on February 14, 2000, the aptly named St Valentine's Day.

Even though Eros is a quite big asteroid, it is still quite a small object and therefore has a very weak gravitational field. As a result, NEAR's orbit around it was very slow and very low; it circled the asteroid at only a

FIGURE 7.25 Eros was investigated by the NEAR Shoemaker spacecraft, which made this picture of the potato-shaped asteroid. [NASA]

couple of kilometers per hour and got as close as 24 kilometers (15 miles) to the surface. This naturally enabled it to do some great science at close distance, and even enabled it to softly touch down on the surface of Eros at the end of its life (more on this in "NEAR Eros").

The next asteroid mission, Deep Space 1, was launched in October 1998. Its primary task was to test new technology for future interplanetary spacecraft, in particular ion propulsion. But just like the later SMART 1 lunar satellite of ESA, NASA's Deep Space 1 also had a scientific goal.

It flew past the 3-kilometer (2-mile) diameter asteroid 9969 Braille on July 28, 1999, at a distance of less than 10 kilometers (6 miles). The spacecraft then moved on and passed the nucleus of comet 19P/Borrelly at a distance of 2,200 kilometers (1,400 miles) in September 2001.

Japan is also joining the asteroid hunt. The ion-propelled MUSES-C probe was launched in 2003, and was renamed Hayabusa once it was put in space. After an Earth swingby on May 19, 2004, the spacecraft made a rendezvous with near-Earth asteroid 25143 Itokawa in September 2005. Asteroids are named by their discoverers, and appropriately this one was named after Hideo Itokawa, the father of rocket science in Japan.

In November Hayabusa released a 600-gram (1.3-pound) lander named Minerva. The little probe was to have photographed the asteroid's surface and recorded temperatures there, but unfortunately contact was lost soon after the lander detached from its mothership.

Nevertheless, the orbiter itself continued to operate reasonably well. For months it made detailed observations while circling the asteroid, then moved in to land on it for a brief period - a bit like a bird of prey swooping down on its victim, but in slow-motion. (In fact, the spacecraft's Japanese name means "peregrine falcon.") Hayabusa was planned to obtain a small sample by shooting a kind of bullet into the surface, then collect some of the expelled debris.

In November 2005 it made two attempts to do this. The first time it touched down, bounced up once, spent 39 minutes resting on the surface and then launched back up in orbit again. Unfortunately it did not shoot its "gun," and thus did not manage to gather any surface material. A week later the second attempt may have been more successful. JAXA, the Japanese Space Agency, said that Hayabusa then probably touched down for a few seconds on the asteroid, and this time the pyrotechnical sample system may have worked successfully.

At the time of writing, Hayabusa is on its way back to Earth. We will not know whether it indeed managed to obtain some powder from the asteroid's surface until its re-entry capsule lands in 2010, hopefully with a few grams of pristine asteroid material. If so, it will be the first sample of an

FIGURE 7.26 Beyond Pluto there are other large planet-like objects that have not been visited by any spacecraft, such as the recently discovered Quaoar and Sedna.

asteroid we can investigate in a laboratory, other than the meteorites found on Earth which we know originate from asteroids. (However, at the time of writing the spacecraft is experiencing major problems with its attitude control, which has already moved the return date from 2007 to 2010.)

NASA's New Horizons Pluto Kuiper Belt Flyby mission was launched in January 2006. Rather than seeking out the asteroids in the inner Solar System, this mission is going much further out. It will first pass Jupiter for a velocity-boosting gravity assist maneuver early in 2007, then reach Pluto and its largest moon Charon in 2015. It will make detailed images and measurements as it passes by at high velocity. After that, the spacecraft is planned to fly on to investigate some of the icy planetoids (large asteroids) that orbit the Sun beyond Neptune, and include Pluto, in a region called the Kuiper Belt.

Earth-based telescopes have already found some planet-like objects beyond Pluto, like Quaoar, a planetoid one-third the diameter of the Moon. The largest one confirmed is Sedna, which has an estimated diameter of about 75 percent of that of Pluto.

However, astronomers have perhaps found a planetoid that is even larger than Pluto. It appears to have a moon, and observations with NASA's Spitzer space telescope have shown that there is methane-ice on its surface. Until it is confirmed, the object is named "2003 UBs^."

As usual, for probes flying this deep into space, the New Horizons mission will incorporate an RTG with radioactive material for electrical power supply. However, there is currently a shortage of the needed plutonium-238, due to a security-related shutdown of the US Department of Energy lab that processes this radioactive material. This means that the probe did not get all the plutonium originally planned.

Whether New Horizons will have sufficient amounts of energy to continue its mission after the Pluto flyby is therefore uncertain. Postponing the launch until all the required plutonium was available would have added three to five years to the probe's transit time and millions of dollars to the mission's cost.

Later in 2006, NASA plans to launch a deep space probe named Dawn. Using ion propulsion, as pioneered by Deep Space 1, Dawn will travel for nine years to reach the two most massive asteroids known, Vesta and Ceres. It will first go into orbit around Vesta and stay there for about nine months. Then, using its solar-electric engine, it will depart and journey further out to reach an orbit around Ceres. Its investigations there are also planned to take about nine months.

Both of the small planets to be visited are located in the main asteroid belt between Mars and Jupiter, but they are very different from each other.

Observations from Earth have revealed that the surface of Ceres is probably rather "primitive," meaning that it has not been altered much by geological processes. It seems to be covered by a layer of dry clay that contains water-bearing minerals, and it possibly has a very weak atmosphere and frost.

Vesta was more active during its early life; it has been resurfaced by basaltic lava flows and may have been (partly) covered by a magma ocean. Wrapped in a layer of basaltic dust, it is also much drier than Ceres.

Vesta has been pounded by smaller space rocks many times during its life, the debris of which have sometimes reached Earth. We know that some of the meteorites found on our planet came from Vesta because they have the exact same composition as the asteroid's surface.

We do not know whether we already have pieces of Ceres in our meteorite collections, because we have not yet been able to determine the composition of Ceres below its clay cover layer. The only way to find out what Ceres is made of is to pay the asteroid a visit, as Dawn will do.

The Dawn mission will investigate the main attributes of the two asteroids, such as their shape, size, mass, composition, density, their magnetic field and the numbers and sizes of craters on their surface. The most important issue that the mission will address is the role of size and water in the evolution of planets.

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