Deep impact

AN ARTIST'S CONCEPTION shows the NASA mission's impactor hurtling away from the spacecraft towards Comet Tempel 1. - BALL AEROSPACE & TECHNOLOGIES CORPORATION/REUTERS

The NASA space mission to investigate the composition of Comet Tempel 1 brings revolutionary findings.

THE Deep Impact space mission of the National Aeronautics and Space Agency (NASA) has been a smashing success. The Deep Impact flyby spacecraft threw its hammer towards the comet Tempel 1 to break it open and see what was inside. The spacecraft scored a direct hit. It took the most detailed pictures ever of a comet nucleus and provided a glimpse into the object's interior. Some revolutionary findings from the analysis of the data received from this mission were released recently.

Why did NASA plan such a mission? Comets are fleeting visitors to the environs of the earth. They spend most of their time in the far-away dark regions of space, beyond the reach of spacecraft and, often, telescopes. At its heart, each comet is a dirty lump of ice and rock. So, when the comet comes close to the sun, the heat transforms the ice into gas and the comet streams a magnificent tail behind it. Comets are leftovers from the formation of the planets four and a half billion years ago. As such, each one represents a fossilised planetary `building block'; investigating them is a way of learning how the planets were formed. In short, the science performed by Deep Impact is about as important as it can ever get. By studying comets, we learn about pristine material left over from the origin of our solar system. We hope to use what we learn to understand ourselves.

Before discussing the salient points of the Deep Impact mission, we may have a brief look at the basic facts of the astronomy of a comet. Every few years, a bright comet is visible in our sky. From a small, bright area called the head, a graceful tail may extend over one-sixth (30o) or more of the sky. The tail of a comet is always directed away from the sun.

In the last few years, several comets appeared in our sky. Comet Hyakutake came in 1996. It was bright enough that all you had to do was step outside and look up, even in a city. In 1997 came Comet Hale-Bopp, which was even brighter. Both provided opportunities for professional astronomers to use the wide range of equipment now available to learn in detail about the working of comets. Hyakutake was observed by a spacecraft when it was too close to the sun to be seen from the earth.

At the centre of a comet's head is its nucleus. The most widely accepted theory of the composition of comets, advanced in 1950 by Fred L. Whipple of what is now the Harvard-Smithsonian Centre for Astrophysics, is that the nucleus is like a dirty snowball a few kilometres across and is apparently made of ices of such molecules as water, carbon dioxide, ammonia and methane, with dust mixed in. It is relatively small and cannot be resolved by any telescope now in use, but it size has been determined with radar observations.

The rest of the head is the coma, which may grow to be over a million kilometres across. The coma shines partly because its gas and dust reflect sunlight towards us and partly because gases liberated from the nucleus are excited enough by sunlight to radiate light.

The Hubble Space Telescope has viewed the inner parts of the heads of comets that came near the earth in recent years (though not all the way to the nuclei) and enabled us to study their rotation. A comet's tail can extend over 150 million km. The tail of Comet Hyakutake affected a spacecraft 550 million km away, meaning that the tail must be at least that long. But the amount of matter in the tail is very small.

Many comets actually have two tails. Both extend generally in the direction opposite to the sun, but they are different in appearance. The dust tail is made of dust particles released from the ices of the nucleus when they are vaporised. It usually curves smoothly behind the comet. The gas tail (also called the ion tail) is composed of ions blown out more or less straight behind the comet by the solar wind. As puffs of ionised gas are blown out and as the solar wind varies, the ion tail takes on a structured appearance. Magnetic fields in the solar wind carry only ionised matter along with them and the neutral atoms are left behind in the coma.

Of all the comets observed so far from the earth, Halley's Comet is the most famous. At each of its apparitions, the world waits eagerly. The last date of its apparition was in 1986. For the first time, a month after its perihelion, an international armada of spacecraft (total five) reached its vicinity. Since it is expensive in terms of fuel to go out of the earth's orbital plane, all the spacecraft intersected the comet when it passed through our orbital plane.

The spacecraft that flew past the nucleus of Halley's Comet in 1986 sent back photographs showing a dark, irregularly shaped body venting gas and dust from active regions on its sunward side. The surface is so dark that it reflects only 4 per cent of the light that hits it. The dark colour of this dust suggests a composition similar to that of carbonaceous chondrite meteorites. The nucleus was measured by the spacecraft closest to it, Giotto (sent by the European Space Agency), to be about 16 km x 8 km x 7 km. The nucleus was potato-shaped with a surface area of 100 square km and showed depressions and hills.

NASA launched the Deep Impact spacecraft on January 8, 2005. It carried a copper bullet, which was fired into the heart of Comet Tempel 1 on July 4, 2005. The weight of the bullet was 372 kg. Comet Tempel 1 was discovered on April 3, 1867, by Ernst Wilhelm Leberecht Tempel in Marseilles, France. Comet Tempel 1 currently circles the sun every 5.5 years. Its orbit lies between Mars and Jupiter, providing the Deep Impact mission with a perfect target for a modest launch vehicle striking at high speed and visible from the earth at impact about 130 million km away.

What were the instruments and data-transmission capabilities of Deep Impact's flyby spacecraft and impactor? There were three purposes of the scientific instruments: (i) to guide the flyby spacecraft to the vicinity of Comet Tempel 1, (ii) to put the impactor on a collision course with the comet, and (iii) to obtain data before, during, and after the impact.

The flyby spacecraft carried primary imaging cameras, the largest space-borne telescopes built specifically for planetary science, and an infrared spectrometer. The impactor's single scientific instrument was the Impactor Targeting Sensor (ITS), which provided sharp views of the comet's nucleus.

The copper bullet impactor was released by the Deep Impact's main flyby spacecraft from a safe distance of 500 km from Comet Tempel 1. The bullet struck the sunlit side of the comet's 14 km-long potato-shaped nucleus at 37,000 km an hour. The kinetic energy unleashed by this impact was estimated to be the equivalent of detonating nearly five tons of TNT. It was powerful enough to produce a crater 200 metres wide and 50 metres deep on the surface of Tempel 1's nucleus.

Owing to such great impact, on July 4, at 06:00 Universal Time, a special kind of celestial fireworks were witnessed from places on the earth where the comet was in a fully dark sky. Such celestial fireworks had never been seen before. Deep Impact's main flyby spacecraft's camera observed and captured images of the crater's formation for about three and a half minutes and then peered into that newly excavated depression for another 10 minutes before speeding away from the comet. Data from the flyby spacecraft and the impactor were radioed back to the earth.

The Deep Impact team has already analysed the data from the crash. Although it will require months of study to get firm results, mission scientists have already learned much about Comet Tempel 1's structure and composition. Some scientists think they have glimpsed the solar system in its earliest days. The close-up images from this mission were the most dramatic.

These views, the highest-resolution images ever taken of a comet's nucleus, reveal a strange surface with enigmatic circular features. There were other surprises too. The comet's rotation period as measured from the spacecraft is 40.7 hours - one hour shorter than what ground-based observations had led scientists to believe. The comet's core reflects a scant 4 per cent of the light it receives. It means the nucleus is darker than charcoal. This agrees nicely with the measured reflectivity of Halley's Comet.

When the impactor struck the nucleus of the comet, the initial flash dazzled the spacecraft's cameras. Immediately afterwards a loop of vapour and a huge plume of dust expanded above the crash site. First there was a small flash, then there was a delay, then there was a big flash. The two-stage nature of the explosion suggests that the washing-machine-sized bullet plowed through a couple of different layers of material. From the volume of dust released from the crater, it was estimated that the crater was greater than 100 metres across.

Findings from the spacecraft's infrared spectrometer are very interesting. The instrument captured a spectrum every 0.7 second. The first thing inferred from the spectrum was the presence of hot water and very hot carbon dioxidesomewhere between 1,000o and 2,000o Kelvin, one-third the surface temperature of the sun. The spectroscopy team also saw evidence of organics, carbon monoxide and volatiles such as hydrogen cyanide. There are hints of silicates from rocks or soil too.

These days interplanetary spacecraft are built to last. Deep Impact will stay in the inner solar system and fly past the earth on December 31, 2007. This puts it in an ideal position to rendezvous with other short-period comets. In late July of 2005, NASA formally called for proposals for additional Deep Impact science projects.

The most fascinating thing coming out of the preliminary analysis of the Deep Impact Space Mission data is this that scientists believe that these dirty snowballs (that is, the nuclei of comets) contain the purest remnants of the solar system's formation. Understanding what makes up comets means understanding the ingredients that came together to form the planets.

In a nutshell, NASA's Deep Impact Space Mission is a grand success.

The author is a senior scientist at the M.P. Birla Institute of Fundamental Research, M.P. Birla Planetarium, Kolkata.