Impactor ejects mighty water mass

By Jonathan Amos
BBC News science reporter, in Leicester

Comets hold materials unchanged since the Solar System's formation

The Nasa projectile that slammed into Comet Tempel 1 last year kicked out at least 250,000 tonnes of water.

The figure comes from UK/US scientists on the Swift telescope, one of many observatories called on to study the US space agency's Deep Impact event.

Swift's X-ray data shows more water was released and over a longer time scale than had previously been thought.

Researchers hope the new information will help them understand better the nature and construction of comets.

"In terms of the science, we got a lot of data that really does support the model of how X-rays are produced at comets, and Swift, because of its agility and the quality of its detectors, will be ideally placed to observe another comet when one becomes visible," said Dr Dick Willingale, of the University of Leicester, who led the Swift study.

Long view

The orbiting Swift's "day job" is to hunt down the colossal bursts of gamma-ray radiation that flash randomly across the cosmos.

However, on 4 July last year, it was among a fleet of space and ground-based telescopes asked to watch what happened when Nasa's Deep Impact probe released a 370kg projectile into the path of the 14km-wide Comet Tempel 1.

Swift's main task is to investigate flashes of gamma-ray radiation

The crash released a plume of material that was examined at a range of different wavelengths.

But whilst the other observatories made relatively quick studies and then turned away, Swift continued to look at the impacted "ice mountain" on and off for more than 60 days. Its patience paid off.

Swift's X-ray Telescope (XRT) saw the comet continue to release water for some 13 days after the initial event, with a peak five days on from the collision.

X-rays provide a direct measurement of the colossal amount of water thrown out as a result of the impact - the Earth-equivalent volume of about 100 Olympic-sized swimming pools.

Exciting time

The radiation traces the interaction between the neutral water molecules and the solar wind, the stream of charged, heavy particles that continually billow away from the Sun.

"The solar wind particles smash into the cometary particles and a process which we call 'charge exchange' occurs," explained Dr Paul O'Brien, also of Leicester University.

Animated guide: Deep Impact

"The upshot is that the heavy ions from the Sun get into an excited state and then they de-excite themselves by emitting photons of light which turn out to be X-rays, typically.

"Essentially, the more material liberated, the more X-rays are produced."

The X-ray power output depends on both the water production rate from the comet and the local conditions of the solar wind in the vicinity of the comet, which at the time was about 130 million km from Earth.

Using data from another satellite called ACE, which constantly monitors the solar wind, the Swift team managed to calculate the solar wind flux at the comet during the X-ray outburst. This enabled them to disentangle the two components responsible for the X-ray emission.

Lifted grains

The question is why the comet continued to eject material for so long after the initial impact.

Comets are continually losing material, especially when they get close to the Sun; and astronomers occasionally see sudden outbursts which are most likely associated with the natural impacts of meteorites.

DEEP IMPACT: 4 JULY The explosive moment of impact on Comet Tempel 1 In pictures

But the long period of raised X-ray emission seen in the case of Deep Impact is a bit of a puzzle, especially since other views of the comet at optical wavelengths suggested outgassing died down relatively shortly after the initial hit.

"I have a hunch that the comet got shook up, so that quite a lot of material was lifted but was still loosely bound to the surface. And you can imagine that if you lift ice grains, it is much easier to sublime (turn straight from solid to gas) them; they get exposed to more radiation from the Sun," said Dr Willingale.

"All this is supposition; all we know is that when we put in the numbers we seem to get an excess coming off of about two and a half times over the quiescent level."

The results of the Swift observations of the Deep Impact event are being discussed here at Leicester University, which is hosting this year's National Astronomy Meeting.

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