Dan Durda, an asteroid expert and president of the B612 Foundation - which aims to anticipate and prevent such impacts - thinks the discovery of HZ51 highlights that at present there are no good options when faced with so little time to prepare. "There really isn't a whole lot we could do," he told New Scientist. "Most of the options that don't resemble a Hollywood movie involve deflection techniques that require many years or decades."

Other than stockpiling food and supplies and evacuating the regions most likely to be affected, he said, we would have to "hunker down and take the impact".

But this is an unusual case, statistically speaking. It is far more likely that Earth's nations would benefit from a much greater lead time before a potential impact, allowing more time for planning.

For example, the second-most imminent threat now on the list is the asteroid Apophis, which has about a 1-in-6000 chance of hitting Earth in 2036 - plenty of time to prevent it.
Altering orbits

The B612 Foundation has been pushing for a mission to place a tracking device on Apophis sometime in the next decade, so that the possibility of impact can be definitively proved or ruled out. The foundation also wants to send a mission to test ways of altering the orbit of a non-threatening asteroid, to test the viability of such methods.

But the chance of an impact by Apophis might be ruled out as early as this weekend, which will be the last chance until 2013 to observe it by radar, from the Arecibo radio telescope in Puerto Rico.

As for the newfound 2006 HZ51, the orbit calculations so far are based on just over 24 hours of observations, and so are likely to change quickly and should not be seen as a serious concern. As Clark Chapman of the Southwest Research Institute in Boulder, Colorado, US, explains: "Almost certainly, observations from one or two more nights will put this to bed as a zero probability."

She said the mountain has significantly increased its activity, with a higher frequency of multi-phased earthquakes as well as those caused by lava fallout.

"All this indicates that magma pressure is building up and an eruption could follow," Sri said.

The alert status of the 2,914-meter (9,560-foot) volcano remained on "standby" however, as it has for more than two weeks, one level below that which would require a mandatory evacuation for more than 29,000 people.

Ratdomo Purbo, who heads the Vulcanology Research and Technology Development Office in Yogyakarta, was quoted by the Koran Tempo newspaper as saying that the dome had now grown some 10 meters (33 feet) high.

Purbo, who could not be reached for comment on Tuesday, was also quoted as saying that should the dome break or burst, it would spew lava accompanied by pyroclastic flows, or heat clouds.

In its last large eruption in 1994, heat clouds known locally as "shaggy goats" reached 600 degrees Celsius and speeds of over 100 kilometers per hour. They killed 66 people on the southern slope of the mountain.

Hundreds of residents have already been relocated to temporary shelters but many living on the volcano's slopes have refused to leave. Merapi, which has been rumbling intermittently over the past four years, looms above a plain in the south of Central Java province.

Indonesia sits on the Pacific "Ring of Fire" noted for its volcanic and seismic activity. The country has more than 100 active volcanoes.

"It needs to be fixed now, and it needs to be fixed right," the engineers wrote in the 82-page report. "The region's future depends on it."

A dike failure could also contaminate South Florida's drinking water and flood thousands of acres of farmland.

"It would be devastating to our economy, environment and quality of life," Bush said in a letter sent Friday to John Paul Woodley Jr., deputy assistant secretary of the Army for civil works.

Messages left for Woodley were not immediately returned Tuesday.

Bush met Tuesday with Homeland Security chief Michael Chertoff, who was in the state capital for a readiness update on the hurricane season, which begins June 1.

"As I understand it, that presents a different set of evacuation issues than in a normal hurricane," Chertoff said. "That's something obviously we want to work very closely with the state on."

Lake Okeechobee is surrounded by the 143-mile Herbert Hoover dike, built in the 1950s in part to prevent another disaster like the 1928 hurricane, in which flooding and storm surge from the lake killed more than 2,000 people.

The report noted that the dike has a 1 in 6 chance of being breached in a hurricane, Bush said.

The 730-square mile lake is the second-largest natural freshwater lake within the contiguous United States, behind Lake Michigan.

The environmental risk assessments were leaked to Indonesian environment group Wahli. "Freeport has known that their operation is endangering the environment but they don't do anything," Wahli spokesman Torry Kuswardono said.

The environmental assessments show that the company's tailings are polluting the world-heritage Lorentz National Park, which stretches from glacier-capped mountains to a tropical marine environment.

Wahli's Igor O'Neill, who examined the documents, said: "Modelling by an expert employed by Freeport confirmed that the tailings are reaching the coastal part of the national park and testing showed that aquatic animals are contaminated with copper."

Mr O'Neill said that while there was no direct proof that the Freeport mine was the source of high levels of copper, animals tested upstream in the rivers free of mine waste showed normal copper levels.

He said dumping the billion tonnes of tailings in rivers rather than using pipes violated Indonesian law, but the Indonesian Government was reluctant to enforce the law, because "they are scared to challenge Freeport".

Acidic waste produced by the mine was so toxic that it violated even the environmental hazard restrictions for factories.

"The mine is breaching Indonesian industrial standards and breaching water quality laws for lakes, streams and rivers. It is definitely illegal," he said.

Indonesian Environment Minister Rachmat Witoelar dismissed Wahli's claims, saying a team of experts from the ministry had inspected the mine recently.

"I say they are abiding by the law, following all directions," he said. "But we are scrutinising them ..."

Mr Witoelar said pollution from the mine was minor, adding that the team had found some damage to the rivers and this was being monitored.

Wahli has called on the Government to suspend operations at the mine until the company abides by environmental laws.

A Freeport spokesman declined to comment.

West Papuan groups have launched several protests against the mining giant in the past few months, accusing it of environmental and human rights abuses. Four security force officers and one civilian were killed during a violent anti-Freeport protest in the West Papuan capital of Jayapura recently.