Speakers at the second annual "Humans 2 Mars Summit," running May 6-8 at George Washington University, include NASA chief Charles Bolden, Apollo moonwalker Buzz Aldrin, and space tourist Dennis Tito, who recently announced his own plan to send a married couple on ain 2015.

The conference is co-sponsored by the Space Policy Institute at George Washington and the non-profit Explore Mars organization, which aims to advance the goal of sending people to the Red Planet within the next two decades. Panels at the meeting this week will examine the challenges scientific, technological, and political of . Among the topics to be discussed are Mars agriculture and food production, propulsion and landing technologies, and spacesuit design and life support systems.

"Our goal for the summit is to not only address the challenges of humans going to Mars but also to propose real solutions," Explore Mars executive director Chris Carberry said in a statement. "With the collaboration of experts in the space and science communities and non-traditional players, we will come out of the summit enlightened, encouraged and ready to plan for a human mission to Mars by 2030."

NASA has said it aims to send astronauts to Mars by the mid 2030s. The space agency is building a giant heavy-lift rocket called the Space Launch System and a new crew capsule called Orion to take people beyond low-Earth orbit.

Private companies and non-profits are also aiming for Mars.

In addition to Tito's Inspiration Mars mission, the Netherlands-based nonprofit Mars One foundation recently announced that it isto establish a colony on the Red Planet starting in 2023. Mars One co-founder Bas Lansdorp will speak at the conference as well.

Registration for the conference is open to the public, and the event will be broadcast live online. You canbeginning at 9 a.m. ET (1300 GMT), courtesy of the summit's webcast.

You can also follow the webcasts directly here: http://h2m.exploremars.org/webcast/. Visit SPACE.com this week for complete coverage from the Humans 2 Mars Summit in Washington.

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MADRID (Reuters) - World number one and defending champion Serena Williams continued her serene progress at the Madrid Open when she eased past Spanish wildcard Lourdes Dominguez Lino 6-2 7-5 in the second round on Tuesday.

Chasing a fourth title of the year and the 50th of her career, Williams had too much power for the slightly-built Dominguez Lino, who nonetheless managed to unsettle her illustrious opponent with some clever drop shots and lobs.

Williams was joined in the third round by Maria Sharapova after the Russian second seed thumped American qualifier Christina McHale 6-1 6-2 in Tuesday's night session.

Sharapova, a former number one who won her 29th career title in Stuttgart last month, will play Sabine Lisicki for a place in the last eight. The unseeded German upset 15th seed Dominika Cibulkova of Slovakia 7-6 7-6.

Williams, 31, said she had enjoyed the test against Dominguez Lino and the experience would stand her in good stead the next time she came up against a player like the 47th-ranked Spaniard, who likes to mix up her shots.

"I made a lot of errors, but her game kind of forces you to make errors," the American told a news conference. "It was a really good claycourt tennis match for me.

"Seeing as I have a few more tournaments on clay, it was perfect to play a match like this, so I know if I face another opponent like that I'll have that experience and I'll know how to play."

A 15-times grand slam singles champion, Williams will play unseeded Frenchwoman Kristina Mladenovic or 13th-seeded Russian Maria Kirilenko for a place in the quarter-finals. (Editing by Clare Fallon and Nick Mulvenney)

PONTE VEDRA BEACH, Florida, May 11 (Reuters) - Saturday is called moving day on the PGA Tour and if early indications bear out, a shootout could be in store as Sergio Garcia and Tiger Woods set out for the third round at the Players Championship.

Garcia, the 2008 champion, held a one-shot lead over world number one Tiger Woods, who won in 2001, in what is generally referred to as golf's fifth major.

Woods has had the better of their long rivalry dating back to 1999. Of the 19 times they have been grouped together on the PGA Tour, the American has shot the lower score 12 times and tied the Spaniard on four occasions.

Eighteen players were within five strokes of the lead at the end of the second round, including world number two Rory McIlroy, Masters champion Adam Scott and defender Matt Kuchar.

Lee Westwood, who began the day on nine-under, had an early hiccup when his tee shot at the first settled next to a tree on the left side of the fairway. Hampered by his restricted stance, he struck the tree with his club and whiffed on the ball.

Joining the fray were some of Saturday's early starters, who showed that the Stadium Course at TPC Sawgrass was still ripe for scoring.

Jeff Maggert put his name just under Woods by going six-under through his first 10 holes to reach nine under par for the championship, two strokes off the pace set by Garcia.

Australian Marc Leishman burst up the leaderboard with three birdies in his first four holes to reach nine-under.

Briton David Lynn used four birdies in his first seven holes to climb to eight under par.

Early hints that it could be a good day for scoring came from a pair of South Africans as Louis Oosthuizen, the 2010 British Open winner, and Branden Grace both fired five-under-par rounds of 67.

American Chris Stroud drew the biggest roar in warm, humid conditions when he used a six-iron for a hole in one on the 186-yard 13th hole.

(Editing by Gene Cherry)

The list, which measures an ecosystem's risk of collapse, will be similar to the group's authoritative , which created internationally accepted criteria for assessing extinction risk.

"The Red Lists for species and ecosystems will together provide a more comprehensive view of the status of the environment and its biodiversity than either can on its own," said lead study author David Keith, a professor at the University of New South Wales in Australia.

"Thefocuses on a high level of biodiversity, the habitats for species, as well as their interactions and dependencies, including food webs, Keith told OurAmazingPlanet in an email interview. These are difficult or impossible to address in Red List assessments of individual species, but very important for the functioning of ecosystems and the services that they provide to support our standards of living."

Through 20 case studies, Keith and an international team of biologists and conservationists designed criteria that could assess the health of all of Earth's varied , from spring-fed limestone caves to sparkling coral reefs.

"This is really a unifying framework," said study co-author Richard Kingsford, also a professor at the University of New South Wales. "The most important thing here, from my point of view, is providing evidence that pushes governments to do things to protect these magnificent parts of the world."

Of the ecosystems examined in the case studies, the most endangered site was the . Drained by a massive irrigation project and further devastated by drought and pollution, the inland sea's ecosystem has collapsed the equivalent of species extinction, the study concludes.The rest of the ecosystem threat categories mirror those for species: critically endangered, endangered, vulnerable and least concern.

North American ecosystems appearing in the case studies included Alaska's giant kelp forests; the Great Lakes' rare Alvar beaches, a legacy of glaciers grinding across the landscape; and Caribbean coral reefs.

The IUCN group that developed the Red List of Ecosystems criteria plans to formally propose the framework to IUCN leadership this year. Funding is in place for listing ecosystems in the Americas, and the organization hopes to have a global list in place by 2025.

Development of ecosystem and speciesare also underway the carrot to the Red Lists' stick to help the IUCN promote conservation by rewarding successes.

Here are the 20 case studies published today, from most to least endangered, with the ecosystem type noted if available.

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WASHINGTON (Reuters) - The U.S. Postal Service lost $1.9 billion in its second quarter, as the agency cut employees' work hours but struggled with an ongoing drop in mail volumes, the cash-strapped agency announced on Friday.

The loss for the quarter ended March 31 was an improvement from the $3.2 billion loss during the same period in 2012, when the mail agency had to set aside twice as much for an annual retiree benefits payment.

The independent government agency relies on sales of stamps and other products rather than taxpayer dollars to fund its operations. First-class mail, its most profitable service, decreased by $198 million in the second quarter of fiscal 2013.

Postmaster General Patrick Donahoe said the Postal Service would require a taxpayer bailout in excess of $47 billion by 2017, unless Congress acted.

The Postal Service needed to save $20 billion annually by 2016, he said.

"We do not want to get to the point where extreme options are the only options," Donahoe said. "But time is of the essence."

The Postal Service, which has been losing money for years, announced in February it planned to end Saturday delivery, a move it said would save $2 billion annually. But Congress thwarted that plan when it voted to maintain a six-day delivery schedule.

The Postal Service, which lost nearly $16 billion in fiscal year 2012, is also buckling under the weight of massive payments into its future retirees' healthcare fund, which were mandated by a 2006 law.

The agency has defaulted on several of the payments and expects to do so again this year, but it must still account for the payments in its quarterly statements.

Donahoe has urged lawmakers to pass legislation allowing the Postal Service to modify its business model and raise revenues. But lawmakers remain gridlocked, and no action has been taken to grant it extra flexibility.

The Postal Service has begun consolidating mail-processing facilities, cutting operating hours and shedding more than 190,000 jobs in the past six years in an effort to stem the mounting losses.

But it still needs Congress to free its hands, Donahoe said.

Republican Senator Tom Coburn of Oklahoma and Democratic Senator Tom Carper of Delaware called on Friday for urgent congressional action to curb the Postal Service's growing financial troubles.

"The reality is that any decline of revenue at this rate is unsustainable and threatens the Postal Service's long-term viability," said Carper, chairman of the Senate Government Affairs Committee.

The National Association of Letter Carriers said gains in the second quarter in the parcel business, a rare bright spot as more Americans shop online, showed that cutting delivery days would be foolish.

Package volumes increased by 9.3 percent or $267 million in the second quarter, compared with the same period last year.

"Instead of the postmaster general's "shrink to survive" strategy - which will only begin a death spiral for the USPS - what is needed is a dynamic business plan for the future to take advantage of the many opportunities for growth, including in the exploding package delivery market," Fredric Rolando, the union's president, said in a statement.

(Reporting By Elvina Nawaguna; Editing by Peter Cooney)

What would happen if the push for Internet sales taxes falls apart? In at least a couple of statesMaryland and Virginiait could mean higher gasoline prices.

One reason the Senates Marketplace Fairness Act came up now is that states are hungry for new revenue. Balanced-budget rules and the slow economic recovery have combined to produce a $55 billion budget shortfall affecting all states this year.

That hunt for revenue makes online shoppers an appealing target; estimates are that collecting sales taxes on Internet purchases could put an additional $23 billion a year in state coffers.

Some states are so anxious for the anticipated revenues theyve already committed the money to various projects. Virginia recently approvedthat assumes $258 million in tax receipts from online salesabout 65 percent of which would go toward infrastructure and abolishing the state gasoline tax.

Maryland is also dependent on congressional action, though to a lesser extent.toward offsetting an increase in its own gas taxes. Plans call for phasing in a hike of 20 cents per gallon between now and 2016, but about 7 cents of that could be offset by Internet sales taxes.

If the Marketplace Fairness Act doesnt make it through the House, both Annapolis and Richmond will be left with revenue shortfalls in programs already approved in state budgets. In Maryland, drivers would be hit with the full 20 cent hike in gasoline taxes.

Theyre going to get the money they want for transportation, said Stephen Lee Davis of Transportation for America, whether its coming through the Internet-sales tax or not.

The consequences for Virginia will be a little more extreme. Instead of abolishing its gas tax as planned, the state will reverse course and raise itprobably the last outcome motorists want.

In the long run, this might be a good thing. Because gas taxes havent kept up with inflation in most places, theyre no longer the moneymakers for states they used to be. Raising gas taxes would correct for that downward trend.

And while they dont affect fuel costs to the same extent that crude oil prices do, gas taxes are between six and 14 times more effective at cutting fuel consumption than new fuel-economy standards, . Thats because, unlike advances in fuel efficiency that take place over years, gas taxes produce immediate changes in behaviorsuch as switching to public transit or ditching the car entirely, both of which could have offsetting social benefits.

The increase along with a jump in homebuilder confidence this month suggests the housing market continues to recover.

The National Association of Realtors said Monday that sales rose 2.1 percent to a seasonally adjusted annual rate of 4.79 million. That's up from 4.69 million in September, which was revised lower.

The sales pace is roughly 11 percent higher than a year ago. But it remains below the more than 5.5 million that economists consider consistent with a healthy market.

As the economy slowly recovers, more people have started looking to buy homes or rent apartments. Prices are steadily climbing, while mortgage rates have been low all year. At the same time, rents are rising, making the purchase of a single-family home or condominium more attractive.

"Altogether, the report is encouraging," said Michael Gapen, an economist at Barclays Capital. "Our view is that housing is in a recovery phase," he added, though it will be restrained by limited credit and modest job gains.

A separate report Monday showed confidence among homebuilders rose this month to its highest level in six and a half years. The increase was driven by strong demand for newly built homes and growing optimism about conditions next year.

The National Association of Home Builders/Wells Fargo builder sentiment index increased to 46, up from 41 in October. Readings below 50 suggest negative sentiment about the housing market. The index last reached that level in April 2006. Still, the index has been trending higher since October 2011, when it stood at 17.

The Realtors' group said Superstorm Sandy delayed some sales of previously occupied homes in the Northeast. Sales fell 1.7 percent there, the only region to show a decline. Those sales will likely be completed in future months, the group said.

The median price for previously occupied homes increased 11.1 percent from a year ago to $178,600, the Realtors' said.

A decline in the number of homes available for sale is helping push prices higher. There were only 2.14 million homes available for sale at the end of the month, the lowest supply in 10 years. It would take only 5.4 months to exhaust that supply at the current sales pace. That's the lowest sales-to-inventory ratio since February 2006.

Prices are also benefiting from the mix of homes being sold. Sales of homes priced at $500,000 and above have jumped more than 40 percent in the past year. Sales of homes and condominiums that cost less than $100,000 fell 0.6 percent.

There have been other positive signals from the housing market. Applications for mortgage loans to buy homes jumped 11 percent in the week ended Nov. 9, compared with a week earlier, the Mortgage Bankers' Association said last week. Purchase applications are up 22 percent in the past year.

Foreclosures are slowing. The number of properties that began the foreclosure process in the first 10 months of the year fell 8 percent compared with the same period last year, RealtyTrac said last week.

And builders broke ground on new homes and apartments at the fastest pace in more than four years in September. The jump could help boost the economy and hiring.

Still, the market has a long way back to full health. Many potential home buyers cannot meet stricter lending standards or produce larger down payments required by banks.

That can be a particular problem for first-time homebuyers. They accounted for 31 percent of sales in October, down slightly from September and below the 40 percent that is common in a healthy market.

Federal Reserve Chairman Ben Bernanke said Thursday that banks' overly tight lending standards may be preventing sales and holding back the U.S. economy.

___

Veiga reported from Los Angeles.

"These are purely allegations and very disappointing," Allegri told a news conference before Milan's home match against Torino on Sunday.

"It upsets me to read that I've signed for Roma. I believe myself to be an honest person and respectful towards my place of work. If anything was to happen the first person to know about it would be the president (Silvio Berlusconi) and CEO (Adriano) Galliani.

"I think this discussion is closed, and thankfully it was closed by the clubs before I had to speak."

Milan earlier denied the reports in Milan-based paper Corriere della Serra that Allegri had agreed a three-year contract with Roma after refusing a deal to join Inter Milan.

"AC Milan categorically denies what was published today in the pages of the Corriere della Sera regarding its manager Massimiliano Allegri," the club said on their website (www.acmilan.com).

Roma and Inter also denied the report.

"AS Roma categorically denies today's report in Corriere della Sera... which refers to an alleged agreement made with Mr Massimiliano Allegri," Roma said.

Inter said there had been no contact between them and Allegri regarding him taking over from under-fire Andrea Stramaccioni.

"Regarding what was reported in today's edition of the Corriere della Sera on the future of the Nerazzurri bench, FC Internazionale categorically denies that has been any contact or requests for contact with AC Milan manager Massimiliano Allegri," Inter said in a statement.

Allegri's position with Milan has never been secure despite a strong second half of the season during which they have only lost one game, to leaders Juventus, and have climbed to third in Serie A.

They are a point ahead of Fiorentina and play three of their last four games against relegation strugglers Torino, Pescara and Siena, making them favourites to take the third Champions league spot.

Opposition candidate Henrique Capriles walked in a crowd of supporters through upscale neighborhoods in the east of Caracasduring a march to celebrate International Workers' Day.

He called for an end to a government crackdown on his backers, and reiterated plans to challenge his narrow election loss in both Venezuelan and international courts.

In downtown Caracas, the government held its own march, featuring songs praising President Nicolas Maduro and his mentor, late president Hugo Chavez.

Both sides appeared to be trying to avoid confrontation by choosing separate locations and calling for peaceful demonstrations, although tensions were running high.

Outside the Justice Ministry, organizers set up a 30-foot-tall inflatable Chavez balloon with its fist raised in the air.Many wore red T-shirts with pro-government slogans.

"This government is defending workers' rights, increasing salaries like it should," said Juan Ramirez, a 49-year-old employee of the state telecommunications company. "Of course, there will have to be more raises to make up for inflation."

Tuesday night's clash erupted when members of the opposition coalition unfurled a banner in the National Assembly denouncing a ruling that strips them of most legislative powers unless they recognize Maduro's April 14 victory.

Assembly member Julio Borges appeared on an independent television station soon after Tuesday night's brawl with blood running down one side of his swollen face. The opposition said at least 17 of its allies and five pro-government deputies were injured.

Opposition lawmaker Ismael Garcia said government loyalists threw the first punches. Pro-government legislators appeared on state TV accusing opposition members of attacking them. Video showed groups of legislators shoving and pushing each other on the floor.

The opposition has refused to accept Maduro's narrow victory, saying the government's 1.49 percent margin resulted from fraud, including votes cast in the names of the thousands of dead people found on current voting rolls.

In retaliation, the government-dominated assembly has barred opposition lawmakers from public speaking and sitting on legislative committees.

Capriles is boycotting an official audit of the election and plans to file a challenge seeking to overturn it in court.

Maduro accused the opposition of provoking Tuesday's violence, which he condemned, and called on the country to work out its disputes peacefully.

National Assembly chief Diosdado Cabello has repeatedly defended barring opposition lawmakers from speaking. He said that if they don't recognize the legitimacy of the presidential election, they are casting doubt on the very system that elected them, thus losing their own legitimacy.

Opposition lawmakers have also lost their seats on legislative commissions.

___

Associated Press writers Michael Weissenstein and Fabiola Sanchez contributed to this report.

Bahrain did not see a repeat of the mass demonstrations that overshadowed last year's race - though young men armed with rocks did clash with police in outlying villages, as they have done regularly since unrest erupted in early 2011.

Protests in the Gulf Arab country, a Western ally that hosts the U.S. Navy's Fifth Fleet, broke out two years ago, with the Shi'ite-led opposition drawing thousands of demonstrators demanding democratic reforms from the Sunni-led government.

Witnesses at the Sakhir desert race circuit, about 30 km (20 miles) southwest of the capital Manama, said there was no sign of unrest in the immediate vicinity but protesters blocked several roads in villages near the capital.

The opposition said the race was being used as a public relations stunt, but the government insisted it was a pure sporting event that should not be 'politicized'.

Security sweeps ahead of this year's contest "thwarted a number of terrorist plots that aimed to affect normal life ... harm the reputation of the nation and commit terrorist acts against policemen", Bahrain's chief of public security, Major-General Tariq Al-Hassan, said according to the BNA agency.

Security forces had found several weapon caches holding 1,000 petrol bombs, 19 mock bombs, bullets and homemade guns, he added.

Hassan said security forces had also handled several incidents of rioting, including "acts of chaos and destruction" inside an industrial secondary school by students who had also blocked nearby roads and attacked cars, pedestrians and policemen, according to BNA.

"Police at no point in time raided the school or attacked it," Hassan said.

Sayed Yousif al-Muhafdha of the Bahrain Centre for Human rights had said on Sunday police had fired teargas at a secondary school in the city where students had been demonstrating.

On Tuesday, Muhafdha added security forces had arrested up to 50 "pro-democracy activists" in the days preceding the race.

Amnesty International said human rights activists reported dozens of protesters were arrested ahead of the race and Human Rights Watch said on April 10 that 20 opposition activists had been arrested in towns near the circuit.

The government has denied those arrests have taken place. It has said several people accused of stealing and burning cars had been detailed.

Widespread unrest forced the cancellation of the 2011 Formula One race and although the event went ahead in 2012, it was overshadowed by violent protests in the country.

(This story is refiled to change the headline)

(Writing by Yara Bayoumy; Editing by Sami Aboudi and Andrew Heavens)