BP has commenced its “top kill” attempt to choke off the oil spewing into the Gulf of Mexico by pumping heavy drilling mud and cement into the breached mile-deep well. If the operation succeeds, BP would then inject cement to seal the well shut. The procedure has previously worked above ground but has never been attempted at such deep sea levels. BP says it hopes to know by later today whether the top kill is succeeding. It’s estimated a 70 percent chance of success, while acknowledging the operation could result in even more breaches of the well, worsening the spill.

Scientists are issuing dire warnings about the scenario of the top kill’s failure. Professor Tad Patzek of the Department of Petroleum and Geosystems Engineering at the University of Texas-Austin told The Hill newspaper that an unchecked leak “would be an environmental disaster of a caliber heretofore unseen by humanity.” President Obama meanwhile has announced he’ll return to the Gulf Coast on Friday, his second visit to the region since the spill began.

President Obama: “A lot of damage has been done already — livelihoods destroyed, landscapes scarred, wildlife affected. Lives have been lost. Our thoughts and prayers are very much with the people along the Gulf Coast. And then let me reiterate: we will not rest until this well is shut, the environment is repaired, and the cleanup is complete. And I look forward to returning there on Friday to review the efforts currently underway and lend my support to the region.”

In other oil spill news, the Coast Guard has ordered all 125 commercial ships helping with the cleanup effort to return to land after several people fell ill.

The Obama administration meanwhile is suspending both new exploratory drilling in the Arctic Ocean as well as lease sales off Alaska and Virginia. In a report to be released today, Interior Secretary Ken Salazar says he won’t consider new drilling permits until next year. The six-month moratorium is a major setback for the oil giant Shell, which had been set to begin drilling in Alaska’s Beaufort and Chukchi seas in July. Native and environmental groups have warned the drilling could chase away sea life and contaminate the ocean.

On Capitol Hill, congressional leaders have agreed to reduce a nearly $200 billion package of jobless benefits and other economic aid. On Wednesday, Democrats announced they would cut the package to $145 billion to address concerns from lawmakers on both sides of the aisle about increasing the federal debt. Unemployment benefits would now be extended through November instead of the end of the year. A vote is expected on the measure today.

The Obama administration and Democratic leaders, meanwhile, are rallying support for a $23 billion aid measure for the nation’s public schools. Over 100,000 teachers face the loss of their jobs if the bill fails in Congress.

Police chiefs from some of the nation’s largest cities have gathered in Washington to voice opposition to Arizona’s recent anti-immigrant law. The measure requires police officers to stop and interrogate anyone they suspect is an undocumented immigrant. On Wednesday, police chiefs from Los Angeles, Houston, Philadelphia and six other cities warned Attorney General Eric Holder that crime will increase if other states pass similar laws. Tucson Chief of Police Roberto Villaseñor said the law undermines police work.

Roberto Villaseñor: “When you enact legislation that makes any subset of that community feel like they are being targeted specifically or have concerns about coming forward and talking to police, that damages our capability to obtain information to solve the crimes that we need to work with. This is not the focus of local law enforcement. Immigration is the focus of the federal government, and by bringing local law enforcement into the picture, it damages that relationship that we have spent years cultivating to try and get us into a position where we can work well with our community.”

The police chiefs’ meeting with Holder has helped fuel speculation the Obama administration is preparing a federal lawsuit to challenge the Arizona law. Lawmakers in fifteen states are currently considering passing anti-immigrant laws similar to Arizona’s.

Five protesters were arrested in Houston on Wednesday at the annual shareholders meeting of the oil giant Chevron. The True Cost of Chevron Network says it organized the protest to call attention to Chevron’s human rights and environmental record. One of the five arrested, author Antonia Juhasz, was detained after questioning Chevron CEO John Watson during an open comment period for proxy holders. Outside the meeting, Maria Lya Ramos of the Rainforest Action Network’s Change Chevron Campaign stood with Ecuadorian activist Maria Jimenez, who was able to briefly address the shareholders inside.

Maria Lya Ramos: “You cannot lie to her. She has seen it. She’s a grandmother of seventy-one. She has had two miscarriages. Her children have become ill. She has lost her sister. Her sister has died because of being in contact with contaminated water. You cannot lie to her, because she is living witness to the contamination of this company. Shame on Chevron!”

In a possible violation of SEC rules, the activists say Chevron allowed entry to just seven of the twenty-seven members of their group who hold legal shareholder proxies. Most of those denied entry were activists who had traveled from Ecuador, Nigeria, Burma and several other countries to speak out against Chevron’s operations in their countries.

A showdown is looming in the Mediterranean Sea as a flotilla of nine humanitarian aid ships approaches the Gaza Strip in an attempt to break the Israeli blockade. Israel is vowing to repel the Free Gaza Movement’s “Freedom Flotilla” after stopping at least three other sailings since January 2009. The ships are expected to reach the Gaza coastline by Friday. Israeli spokesperson Yigal Palmor vowed Israel would stop the ships.

Yigal Palmor: “This is not in the cause — for the cause of the Palestinian people, not for the cause of peace. This is just for the cause of provocation by people who care not about humanitarian aid or about the prospects of peace. And if they try to force their way into Gaza, after they have been warned time and again not to do so, after they have been warned that this is against international law, well, then they will have to be stopped.”

The flotilla is the largest to attempt to reach the Gaza Strip since Israel imposed the blockade on the coastal territory three years ago.

In Peru, the jailed American activist Lori Berenson is expected to be released from prison today after being granted parole earlier this week. Berenson has been jailed for over fourteen years after hooded Peruvian military judges convicted her of collaborating with the rebel group MRTA. She has long maintained her innocence. (Related coverage: After Over 14 Years in Peruvian Prison, Jailed US Activist Lori Berenson Ordered Freed on Parole)

And in other news from Peru, indigenous leader Alberto Pizango has been arrested after returning from an eleven-month political asylum in Nicaragua. Pizango has been wanted in Peru on sedition charges after leading protests opposing laws that encourage foreign mining and energy companies to invest billions of dollars in the Amazon rainforest.