Scientists are nervously readying for the first-ever landing of a spacecraft on a comet, scheduled for tomorrow morning.

The European Space Agency is doing a series of checks today to decide whether to give the go ahead for the Rosetta spacecraft to drop its Philae lander toward Comet 67P/Churyumov-Gerasimenko.

If all goes well, the launch will take place 3:35 a.m. ET Wednesday morning.

The 100-kilogram probe will spend seven hours drifting toward the surface of the four-kilometre-wide, duck-shaped comet, which is currently between the orbits of Mars and Jupiter. Philae is scheduled to "land" at 10:30 a.m. ET. Data confirming the launch and landing will arrive at Earth half an hour after the events take place.

The space agency will begin live online coverage of the comet landing starting 2 p.m. ET today. The landing will be streamed live on NASA TV starting 9 a.m. ET Wednesday. It will also be streamed live on CBCNews.ca.

The day will be a tense one, said Ian Wright, a planetary scientist at the Open University in England, who is the principal investigator of one of the instruments on the lander.

Images of the comet sent back by Rosetta have shown in "exquisite and nerve-racking detail just how varied the surface is," Wright told CBC Radio's Quirks & Quarks. "There isn't anywhere really on the surface that you'd say, 'that's an absolutely lovely flat area to land on.' Everything is challenging."

The tiny comet has just 1/100,000th the gravity of Earth, so the lander will rely on screws and harpoons to anchor itself to the surface. But if the surface is too soft, the lander will sink in, and if it is too hard, the probe may bounce off and be unable to anchor.

Fingers crossed

"Fingers still need to be crossed that we land somewhere nice and suitable," Wright said.

If the operation is successful, Philae's Ptolemy instrument, which Wright is in charge of, should soon start sending data to Earth. The device is designed to drill into the comet and detect and analyze what it's made of.

Scientists generally know that comets are made of the original material from which planets such as Earth formed, and that they contain ice and carbon dioxide.

Photos show 67P/Churyumov-Gerasimenko is covered in a jet black layer of sooty material. But scientists don't have any details.

"How deep is this black layer, for instance? Nobody really knows," Wright  said. "By drilling through it, we'll hopefully find out."

If all goes well, Philae will keep sending back data until May, while Rosetta is expected to keep studying the comet until next August.