On July 4, 2005, NASA’s Deep Impact space probe successfully landed on the comet Tempel 1. 

It didn’t so much land as purposely crashed a probe into the comet allowing scientists to peer deep into a comet for the first time.  

Deep Impact Probe on Tempel 1 Comet
Liberia issued this souvenir sheet in 2006 as part of their Space Exploration (2006) set

Deep Impact launched from Cape Canaveral Air Force Station on January 12, 2005, making its way through space, travelling 429 million km (267 million mi) to reach the comet. When it was in range DI released a probe, called an impactor to literally slam full force into the comet, causing it to go deep into it.    

Deep Impact, which released an impactor on comet Tempel 1 to expose materials on its surface, revealed a number of new findings about comets and their composition, including evidence of water ice and organic materials. Researchers now believe that comets may have transported these compounds to Earth at one time, playing an essential role in the formation of the solar system and life on Earth.

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Researchers discover a number of surprising facts about comet Tempel 1 from the Deep Impact experiment: Tempel 1 has a very fluffy structure made up of a fine dust that is weaker than a bank of powder snow, but that’s held together by gravity; what appear to be impact craters can be seen on the surface of the comet; a huge increase of carbon-containing materials were detected when analyzing the comet’s ejection plume, indicating that comets contain a substantial amount of organic material and may have brought that material to Earth at one time; and the comet’s interior is well shielded from solar heating, meaning that the ice and other material deep within the comet nucleus may be unchanged from the early days of the solar system. 
Deep Impact – Asteroid & Comet Missions – NASA Jet Propulsion Laboratory | NASA Jet Propulsion Laboratory (JPL)

On-board Tempel 1 were sensors and high and medium resolution instruments to help process the data gathered by the impactor.

Although it’s main mission ended on August, 2005, like much of what NASA built, it worked long past it’s initial mission. It was used to flyby and examine 103P/Hartley 2 comet in 2010. Communication was finally lost with the probe August 2013, with NASA stopping all efforts to reconnect in September 2013. NASA has a number of excellent pages on Deep Impact and the work it performed, all articles are informative but written in a form that is easy for laypeople to understand.. Deep Impact (EPOXI) – NASA Science  

Ah, the USA, before the Party of Hate and Ignorance ran roughshod over education. 

Last year, the Stamp of the Day was a tribute to the good people in the USA. Today, it’s a celebration of what they have achieved – a spirit of exploration and curiosity. No fear of looking into the future and taking it on. Have a safe July 4th my American readers. There are millions of us wishing you strength and perseverance.  Fight the fascists at every chance. Education and unity is the key. Don’t let the bastards win.