On Friday, September 30, 2016, the Rosetta space probe will crash land on a comet. It is not the first time a mission has landed on a small solar system body. In fact, 15 years ago, on February 12, 2001, NASA spacecraft NEAR*, touched down on the surface of asteroid EROS. It was not originally intended to land, but controllers decided to take it down to get closer and closer images of the surface, knowing that contact with the spacecraft would soon be lost anyway. It sent back photos from as close as a few meters from the surface, but contact was lost on touchdown.
Then just two years ago in 2014, a small probe called Philae, a lander from the very same Rosetta mission, scheduled to contact the the comet on Friday, “soft” landed on the surface of a comet for the first time. In this case, Philae was designed and intended to land, which it did, although with complications. Photos and other data were received, but contact was spotty and was lost permanently a few months later.
Now, this week, the Rosetta mothership itself also will touch down on comet (67P/Churyumov–Gerasimenko), although like the NEAR-Shoemaker probe, it was never designed nor intended to do so. It will take photos and readings on the way down, but all contact is expected to be lost with first contact with the surface of the comet. It may be more accurate to refer to the “landing” as a controlled impact.
Too far and faint to be seen in Earth-telescopes, the comet and spacecraft are passing through the area of sky between Spica and Gamma Virginis in the constellation Virgo. (For northern observers, this area of sky sets very shortly after sunset.) The comet and probe will be roughly 719 million kilometers (447 million miles) at the time of spacecraft impact.
The “collision manoeuvre” will begin at 20:50 GMT on Thursday, September 29. That’s 2:50 pm MDT or 4:50 pm EDT. The slow descent will take about 14 hours, with the expected cometary contact within 20 minutes of 10:40 GMT on Friday. This is 4:40 am MDT or 6:40 am EDT. You can follow this online here: http://go.nasa.gov/2cLhy0q
The cometary nucleus is approximately 4.3 by 4.1 km (2.7 by 2.5 mi) in size, and composed primarily of dust-covered ice. 67P/Churyumov–Gerasimenko is a short period comet, reaching its closest point to the Sun every 6.45 years, although never getting bright enough to be seen without a telescope. In fact, in its current orbit it never gets as close to the Sun as the Earth, and is essentially just a cometary nucleus without a visible coma or tail. Its orbit has been significantly altered by the gravity of Jupiter, but it likely originated much farther out in the Kuiper Belt.
(All images courtesy of the European Space Agency, ESA)
* Near Earth Asteroid Rendezvous, later rechristened NEAR-Shoemaker in honor of pioneering Earth-space science Eugene Shoemaker