NAVCAM/Rosetta/ESA, CC BY-SA IGO 3.0
For nearly 26 months, a European Space Agency craft has been orbiting a comet. At 7:19 a.m. Eastern time on September 30, that came to an end. As it prepared to land on the comet’s surface and shut down, the Rosetta spacecraft snapped its last photos and gathered its last data. Then it quickly sent these — the most detailed views of a comet ever — to researchers back home.
The last image was taken 51 meters ( 167 feet) above the surface of comet 67P/Churyumov-Gerasimenko. It showed the spacecraft falling toward a gravelly surface. The landing site has been dubbed Sais. That's the name of the ancient Egyptian town believed to be the original home of the Rosetta stone, for which the mission was named. “We can say Rosetta has come home,” said Patrick Martin . This planetary scientist is the Rosetta mission manager for the European Space Agency.
The spacecraft launched more than 12 years ago. Rosetta arrived at 67P on August 6, 2014. Some three months later, a lander named Philae detached from the orbiter and dropped to the comet’s surface. It was a rough landing: Philae bounced twice. Then it nicked a ridge before coming to rest on its side in the shadow of a cliff. With too little sunlight to charge its battery, Philae went to sleep about 2.5 days later. Before it did, it sent back to Earth some pictures of its new home.
Unlike Philae, Rosetta was never designed to survive a touchdown on the comet. Though it was due to land at about a walking pace — 3 kilometers (1.9 miles) per hour — this “controlled impact” was expected to likely break apart the spacecraft.
Now that it's over, “Feelings are mixed,” says mission lead Matt Taylor. He works for the European Space Research and Technology Center in Noordwijk, the Netherlands. He says there is “sadness that this is over, but great joy on what [we’ve] achieved.”
Before bidding the spacecraft goodbye, here’s a look back at five mission highlights.
- One comet, two comet? One surprise emerged as researchers got their first good look at the comet. Described as resembling a rubber duck, comet 67P has two distinct lobes. Some planetary scientists suspect that 67P was once two comets that smooshed together.
- Not just an iceball. Comets are not just big balls of ice, as once thought. Towering cliffs, dusty dunes, shadowy pits — the landscape on 67P is a hodgepodge of terrains. Some of them have been scarred by erosion. Dust has blanketed others under seasonal flows of fine dust. Comets “are much more dynamic than a lot of surfaces in the solar system,” Sunshine says.
- Its water is weighty. Water on the comet is unlike Earth’s. This suggests that comets provided little help in bringing H2O to our planet. The ratio of deuterium (Dew-TEER-ee-um) — a heavy form of hydrogen — to regular hydrogen in the comet’s water is roughly three times as high as that on Earth. Comets as a whole, however, show a large range in this ratio of hydrogen isotopes. This seems to suggest that comets did not all develop the same way.
- Comet 67P carts around a cocktail of chemicals. These include oxygen and noble (inert gases such as argon). Both indicate a birthplace that was cold and far from the sun. Organic (carbon-based) molecules are also present. While asteroids probably delivered the bulk of water to Earth, “comets do have complex organics and could have brought those to Earth and provided the seeds of life,” says Taylor.
- It's full of holes. The interior of the comet is quite porous. This suggests that 67P was assembled gently 4.6 billion years ago. If true, the comet’s building blocks weren’t altered by being smashed together. And that would support the long-standing idea that comets are time capsules, preserving samples from when our solar system was forming.
Don’t worry. Comet science won’t end with Rosetta. Ground-based telescopes will continue to study these long-distance orbiting bodies from afar. And next year NASA will consider proposals for not only flying a spacecraft to a comet, but also plucking a piece off its surface and bringing it back to Earth.
“As for Rosetta data, there is loads of it,” says Taylor. “There is decades of work to do. So Rosetta isn’t over — it’s just begun.”
Indeed, notes Jessica Sunshine, “Every time you look at a body and increase the resolution ... it’s another world.” She’s a planetary scientist at the University of Maryland in College Park. She says of 67P, poring over those data, “It’s going to be very interesting to see what this place looks like.”
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argon An element first discovered on August 13, 1894, by Scottish chemist Sir William Ramsay and English physicist John William Strutt, better known as Lord Rayleigh. Argon was the first “noble” gas, meaning one that does not react chemically with other elements. For its discovery, Ramsey would receive the 1904 Nobel Prize in chemistry and Rayleigh the 1904 Nobel Prize in Physics.
asteroid A rocky object in orbit around the sun. Most orbit in a region that falls between the orbits of Mars and Jupiter. Astronomers refer to this region as the asteroid belt.
carbon The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.
chemical A substance formed from two or more atoms that unite (become bonded together) in a fixed proportion and structure. For example, water is a chemical made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O. Chemical can also be an adjective that describes properties of materials that are the result of various reactions between different compounds.
comet A celestial object consisting of a nucleus of ice and dust. When a comet passes near the sun, gas and dust vaporize off the comet’s surface, creating its trailing “tail.”
deuterium An isotope of hydrogen which consists of a proton, neutron and electron. The proton-neutron nucleus is also referred to as deuteron.
dynamic An adjective that signifies something is active, changing or moving. (noun) The change or range of variability seen or measured within something.
erosion The process that removes rock and soil from one spot on Earth’s surface, depositing it elsewhere. Erosion can be exceptionally fast or exceedingly slow. Causes of erosion include wind, water (including rainfall and floods), the scouring action of glaciers and the repeated cycles of freezing and thawing that occur in many areas of the world.
hydrogen The lightest element in the universe. As a gas, it is colorless, odorless and highly flammable. It’s an integral part of many fuels, fats and chemicals that make up living tissues.
inert Inactive or having no chemical or physical effects.
isotopes Different forms of an element that vary somewhat in weight (and potentially in lifetime). All have the same number of protons but different numbers neutrons in their nucleus. As a result, they also differ in mass.
lander A special, small vehicle designed to ferry humans or scientific equipment between a spacecraft and the celestial body they will explore.
lobe A rounded and somewhat flat projection. Many leaves, for instance, have lobed edges.
molecule An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).
NASA Short for the National Aeronautics and Space Administration. Created in 1958, this U.S. agency has become a leader in space research and in stimulating public interest in space exploration. It was through NASA that the United States sent people into orbit and ultimately to the moon. It has also sent research craft to study planets and other celestial objects in our solar system.
orbiter A spacecraft designed to go into orbit, especially one not intended to land.
organic (in chemistry) An adjective that indicates something is carbon-containing; a term that relates to the chemicals that make up living organisms. (in agriculture) Farm products grown without the use of non-natural and potentially toxic chemicals, such as pesticides.
oxygen A gas that makes up about 21 percent of the atmosphere. All animals and many microorganisms need oxygen to fuel their metabolism.
planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and it must have cleared other objects out of the way in its orbital neighborhood. To accomplish the third feat, it must be big enough to pull neighboring objects into the planet itself or to sling-shot them around the planet and off into outer space. Astronomers of the International Astronomical Union (IAU) created this three-part scientific definition of a planet in August 2006 to determine Pluto’s status. Based on that definition, IAU ruled that Pluto did not qualify. The solar system now includes eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
porous The description of a substance that contains tiny holes, called pores , through which a liquid or gas can pass. (in biology) The minute openings in the skin or in the outer layer of plants.
range The full extent or distribution of something. For instance, a plant or animal’s range is the area over which it naturally exists. (in math or for measurements) The extent to which variation in values is possible. Also, the distance within which something can be reached or perceived.
ratio The relationship between two numbers or amounts. When written out, the numbers usually are separated by a colon, such as a 50:50. That would mean that for every 50 units of one thing (on the left) there would also be 50 units of another thing (represented by the number on the right).
solar system The eight major planets and their moons in orbit around the sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.
sun The star at the center of Earth’s solar system. It’s an average size star about 26,000 light-years from the center of the Milky Way galaxy. Or a sunlike star.
technology The application of scientific knowledge for practical purposes, especially in industry — or the devices, processes and systems that result from those efforts.
telescope Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.
terrain The land in a particular area and whatever covers it. The term might refer to anything from a smooth, flat and dry landscape to a mountainous region covered with boulders, bogs and forest cover.
Journal: M. Fulle et al. Unexpected and significant findings in comet 67P/Churyumov–Gerasimenko: an interdisciplinary view. Monthly Notices of the Royal Astronomical Society. Published online September 8, 2016. doi: 10.1093/mnras/stw1663
Blog: How to follow Rosetta’s grand finale. Rosetta blog. Published online September 27, 2016.
Blog: Comet watch 18 September — a new view of Rosetta’s impact site. Rosetta blog. Published online September 23, 2016.