of some comets may be fragile, a conclusion supported by the observation of comets splitting apart. Splitting comets include 3D/Biela in 1846, Shoemaker–Levy 9 in 1992, and 73P/Schwassmann–Wachmann from 1995 to 2006. Greek historian Ephorus reported that a comet split apart as far back as the winter of 372–373 BC. Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact. Comets 42P/Neujmin and 53P/Van Biesbroeck appear to be fragments of a parent comet. Numerical integrations have shown that both comets had a rather close approach to Jupiter in January 1850, and that, before 1850, the two orbits were nearly identical. Albedo Cometary nuclei are among the darkest objects known to exist in the Solar System. The Giotto probe found that Comet Halley's nucleus reflects approximately 4% of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly's surface reflects only 2.5–3.0% of the light that falls on it; by comparison, fresh asphalt reflects 7% of the light that falls on it. It is thought that complex organic compounds are the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like tar or crude oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing. Roughly six percent of the near-Earth asteroids are thought to be extinct nuclei of comets (see Extinct comets) which no longer experience outgassing. Two near-Earth asteroids with albedos this low include 14827 Hypnos and 3552 Don Quixote. Discovery and exploration The first relatively close mission to a comet nucleus was space probe Giotto. This was the first time a nucleus was imaged at such proximity, coming as near as 596 km. The data was a revelation, showing for the first time the jets, the low-albedo surface, and organic compounds. During its flyby, Giotto was hit at least 12,000 times by particles, including a 1-gram fragment that caused a temporary loss of communication with Darmstadt. Halley was calculated to be ejecting three tonnes of material per second from seven jets, causing it to wobble over long time periods. Comet Grigg–Skjellerup's nucleus was visited after Halley, with Giotto approaching 100–200 km. Results from the Rosetta and Philae spacecraft show that the nucleus of 67P/Churyumov–Gerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation