Comet Hyakutake sweeps past Earth
On 25 March 1996 this comet passed about 15 million km from Earth. Because it came so close, its tail stretched nearly 80 degrees across the sky.
Available views: Orbit ✓ · Sky — (not visible in the sky)
The story
On 30 January 1996 the Japanese amateur astronomer Yuji Hyakutake spotted a new comet through a pair of large binoculars. He had actually been searching for another comet that he had discovered a few weeks earlier. The new comet was heading toward Earth quickly, and its orbit showed that it would pass very close about two months later.
On 25 March the comet came within 0.1 AU of Earth. That is one tenth of the distance between Earth and the Sun, or about 15 million km, roughly forty times the distance to the Moon. Very few comets in the previous two hundred years had passed closer.
Because it was so near, the comet looked large and bright. Away from city lights, people could see its blue-green head and silvery tail with the naked eye, and the tail reached nearly 80 degrees across the sky. The comet also moved quickly against the background stars, so that anyone watching for a few hours could notice its position change.
Scientists had surprises too. Hyakutake was the first comet found to give off X-rays. On 1 May the Ulysses spacecraft passed through its tail by chance, and its measurements showed that the tail was at least 570 million km long. The comet will not return for about 70,000 years.
When and where to see it
Comet Hyakutake is now very far from the Sun and cannot be seen with any telescope. One trip around its orbit takes about 70,000 years, so its next return lies just as far in the future. To see it again we have to rely on records and simulations.
Choose Hyakutake in the sky view's event list to replay the northern sky of late March 1996. At that time the comet passed close to Polaris, the pole star. For that reason it never set for observers in the northern hemisphere, and by changing the time you can watch it move quickly among the stars.
In the orbit view's event replay, the comet's path cuts inside Earth's orbit on its way toward the Sun. Slow the playback to see first the close pass by Earth on 25 March and then the closest approach to the Sun on 1 May.
See this for yourself — replay the moment in the orbit view
Past returns and history
Yuji Hyakutake had discovered another comet, C/1995 Y1, in December 1995. That comet never became bright enough to see with the naked eye. While scanning the sky to observe it again, he found the great comet of 1996 nearby. He died in 2002 at the age of 51.
After passing Earth, the comet reached its closest point to the Sun on 1 May, at 0.23 AU. That is closer to the Sun than Mercury's orbit. As it headed toward the Sun it sank lower in the northern sky, and after May it was hard to see from the northern hemisphere.
Orbit calculations show that the comet's previous trip close to the Sun was about 17,000 years ago. On this visit the pull of the planets stretched its orbit, so the next visit will come after about 70,000 years. Scientists think comets with such long orbits come from the Oort cloud, the outermost part of the solar system.
Because it came so close, it was observed in great detail. Photographs caught the tail breaking off and forming again when the solar wind grew strong. The Hubble Space Telescope saw small fragments breaking away near the nucleus. Even so, the nucleus itself did not fall apart and survived its pass around the Sun.
Why it matters
Hyakutake shows that a comet's brightness does not depend on size alone. Its nucleus was only about 4 km across, which is not large for a comet. It became a great comet because it came so close to Earth. Hale–Bopp, which arrived the following year, was the opposite case: it stayed far away but had a huge nucleus. Taken together, the two comets show that brightness depends on both size and distance.
The X-ray discovery opened a new area of comet research. Nobody expected a cold lump of ice to give off X-rays. Scientists found the cause in the solar wind. Fast charged particles streaming from the Sun collide with the gas around a comet and grab electrons from it, and this process releases X-rays. Today researchers use these X-rays to study how the solar wind and comets interact.
The accidental tail crossing by Ulysses is also an important record. A comet's ion tail is carried by the solar wind and points away from the Sun for a very long way. A length of 570 million km is 3.8 times the Earth–Sun distance, and it proved that comet tails can be much longer than they look in photographs.
FAQ
How close did it come?
On 25 March 1996 it passed 0.1 AU from Earth, about 15 million km, or roughly forty times the distance to the Moon.
How long was the tail?
In the sky it stretched nearly 80 degrees. Data from the Ulysses spacecraft showed its real length was at least 570 million km.
Will it come back?
Not for anyone alive today. One trip around its orbit takes about 70,000 years.
How does it differ from Hale–Bopp?
Hyakutake was bright because it came close despite a small nucleus. Hale–Bopp was bright because its nucleus was huge, even though it stayed far away. They appeared a year apart.
Related
- Comet Hale–Bopp perihelion
- Comet NEOWISE — the great comet of 2020
- Halley's Comet perihelion (1986)
- Earth — object index
- The Sun — object index
Times and figures are approximate — verify with official sources when planning travel. Simulation uses J2000 mean elements.