Comet Hyakutake
What is Comet Hyakutake?
Comets· Nucleus ~4 km. The great comet of spring 1996 that swept within 0.1 AU of Earth, its tail stretching halfway across the sky. A small nucleus made spectacular by sheer closeness, and the first comet ever seen in X-rays. Replay its orbit and the night of 26 March 1996 in the simulators.
Where Comet Hyakutake is in your sky right now: https://nowsky.org/solar/sky?sel=comet:hyakutake&loc=seoul&sd=1996-03-26T13:00Z — location and time set automatically, no login.
Past visits and the next return
Orbit and period
| Nucleus | ~4 km |
|---|---|
| Closest to Earth | 1996-03-25, 0.10 AU |
| Period | ~70,000 yr |
| Record | tail 570 million km — longest measured |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Frequently asked
- How close does Comet Hyakutake get to the Sun?
- 1996-03-25, 0.10 AU.
- How long is the orbital period of Comet Hyakutake?
- ~70,000 yr.
- In one line, what is Comet Hyakutake?
- Comets — The great comet of spring 1996 that swept within 0.1 AU of Earth, its tail stretching halfway across the sky.
- Where is Comet Hyakutake in the sky right now?
- Open the sky view for your location and time and it shows the position in the sky right now.
A comet that shines in X-rays
When Germany's ROSAT X-ray satellite pointed at Hyakutake on 27 March 1996, nobody expected a signal — there was no reason for a lump of ice and dust to emit X-rays. Yet a bright crescent of X-rays appeared on the sunward side, a hundred times brighter than any prediction. It was among the most surprising results in the history of comet observation.
The answer came a year later: when highly charged oxygen and carbon ions in the solar wind meet the neutral gas a comet releases, they steal electrons, and as those electrons drop into lower orbits they give off X-rays. This "charge exchange" has since been confirmed around Mars and Jupiter and at the edge of the heliosphere, and X-ray brightness has become a tool for measuring the composition of the solar wind. Nearly every bright comet observed since Hyakutake has shone in X-rays.
The comet in depth
Comet Hyakutake was discovered on 30 January 1996 by Japanese amateur astronomer Yuji Hyakutake, using binoculars. Just two months later, on 25 March, it passed 0.10 AU (15 million km) from Earth — among the closest cometary approaches of the past two centuries. Its nucleus was small, about 4 km, but proximity pushed it to magnitude 0.
At its peak the tail spanned more than 80 degrees, stretching from the Big Dipper to the horizon, and the Ulysses spacecraft crossed tail material 570 million km from the nucleus — the longest comet tail ever measured. Germany's ROSAT satellite detected X-rays from the comet, a great surprise, later explained as solar-wind ions stealing electrons from cometary gas. Its close pass by the Sun reshaped the orbit, so the next return is roughly 70,000 years away.
Track Hyakutake in the orbit view to watch it dive toward the Sun and get flung far out again. In the Sky view, move to Seoul on the night of 26 March 1996 to see where it hung near Polaris.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.