C/1999 S4 (LINEAR)
First comet caught in the act of disintegrating.
C/1999 S4 (LINEAR) was a hyperbolic comet first spotted by the Lincoln Near-Earth Asteroid Research survey on 27 September 1999. Its nucleus measured roughly 0.9 km across. Before it fell apart, the nucleus was shedding dust and water at a rate of about 1 cm per day. The comet grew brighter around 5 July 2000, then experienced a minor breakup. It brightened again around 20 July 2000, and soon disintegrated. Optical and most radio data indicate the main nuclear decay began on 23 July 2000. The resulting dust cloud expanded at roughly 20 m/s, while the fragments moved outward at about 7 m/s. While other comets have vanished, this one was the first caught in the process. Besides water, scientists detected traces of ethane, methane, hydrogen cyanide, and carbon monoxide, as well as ammonia, acetylene, formaldehyde, and methanol.
For long-period comets, the orbit is best determined using an osculating epoch after leaving the planetary region, calculated relative to the Solar System's center of mass. According to JPL Horizons, the barycentric orbital elements for 1 January 2010 give a semi-major axis of 700 AU, an aphelion distance of 1400 AU, and a period of roughly 18,700 years. The comet made its closest approach to Earth on 22 July 2000, coming within 0.372 AU (about 55.7 million km). It reached perihelion on 26 July 2000, at a distance of 0.765 AU (about 114.4 million km) from the Sun.
Quick Facts
- Discovery Ref
- MPEC_1999-T02
- Discoverer
- LINEAR (704)
- Discovery Site
- Socorro, New Mexico
- Discovery Date
- 27 September 1999
- Designations
- CJ99S040MPEC_1999-T02
- Orbit Ref
- barycenter · jpl
- Epoch
- 13 February 2000 (JD 2451587.5)
- Observation Arc
- 300 days
- Obs
- 1,230
- Perihelion
- 0.765 AU
- Aphelion
- ~1400 AU
- Semimajor
- ~700 AU
Facts from the source article.
Lore & Background
The comet's nucleus erosion rate was about 1 cm per day before breakup. It brightened near 5 July 2000, underwent a minor fragmentation event, then brightened again around 20 July 2000, and disintegrated. Published optical and most radio data support that the main nuclear decay started 23 July 2000. The dust cloud expanded at about 20 m/s while fragments expanded at around 7 m/s. Other comets have disappeared, but Comet LINEAR is the first one to have been caught in the act. In addition to H2O, traces of C2H6, CH4, HCN, and CO were detected, along with traces of NH3, C2H2, H2CO, and CH3OH.
Reader's Guide
C/1999 S4 (LINEAR) holds significance as the first comet observed during its disintegration, providing direct evidence of cometary decay. Its nucleus, about 0.9 km in diameter, eroded at about 1 cm per day before breakup. The comet brightened near 5 July 2000 and again around 20 July 2000, with the main nuclear decay starting 23 July 2000. The dust cloud expanded at about 20 m/s and fragments at about 7 m/s. Its legacy includes detection of various molecules beyond water, such as C2H6, CH4, HCN, CO, NH3, C2H2, H2CO, and CH3OH. The comet's barycentric orbit, calculated for epoch 2010-Jan-01, yields a semi-major axis of 700 AU, aphelion of 1400 AU, and period of about 18,700 years. It made its closest approach to Earth on 22 July 2000 at 0.372 AU and reached perihelion on 26 July 2000 at 0.765 AU.
Did You Know?
- The comet's nucleus was estimated to be about 0.9 km in diameter.
- It was the first comet caught in the act of disintegrating.
Frequently Asked Questions
Who is C/1999 S4 (LINEAR)?
It is a hyperbolic comet first detected on 27 September 1999 by the Lincoln Near-Earth Asteroid Research survey. Its nucleus spanned roughly 0.9 km across before it broke apart.
Why is C/1999 S4 (LINEAR) important to comet history?
It earned the distinction of being the first comet ever caught in the act of disintegrating while under continuous observation. Prior to its breakup, the nucleus was actively shedding dust and water at a rate of about one centimeter per day.
How close did C/1999 S4 (LINEAR) come to Earth?
It passed within 0.372 AU of Earth on 22 July 2000, just four days before reaching perihelion at 0.765 AU from the Sun on 26 July.
What made C/1999 S4 (LINEAR) different from a typical periodic comet?
Its hyperbolic orbit meant it was on a one-way trajectory through the inner solar system rather than a repeating cycle. Combined with its rapid disintegration, this made it a uniquely short-lived visitor that could never return.
More in Named Comets, Part 2 1-24
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