C/2017 O1 (ASASSN)
A comet discovered by ASAS-SN that brightened rapidly then faded.
C/2017 O1 (ASSN) is a comet discovered by the All Sky Automated Survey for SuperNovae (ASAS-SN) on July 19, 2017, in the southern constellation Cetus. It was notable for a rapid brightening event observed within days of its detection, followed by a subsequent fading.
Quick Facts
- Discovery Ref
- MPEC_2017-O45
- Discoverer
- All Sky Automated Survey for SuperNovae
- Discovery Site
- Cerro Tololo, Chile
- Discovery Date
- 19 July 2017
- Designations
- ASASSN1 / CK17O010
- Orbit Ref
- jpl
- Epoch
- 13 October 2017 (JD 2458039.5)
- Observation Arc
- 601 days (1.65 years)
- Obs
- 2,877
- Aphelion
- 837.04 AU
- Perihelion
- 1.4987 AU
- Semimajor
- 419.27 AU
Facts from the source article.
Lore & Background
The comet was first detected by the ASAS-SN survey using the quadruple 14-cm 'Cassius' telescope on Cerro Tololo, Chile. The team assigned it the temporary designation ASASSN1. In the discovery images, it had a V band magnitude of 15.3 and was moving between three 90-second dithered images with no counterpart in the Minor Planet Center database. Follow-up observations from the Savannah Skies Observatory 9.7 hours later recovered the comet 0.21 degrees away, revealing a halo at least 25 arcsec in radius, a compact core, and no clear tail. Further imaging with the Las Cumbres Observatory 1m telescope at CTIO 24.7 hours after discovery gave a g-band magnitude of 15.2. The comet was found 0.77 degrees from its original location 35.7 hours after discovery. Three days after discovery, the Cassius unit re-observed the field and found the comet had significantly brightened to V band magnitude 11.9, and its coma had increased to 2.5 arcmin. Six days after discovery, the northern ASAS-SN unit 'Brutus' on Haleakala, Hawaii, observed the comet at V~12.2, indicating it had faded. Its nucleus has an estimated effective radius of 0.919 ± 0.092 km.
Reader's Guide
The comet's significance lies in its rapid brightness variation observed within the first week after discovery, from magnitude 15.3 to 11.9 and then back to 12.2, demonstrating the dynamic behavior of cometary activity. The discovery was made possible by the ASAS-SN survey's automated detection of moving transients, and follow-up observations from multiple observatories confirmed its cometary nature. The comet's trajectory took it from Cetus in July into Taurus in September, then through Perseus and Camelopardalis in late September and early October, reaching perihelion on October 14 and closest approach to Earth on October 18. It continued moving toward the north pole and northern Cepheus into December. The comet was bright enough to be observed with binoculars, as noted in a September 2017 guide. Its legacy includes being a well-documented example of a comet discovered by a robotic survey and tracked by amateur and professional astronomers, with its nucleus size estimated from observations.
Did You Know?
- Three days after discovery, the comet brightened from V magnitude 15.3 to 11.9.
- Its coma increased to 2.5 arcmin within three days of discovery.
Frequently Asked Questions
What is C/2017 O1 (ASASSN)?
It is a long-period comet first flagged by the All Sky Automated Survey for Supernovae, a robotic survey built to catch brief transient events. The 'ASASSN' tag in its designation simply credits that automated system as the discoverer rather than a human observer.
How and where was C/2017 O1 (ASASSN) discovered?
The ASAS-SN survey detected it on July 19, 2017, while scanning the southern constellation Cetus. At the moment of detection it was sitting at a V-band magnitude of roughly 15.3, far too faint for the unaided eye.
What made C/2017 O1 (ASASSN) notable to observers?
Within a handful of days of its discovery the comet spiked sharply in brightness and then faded back down just as quickly. That rapid brighten-then-dim behavior turned it into a useful short-window target for time-resolved photometry and activity studies.
How large is the nucleus of C/2017 O1 (ASASSN)?
Measurements place its effective radius at about 0.92 kilometres, with an uncertainty of roughly 0.09 km. That makes it a small-nucleus long-period comet, well under a kilometre across.
When did C/2017 O1 (ASASSN) reach perihelion and pass closest to Earth?
It swung past the Sun on October 14, 2017, and made its nearest approach to Earth four days later on October 18. Because of its modest peak brightness and small nucleus, it never became a widely visible naked-eye spectacle.
More in Named Comets, Part 4 1-24
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