  A stellar occultation by the large trans-Neptunian object (90482) Orcus was
predicted to occur on 2017 March 07. Observations were made at five sites in
North and South America. High-speed, visible-wavelength images were taken at
all sites, in addition to simultaneous K-band images at one location.
Solid-body occultations were observed from two sites. Post-event reconstruction
suggested an occultation of two different stars observed from two different
sites. Follow-up, speckle imaging at Gemini Observatory revealed a second star,
which verified that the occulting body in both cases was Orcus` satellite,
Vanth. The two single-chord detections, with an anomalously large timing delay
in one chord, have lengths of 291+/-125 km and 434.4+/-2.4 km. The
observations, combined with a non-detection at a nearby site, allow a tight
constraint of 443+/-10 km to be placed on Vanth`s size (assuming it is
spherical). A 3-{\sigma} upper limit of 1-4 {\mu}bar (depending on constituent)
is found for a global Vanth atmosphere. The immersion and emersion profiles are
slightly different, with atmospheric constraints 40 percent higher on immersion
than on emersion. No rings or other material were detected within ten thousand
kms of Vanth, and beyond 8010 km from Orcus, to the tightest optical depth
limit of approximately 0.1 at 5 km scale. The occultation probed as close as
5040 km from Orcus, placing an optical depth limit of approximately 0.3 at 5 km
scale on any encircling material at that distance.
