From Cape Town to Hawai’i to Sedna: The most remote observations of the most remote planetary object

On 24 January 2015 (SAST), observers at the SAAO in Cape Town used NASA’s 3-m Infrared Telescope Facility (IRTF) on Mauna Kea, Hawai’i to take spectra of minor planet 90377 Sedna, the Solar System’s most distant known object.

Dr. Amanda Gulbis and visiting Prof. David Trilling worked with IRTF astronomer Dr. Bobby Bus and telescope operator Mr. Brian Cabreira to control the telescope and instruments from the SAAO for the entire night in Hawai’i, 18:30-06:30 HST (conveniently corresponding to the day in Cape Town, 06:30-18:30 SAST). The IRTF was a pioneer of the remote observing technique and regularly has astronomers taking data with the telescope from locations around the world. Cape Town, at ~18,400 km from Mauna Kea, is now the most distant location on Earth from which a night of successful observations has been obtained at the IRTF. Note that the distance between antipodal points on Earth is ~20,000 km.

Artist’s conception of the view of our Sun from Sedna.

Artist’s conception of the view of our Sun from Sedna. (Credit: Adolf Schaller, ESA, NASA)

Sedna is a trans-Neptunian object, currently located at 86 AU (approximately 13 billion km) from Earth. At the time of the observations, Sedna had an apparent magnitude of 21. Its orbit is highly elliptical.  Other than long-period comets, Sedna has the most distant aphelion for a known Solar System object at 973 AU and the longest known orbital period at ~12,000 years.   Sedna is approximately 1000 km in diameter and is thus a candidate for designation as a dwarf planet. It is also one of the reddest objects in the Solar System, with the color thought to be due to organic molecules formed on the icy surface as a result of long-term exposure to radiation. The spectra taken at the IRTF are being used to study the surface composition of Sedna and other trans-Neptunian objects.

Photograph of NASA’s 3-m IRTF on Manua Kea, Hawaii’i.

Photograph of NASA’s 3-m IRTF on Manua Kea, Hawaii’i. (Credit: A. Gulbis)

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