How Successful will TESS be?

As pictured in this Astronomy Picture of the Day for April 21, in a brilliantly successful launch, the Transiting Exoplanet Survey Satellite (TESS) was launched atop a SpaceX Falcon 9 into a highly elliptical, High Earth Orbit.

Contrary to many news reports, TESS is not a telescope, nor does it have any telescopes onboard. It is not a replacement for Kepler, a one-meter orbiting telescope and observatory, nor was it ever intended to be.

The scope of TESS’s mission is quite different – by design, it consists of 4, 10 cm (4 inch) diameter wide-field cameras whose fields of view overlap. It’s mission is to measure small flux changes of bright stars in the solar neighborhood, flux changes presumably caused by transiting planets. The only similarity between Kepler and TESS is that any candidate systems are to be followed up either by large, ground-based telescopes or orbiting platforms such as the Hubble Space Telescope or the James Webb Space Telescope (JWST), due to launch in about two years (May, 2020).

A chronological representation of current and future orbiting observing platforms. TESS will be followed by the James Webb Space Telescope, scheduled (for now) to be launched in May, 2020.

Kepler vs. TESS

When comparing the two missions and their respective objectives, its instructive to note that the magnitude or brightness of any star or object to be observed is limited by the instrument’s aperture or diameter. The diameter of the Kepler Space Telescope is one meter; the diameter of each of TESS’s 4 cameras is 10 cm or 1/10 the diameter of Kepler! This corresponds to a 100 fold reduction in limiting magnitude or brightness when comparing TESS to Kepler; Kepler can observe targets 100x fainter than TESS!

The original mission of Kepler was to find evidence of transiting Earth-like planets orbiting their host stars in the “habitable zone”, the region around any star where the conditions are such that water could exist in a liquid state. The region of interest was centered on the Lyra-Cygnus region of the Milky Way, where a target list of 150,000 stars, culled from a list of 450,000 sun-like stars down to 16th magnitude, were to be continuously monitored over a period of three years. These mission parameters are quite different from those of TESS where the target stars are selected from only the brightest stars in the solar neighborhood, monitoring changes in brightness as evidence of any planet transiting its host star, in any orbit.

The burning question is “How Successful will TESS be” and how will it compare to the stunning success of Kepler whose usable life has been cut short by the earlier failure of two reaction wheels and now, the imminent loss of control propellant?

What do you think?

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