If you are dazzled by the recent flurry of exoplanet discoveries by the Kepler Space Telescope and other ground based telescopes, wait till the Transiting Exoplanet Survey Satellite (TESS) is launched later this year, most likely in April.
The Transiting Exoplanet Survey Satellite (TESS) is a space telescope, designed to search for exoplanets using the transit method. TESS will perform an all-sky survey of over 500,000 of the brightest nearby stars, surveying a region 400 times larger than that surveyed by Kepler. Follow-up observations of planets discovered by TESS will be performed by other ground and space based telescopes to determine planet masses, sizes, densities, and atmospheric properties.
Unlike its predecessor, the Kepler Space Telescope, which surveys a small fixed area of faint and distant stars in our galaxy, TESS will survey the entire sky in a region closer to Earth over its two-year mission.
The Transit Method of Detecting Exoplanets
When a planet passes in front of a star as viewed from Earth, the event is called a “transit”. Tiny dips in the brightness of a star caused by the transit can be measured by sensitive photo-detectors on telescopes such as Kepler and TESS. Once detected, the planet's orbital size can be calculated from the orbital period (through repeated observations) and the mass of the star using Kepler's Third Law of planetary motion. The size of the planet is found from the depth of the transit (how much the brightness of the star drops) and the size of the star. From the orbital size and the temperature of the star, the planet's characteristic temperature can be calculated. From this the question of whether or not the planet is habitable (not necessarily inhabited) can be answered.
Note that TESS will not be able to analyze planetary characteristics such as atmospheres and its components. After the TESS Science Team identifies the best candidates for rocky planets, large telescopes, such as the James Webb Space Telescope or ground based telescopes, equipped with spectroscopy instruments, will be used to characterize atmospheres and other properties.
The TESS Instrument
The sole instrument on TESS is a package of four wide-field-of-view 16.8 megapixel CCD cameras. Each has a 24° × 24° field of view, a 100 mm effective pupil diameter and a lens assembly with seven optical elements.
TESS will observe a large number of M dwarfs; most nearby stars are M dwarfs and planets are easier to detect around these small stars (the planets induce larger transit signals). Because M dwarfs are relatively cool and red, the TESS instrument will be more sensitive to red wavelengths compared to Kepler.
Blurry Vision?
NASA discovered in July 2017 that the focus of the four cameras on spacecraft will shift a little when the spacecraft cools to operating temperatures (-75o Celsius) in its final orbital position.
After a thorough engineering evaluation, NASA concluded that TESS can fully accomplish its science mission with the cameras as they are. NASA stated that the out-of-focus area is limited to the outer edges of the image, and that “recent testing shows that the camera focus towards the image center is better than originally designed.”
George Ricker, principal investigator for TESS at MIT said — “This is a photometry mission, not an imaging mission. What this means is that it’s not important to have a sharp focus across the entire field of view. This was never part of the design. But it is important that the focus be stable, and that’s what we’ve been able to establish.”
See spacenews.com/… and smd-prod.s3.amazonaws.com/… for details.
The P/2 Orbit
To carry out its survey of extrasolar planets in both celestial hemispheres, TESS needs to occupy a very particular position in space, a highly stable place that maximizes sky coverage and gives the observatory a mostly unobstructed view of the cosmos, all from a low-radiation, thermally benign environment. www.nasa.gov/...
After exhaustive studies, the TESS team chose a never-before-used 2:1 lunar-resonant orbit known as P/2. This high-Earth, highly elliptical orbit has a period of 13.7 days, exactly half that of the Moon. The spacecraft apogee (closest point to Earth) will be 108,000 km away, while perigee will be 373,000 km from Earth. The moon is ~384,400 km away. The High-Earth-Orbit (HEO) is inclined by 37 degrees relative to the moon orbital plane.
Perturbations caused by the Moons’ gravity cancel each other out over time; during one orbit, the moon is to the “left” of orbital plane of TESS, while during the next orbit, the moon is to the “right”. This will result in a very stable orbit and not require any in-orbit maneuvers, which may extend TESS’s operational life into decades, far beyond its two-year primary mission.
The following video shows the orbit insertion maneuvers and final orbit of TESS -
Surveying the Sky
The survey is broken up into 26 observation sectors, 24° × 96° each (using 4 cameras). Sectors overlap at the ecliptic poles allowing additional time for sensitive observations of exoplanets in that region of the celestial sphere. The spacecraft will spend two 13.7 day orbits observing each sector, mapping the northern hemisphere of sky in its first year of operation and the southern hemisphere in its second year. Each of TESS's cameras will take one-minute exposures focused on specifically targeted stars, and 30-minute full-frame exposures to search for other transient events. The exposure time for each image is 2 seconds; longer exposures are created by integrating data from multiple such 2-second images.
The 30 minutes exposures will expand the transit search to any star in the fields of view that is sufficiently bright, regardless of whether it was selected ahead of time. This will reduce the impact of any imperfections in the target star catalog, and allow the search sample to extend beyond the pre-selected dwarfs that are the focus of the mission. arxiv.org/...
Note that survey strategy limits the orbital periods of detected exoplanets to ~10 days in general, and ∼ 40 days for areas surrounding the ecliptic poles.
Data Collection and Transmission
Data will be collected over the 2 week orbital period and stored in a 192 gigabyte solid state recorder. The data will be transmitted to Earth when the spacecraft is closest to Earth. Transmission rate will be about 100 Mbps over a Ka-band link. At 100 Mbps, it will take 4.2 hours to transmit 192 gigabytes. Data will sent to the NASA Deep Space Network (DSN) sites at Canberra, Madrid and California, depending on the orientation of the spacecraft with respect to these sites, from where data will sent to processing centers in the U.S.
TESS vs Kepler
Feature |
TESS |
Kepler |
Launch Date |
April 2018 (expected) |
March 7, 2009 |
Exoplanets discovered |
500,000 stars will be monitored
Expected to catalog 3,000 transiting exoplanet candidates, including a sample of ~500 planets less than 2x Earth-size
|
4,496+622 candidates
2,341+197 confirmed
Confirmed exoplanets less than twice Earth-size in the habitable zone: 30
|
Orbit |
Earth-centric orbit, 2:1 Lunar resonant |
Heliocentric orbit
Lags behind Earth
|
Mission duration |
2 years (will likely be extended) |
Almost 9 years so far |
Celestial Sphere Scanned |
Almost full sky; 400x coverage of Kepler
|
Fixed field of view, 115 square degrees, around 0.25% of the sky |
Star distances |
200 light years
|
3,000 light years
|
Data download |
Every 13.7 days |
Once a month |
Brightness of stars surveyed |
30-100 times brighter than those surveyed by Kepler |
|
Payload Cost |
$200 million |
$550 million |
Kepler
Kepler was the predecessor to TESS and has been instrumental in the discovery of exoplanets in recent years. The Kepler spacecraft has been in space for almost 9 years now. It is running out of the hydrazine fuel used by thrusters to maintain the spacecraft’s orientation. The spacecraft mission will end sometime in 2018, although the treasure-trove of information will continue to be studied for many years.
Beyond TESS
The James Webb Space Telescope (JWST) will launch in 2019, one year after TESS, providing a unique opportunity for in-depth study of TESS discoveries.
The European Space Agency is scheduled to launch the CHaracterising ExOPlanets Satellite (CHEOPS) in late 2018. It will use a sun-synchronous low-earth-orbit (~700 km altitude). The main goal of CHEOPS will be to accurately measure the radii of the exoplanets for which ground-based spectroscopic surveys have already provided mass estimates.
The NASA Wide Field Infrared Survey Telescope (WFIRST) mission was scheduled for launch in the mid-2020s, but its future is in doubt, since funding was eliminated in the 2018 White House budget proposal.
Status
TESS integration and testing is complete. It was transported by truck from Orbital ATK in Dulles, VA, to Kennedy Space Center where it will be prepped for launch on board a SpaceX Falcon 9 rocket.
The launch will take place no earlier than April 16.
NASA has recently certified the current version of the SpaceX Falcon 9 to launch some categories of science missions, including TESS.
Epilogue
These are exciting times in the search for exoplanets and life in outer space. Exoplanets are far numerous than previously thought, simply because technology did not exist before to detect these faint, small and distant objects. We have searched in a tiny fraction of space so far. TESS will greatly expand the catalog of exoplanets which can be further studied by future telescopes.
George Ricker, principal investigator for TESS at MIT said — “TESS is tiny, but it punches above its weight. It’s a finder scope for JWST.”
References
- NASA TESS Site — heasarc.gsfc.nasa.gov/… and tess.gsfc.nasa.gov/…
- TESS wiki — en.wikipedia.org/…
- TESS at MIT — tess.mit.edu/...
- The Transiting Exoplanet Survey Satellite — paper at arxiv.org/…
- New Explorer Mission Chooses the ‘Just-Right’ Orbit — www.nasa.gov/…
- NASA Exoplanet site — exoplanets.nasa.gov/...
- NASA Kepler site — www.nasa.gov/… and tess.gsfc.nasa.gov/…
- Discovery alert! 95 new planets found with NASA telescope — exoplanets.nasa.gov/...
- The Ultracool Dwarf (Star) and the Seven Exoplanets — www.dailykos.com/...
- Space Exploration - The Year Ahead (2018) — www.dailykos.com/…
- Why Go To Mars? And other Planets and Moons. — www.dailykos.com/…
- The James Webb Space Telescope — www.dailykos.com/...