From NASA Spaceflight: TESS

NASA Spaceflight

NASA Spaceflight

April 16, 2018
Chris Gebhardt

TESS background/overview:

NASA/TESS

The original idea for TESS goes back to 2005 when Dr. George Ricker was the Principle Investigator High Energy Transient Explorer (HETE) – the first satellite mission dedicated to the study of gamma-ray bursts. Slowly, the idea evolved in 2008 and 2009, with Dr. Ricker, now TESS’s Principal Investigator at MIT (Massachusetts Institute of Technology), saying “We wanted to initially try to do this as a privately funded system, and MIT was very helpful for us. We had support from Google for some of the studies that were originally going to be done.”

That led to a collaboration with NASA Ames to create a proposal for a small-class explorer exoplanet mission that was ultimately not selected for flight. That then led to a partnership with Orbital ATK and the Goddard Space Flight Center in Greenbelt, Maryland, for a revised mission proposal over 2011 and 2012.

TESS was officially selected for inclusion in NASA’s Medium Explorer mission program on 5 April 2013, and with just over five years of design and build operations, now stands ready to launch. “It’s been a long time coming. It’s been 13 years, but for the last five years, basically, pretty much [everything with the mission has been] the same,” said Dr. Ricker.

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TESS undergoes final pre-launch processing at the Kennedy Space Flight Center. Credit: Chris Gebhart for NSF/L2

While TESS is generally perceived as a follow-on to NASA’s Kepler planet hunting satellite, it will perform a very different kind of mission. Where Kepler was a prolonged, deep, and narrow field observatory that looked continuously at specific stars in one quarter of 1% of the sky at an optimal range of 2,000 to 3,000 light years distance, TESS will perform a wide- and shallow-field survey covering 85% of the sky with an optimal distance stretching to 300 light years.

TESS will accomplish its observations by using the sole science instrument onboard: a package of four wide-field-of-view CCD cameras with a low-noise, low-power 16.8 megapixel CCD detector. Each camera as a 24° x 24° field of view, a 100 mm (4 in) pupil diameter, a lens assembly with seven optical elements, and a bandpass range of 600 to 1,000 nm.

When functioning together – as designed – the four cameras have a 24° x 96° field of view.

The overall spacecraft is built on a LEOStar-2 satellite bus by Orbital ATK. The spacecraft bus is capable of three-axis stabilization via four hydrazine thrusters as well as four reaction wheels. This provides TESS’s cameras with greater than three-arc-second fine pointing control – necessary for the sensitive light observations TESS will perform once in its science orbit.

The data collected during TESS’s observational campaigns – as well as general spacecraft communications – will route through a Ka-band antenna with a 100 Mbit/s downlink capability. The entire craft is powered by two solar arrays capable of generating 400 watts.

“There’s more than 100 scientists and other personnel cooperating on the mission,” said Dr. Ricker, “and as far as the mission itself is concerned, all the work that was involved in designing, developing, and building the hardware, we’ve estimated that there’s more than a million person-hours that have gone into that over the past five years.”

Launch and Orbit:

The launch phase of the mission will see a Falcon 9 deliver TESS into a lunar transfer orbit, sending the craft to a precise point when the moon’s gravity will grab TESS and fling it out into a farther orbit than it’s initially launched into.

At 350 kg (772 lb), TESS is the lightest-known payload to have ever launched on a Falcon 9. After lifting off from SLC-40 at the Cape Canaveral Air Force Station, FL, the Falcon 9 will fly due east from the pad. The first stage, after 2 minutes 29 seconds of powered flight, will separate from the second stage and perform a landing on the Of Course I Still Love You drone ship in the Atlantic.

SpaceX will also attempt to recover the payload fairing, but as there is no fairing catching boat – yet – on the east coast, the fairing will parachute into the ocean for intact recovery, serving primarily as a test of the new recovery systems.

For the launch, after stage separation, the second stage will continue to fire its single MVac (vacuum optimized Merlin engine) until SECO-1 (Stage Engine Cut Off -1) at 8minutes 22 seconds into flight. This will be followed by a 32 minute 33 second coast of the stage and TESS before the second stage engine re-starts for a burn to send TESS into a Lunar Transfer Orbit.

Shortly after SECO-2, TESS will separate from the top of the Falcon 9 second stage at 48 minutes 42 seconds after launch having been placed into a super synchronous transfer orbit of 200 x 270,000 km (124 x 167,770 mi). The second stage will then perform a third burn to inject itself into a disposal hyperbolic (Earth-escape) orbit.

Over the first five days, TESS’s control teams will check out the overall health of the spacecraft before activating TESS’s science instruments 7-8 days after launch. TESS will then perform a final lunar flyby on 16 May – one month after launch, a lunar gravity assist which will change the the craft’s orbital inclination to send it into its 13.7 day, 108,000 x 373,000 km (67,000 x 232,000 mi) science orbit of Earth – an orbit that is in perfect 2:1 resonance with the moon.

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The maneuvers and encounters Leading to the final TESS orbit. PLEA and PLEP are the post lunar-encounter-apogee and perigee, respectively. Credit Ricker et al. 2015.

The specific orbit, referred to as the P/2 lunar resonant orbit, will place TESS completely outside the Van Allen Radiation belts, with TESS’s apogee (farthest point in orbit from the Earth) approximately 90 degrees away from the position of the Moon. This will minimize the Moon’s potential destabilizing effect on TESS and maintain a stable orbit for decades while also providing a consistent, good camera temperature range for the observatory’s operations.

Moreover, this orbit will provide TESS with unobstructed views of both the Northern and Southern Hemispheres. For almost all of its orbit, TESS will be in data gathering mode, only transmitting its stored data to Earth once per orbit during the three hours of its closest approach to Earth, or perigee. Assuming an on-time launch, TESS will enter operations on 12 June.

Overall, TESS has daily launch opportunities from 16-21 April, no launch opportunity on the 22nd (per NASA documentation), and then daily opportunities again from 23-26 April. There is no opportunity on 22 April because the amount of time between the consecutive daily opportunities on 21 and 23 April is just slightly longer than 24 hours, thus barely skipping over all times on the 22nd.

However, if for some reason TESS is not off the ground by 26 April, the exoplanet hunter must stand down launch operations so that NASA’s Launch Services Provider (LSP) group can shift gears to support the agency’s InSight mission launch to Mars from Vandenberg Air Force Base, California.

The LSP does not have a large enough staff to support two missions from both coasts, and since InSight has a short interplanetary launch window it must launch within, InSight would get priority over TESS. After InSight, TESS has additional launch opportunities in both May and June.

Mission:

Once its checkout phase is complete, TESS will begin its 26 observational campaigns (13 for each hemisphere) to survey 85% of the sky for transiting exoplanets near Earth. Observations will start with the Southern Hemisphere, and those 13 campaigns will last approximately one year.

According to Dr. Ricker, choosing to survey the Southern Hemisphere first was “a function of the follow-up resources that are currently available. Many of the most powerful telescopes that ground-based astronomers use are located in the Southern Hemisphere.”

TESS will then be re-aimed to perform the 13 observational campaigns needed to cover the Northern Hemisphere. During all 26 campaigns, the entire south and north polar sky regions will receive near-continuous year-long assessments from TESS’s cameras – as each observation campaign for the Southern and Northern Hemispheres overlap completely at their respective pole.

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Dr. Ricker shows the number of exoplanets TESS is predicted to find within 100 parsecs (326 lightyears) of Earth. Credit Ricker et al. for NSF/L2

Every 13.7 days, when TESS swings closest to Earth, the craft will downlink its observation data to scientists at MIT who will process it and make it available to other scientists and the public. Specifically, TESS’s team will focus on the 1,000 closest red dwarf stars to Earth as well as nearby G, K, and M type stars with apparent magnitudes greater than 12.

Over its primary 2 year mission, TESS will observe about half a million stars in an area 400 times larger than the Kepler mission and is expected to find 20,000 exoplanets – including 500-1,000 Earth-sized planets and Super-Earths.

These planets will be added to the growing number of known exoplanets. According to NASA’s Exoplanet Archive hosted by CalTech, as of 12 April 2018, there are 3,717 known exoplanets with 2,652 of those found by the Kepler Space Telescope.

TESS’s primary mission duration is two years, during which all of its science objectives are scheduled to be completed. While a mission extension is never a guarantee, TESS can be extended for additional observations based on its design and orbit. “We can extend, because the orbit will be operating and aligned for more than two decades,” said Dr. Ricker. “Now, as is the case for many Explorer missions, we fully expect that there will be an extended mission for TESS, so we pre-designed the satellite and the operation so that it can go on for a much longer time.”

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