The successful launch of the James Webb Space Telescope (JWST) has placed the orbiting observatory on the path to become the premier space platform for the next decade and beyond!
On Christmas Day, 2021 the world witnessed the launch of JWST, by all accounts a brilliant launch in its precision and timing! That launch put this next generation orbiting observatory on the path to becoming the premier orbiting platform well into the next decade and, perhaps, beyond.
If all goes according to plan within the next 21 days, JWST will be in a ‘Halo’ orbit around the L2 point 1.5 million km distant in a direction opposite that of the sun, one of 5 points where all gravitational interactions between the sun, the earth and the moon are in balance.
The deployment of JWST has presented an extraordinary teaching opportunity in basic physics but before we discuss any of that, let’s take a look at what has occurred thus far and where it is now.
Necessary First Steps
Before the telescope can be inserted into its Halo orbit around the L2 or LaGrange 2 point, an extraordinarily transformation has to occur. Because of the telescope’s size and the limited space available of any launch vehicle (the payload bay of the Ariane 5 launch vehicle is the largest), JWST was launched in its stowed or folded state. Thus, a complex choreography has to occur, a choreography transforming it from its folded state to a 6.5 meter precision Infrared/Optical Observatory. This Choreography is presented here with a timeline here and full video here. Images of the launch can be found here.
Because JWST is primarily an infrared observatory, its operating environment has to be very cold. In fact, a 4 degree Kelvin (4 degrees above absolute zero) internal temperature has to be maintained in one of the primary IR cameras. The telescope’s gold-plated mirrors, optical assembly and all science and imaging equipment can never be exposed to direct sunlight. Doing so could permanently impair the equipment and prevent JWST from fulfilling its primary mission. To maintain the telescope’s cold operating environment, a complex sun-shield has been designed to prevent any warming by the sun.
Where is JWST now?
As of the publication date of this article, JWST is 693,000 km from earth, traveling at 738 meters/second (0.738 km/sec). Following a brilliant launch aboard an Ariane 5 rocket, the telescope was accelerated to 9.7 km/sec or 1.5 km/sec below earth’s escape velocity of 11.2 km/sec by the the Ariane 5 second stage booster. Full details regarding JWST’s position and status are presented here with a timeline here and full video here. News and status reports can be found here.
Deployment of JWST as a Physics Teaching Lab
The publicity of the mission has engendered widespread public interest in some of the basic physics of the mission and its deployment. Discussions on gravity, acceleration, velocity, energy, power and thermodynamics are some areas of interest. We often get questions regarding anyone one of these subject areas. The answer to two questions we received follows and pertains to the kinematics (the study of motion) of JWST’s deployment and how long it will take to reach the L2 point.
Question: I was wondering how JWST slows down from its launch speed
The quick answer to your question, K***: the acceleration of Earth’s gravity: -9.81 meters/sec2 (note the ‘-‘ as part of the acceleration – this is the value at sea level and it decreases with altitude). Force (+/-) always precedes an acceleration and this acceleration results from the force of Earth’s gravity (resulting from its mass of 5.98×1024 kg and radius of 6400 km). Classic Newtonian gravity is formulated as: F = GMm/r2 and describes the mutually attractive force that exists between two masses (M and m) where G is the Universal Gravitational Constant, not to be confused with g, the acceleration of earth’s gravity of -9.81 m/sec2 and r, the distance separating the two masses. Gravity is always radially directed *towards* the center of mass, hence the negative sign and is why JWST is slowing down as it continues its journey towards L2.
You may often hear NASA commenting on a launch as “the vehicle is climbing out of the gravity well”, a metaphor of sorts used to describe what’s happening – in actuality it really is a well when one considers gravity as a warp (dependent on mass) in the space-time fabric.
Because of Webb’s unique requirements to be forever shielded from the sun, the launch parameters specifically called for a slight under-thrust, below Earth’s escape velocity of 11.2 km/sec. If you watched the launch, ESA provided the launch details and the final velocity of Webb at SEco (second engine cut off) was ~ 9.7 km/sec, 1.5 km/sec *below* what would be required to completely escape earth’s gravitational pull.
Why did they do this?
If, by some chance, a slight overthrust occurred during second stage boost, there would be no way to slow the craft down without applying reverse thrust during cruise phase, something that was strictly prohibited in the launch and design specs. If the craft was turned around (to orient the main engines in the direction of motion), there would be a danger that the the mirror and/or IR equipment would be permanently altered or damaged from exposure to direct sunlight (the prevailing temperature above the atmosphere in direct sunlight is ~394 K (100 C) or the boiling point of water). To provide the necessary energy (speed) to reach L2, 3 mid-course burns using Webb’s main engines will be executed, the first one already completed Saturday night, 12/26/2021 ~ 9:00 PM, EST (65 minute duration).
Question: How long will it take Webb to reach L2?
It will take 29 days from launch to reach the Halo Orbit insertion point around L2. An additional 130 days are needed for full commissioning before science operations can begin.
Question: Why are the mirrors coated with gold?
Because JWST is primarily an infrared telescope, the optical surfaces are coated with a material whose reflective properties are most efficient in the Infrared part of the Electromagnetic Spectrum; Gold is that element. Gold is highly reflective in those parts of the Infrared spectrum JWST will be observing in. The amount of gold used was 48 grams with a coating thickness of 100 nanometers (500 x thinner than a human hair).
Comment: It seems like a poor design if a multi-billion-dollar mission which took years to develop either can’t restart itself on its own or there are insufficient energy resources to complete the mission. Or maybe its own power source, yes weight could be a problem. As an engineer disappointing to hear this and as an enthusiast and taxpayer. A*** J
JWST development began in 1996 (25 years ago) based on the same concept that gave us all the orbiting telescopes to date (HST, Chandra, Kepler, Spitzer, etc). There are many details and compromises that have resulted in the final design we have today with emphasis on ‘compromise’. For example, they originally wanted a greater aperture but had to settle on 6.5 meters since there isn’t a launch vehicle to date capable of delivering anything larger to orbit.
JWST is primarily an Infrared telescope. The cryocooler developed for the main IR camera chills the sensors down to 4 Kelvin (4 degrees above absolute zero)! This requirement is one reason why so much attention is paid to ‘getting it right the first time’ – the onboard Helium (contained in the cryocooler as a closed system) necessary is a perishable resource and essential for the proper operation of the cryocooler. Not sure where you heard there were ‘insufficient resources to complete the mission’. The only perishable resources on board are the Helium (as mentioned), the Hydrazine and di-nitrogen tetroxide used as propellant during orbital recalibration every 21 days and for other necessary orbital maneuvers in conjunction with the reaction wheels. There is enough propellant onboard for a minimum mission lifetime of 5 years and maximum of 10 years. There is even discussion of developing robots to refill the propellant tanks, indefinitely extending the mission.
The solar array provides 2 KW of DC power, enough to operate the entire observatory with battery backup to supply limited power in the event of a contingency (an eclipse, for example, something that is prohibited according to mission specs).
Development of JWST began in the mid-1990s with the technology available at the time. As development continued into the 2000s until now, the technology continued to improve. JWST thus represents both the old and the new with some systems remaining as they were when developed with older technology – it was deemed too costly (with limited improvement) to rebuild/redesign some older systems, a reasonable course of action in my view as a scientist, educator and taxpayer.
JWST also represents the first of its kind in terms of where observational/space-based astronomy is going. HST (and to a lesser extent Chandra, Kepler, Spitzer, etc) demonstrated that space-based astronomy works and works well and now JWST is taking that success to the next level.
Excess Fuel Likely to Extend JWST Lifetime Expectations
After a brilliantly successful launch of the James Webb Space Telescope on Christmas Day, 2021, brilliant in its precision and accuracy and, following completion of two of the three mid-course correction maneuvers, the JWST team has determined there should be enough propellant for science operations to continue well beyond the previously planned 10-year science lifetime. The minimum baseline for the mission is five years.
The analysis shows that less propellant than originally planned for is needed to correct Webb’s trajectory toward its final orbit around the second Lagrange point known as L2, a point of gravitational balance on the far side of Earth away from the Sun. Consequently, Webb will have much more than the baseline estimate of propellant – though many factors could ultimately affect Webb’s duration of operation.
Astronomy for Change Recommended Videos
What Will the James Webb Space Telescope See Out There? By New York’s American Museum of Natural History
James Webb Space Telescope Launch and Deployment By Northrop Grumman Aerospace
JWST Deployment Sequence By Northrop Grumman Aerospace
James Webb Space Telescope Deployment Sequence (Nominal) By The National Aeronautics and Space Administration (NASA)
Please watch this space for additional stories and content about JWST.
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