A Solar Orbit
The James Webb Space Telescope will not be in orbit around the Earth, like the Hubble Space Telescope is - it will actually orbit the Sun, 1.5 million kilometers (1 million miles) away from the Earth at what is called the second Lagrange point or L2. What is special about this orbit is that it lets the telescope stay in line with the Earth as it moves around the Sun. This allows the satellite's large sunshield to protect the telescope from the light and heat of the Sun and Earth (and Moon).
Why Does the Direction of the Earth and Sun Matter?
Webb primarily observes infrared light, which can sometimes be felt as heat. Because the telescope will be observing the very faint infrared signals of very distant objects, it needs to be shielded from any bright, hot sources. This also includes the satellite itself! The sunshield serves to separate the sensitive mirrors and instruments from not only the Sun and Earth/Moon, but also the spacecraft bus.
The telescope itself will be operating at about 225 degrees below zero Celsius (minus 370 Fahrenheit). The temperature difference between the hot and cold sides of the telescope is huge - you could almost boil water on the hot side, and freeze nitrogen on the cold side!
To have the sunshield be effective protection (it gives the telescope the equivalent of SPF one million sunscreen) against the light and heat of the Sun/Earth/Moon, these bodies all have to be located in the same direction.
This is why the telescope will be out at the second Lagrange point.
What is L2?
Joseph-Louis Lagrange was an 18th century mathematician who found the solution to what is called the “three-body problem.” That is, is there any stable configuration, in which three bodies could orbit each other, yet stay in the same position relative to each other? As it turns out, there are five solutions to this problem - and they are called the five Lagrange points, after their discoverer. The L1, L2, and L3 points are all in line with each other - and L4 and L5 are at the points of equilateral triangles.
Webb at L2
If Webb is orbiting the Sun further out than Earth, shouldn't it take more than a year to orbit the Sun? Normally yes, but the balance of the combined gravitational pull of the Sun and the Earth at the L2 point means that Webb will keep up with the Earth as it goes around the Sun. The gravitational forces of the Sun and the Earth can nearly hold a spacecraft at this point, so that it takes relatively little rocket thrust to keep the spacecraft in orbit around L2.
And Webb will orbit around L2, not sit stationary precisely at L2. Webb's orbit is represented in this screenshot from our deployment video (below), roughly to scale; it is actually similar in size to the Moon's orbit around the Earth! This orbit (which takes Webb about 6 months to complete once) keeps the telescope out of the shadows of both the Earth and Moon. Unlike Hubble, which goes in and out of Earth shadow every 90 minutes, Webb will have an unimpeded view that will allow science operations 24/7.
Communicating with Webb
Webb's position out at L2 also makes it easy for us to talk to it. Since it will always be at the same location relative to Earth-in the midnight sky about 1.5 million km away - we can have continuous communications with it as the Earth rotates through the Deep Space Network (DSN), using three large antennas on the ground located in Australia, Spain and California. During routine operations, Webb will uplink command sequences and downlink data up to twice per day, through the DSN. The observatory can perform a sequence of commands (pointing and observations) autonomously. Typically, the Space Telescope Science Institute will upload a full week's worth of commands at a time, and make updates daily as needed.
How long will it take Webb to get to L2?
It will take roughly 30 days for Webb to reach the start of its orbit at L2, but it will take less than a day to get far away from Earth and much of the way there. Getting Webb to its orbit around L2 is like reaching the top of a hill by pedaling a bicycle vigorously only at the very beginning of the climb, generating enough energy and speed to spend most of the way coasting up the hill so as to slow to a stop and barely arrive at the top.
Timeline of Events After Launch:
- In the first hour: Starting at liftoff, the Ariane rocket will provide thrust for a little over 8 minutes. Webb will separate from the Ariane V launch vehicle a half hour after launch and we will deploy the solar array immediately afterward. We will also release several systems that were locked for launch.
- In the first day: Two hours after launch we will deploy the high gain antenna. Twelve hours after launch there will be the first trajectory correction maneuver by small rocket engines aboard Webb itself.
- In the first week: The second trajectory correction maneuver will take place at 2.5 days after launch. We will start the sequence of major deployment just after that. The first deployments are the fore and aft sunshield pallets, followed by the release of remaining sub-system launch locks. The next deployment is the telescope in which the telescope and the spacecraft bus move apart from each other by about 2 meters when the deployable tower assembly extends. The full sunshield deployment with unfolding and tensioning of the membranes can then be initiated. At 6 days we deploy the secondary mirror, followed by the side wings of the primary mirror.
- In the first month: As the telescope cools down in the shade of the deployed sunshield, we will turn on the warm electronics and initialize the flight software. At the end of the first month, we will do the mid-course correction that ensures that Webb will achieve its final orbit around L2. Although the telescope cools to near its operating temperature, the Integrated Science Instrument Module (ISIM)is warmed with electric heaters to prevent condensation on the instruments as residual water trapped in the materials making up the observatory escapes to the vacuum of space.
- In the second month: At 33 days after launch we will turn on and operate the Fine Guidance Sensor, then NIRCam and NIRSpec. The first NIRCam image will be of a crowded star field to make sure that light gets through the telescope into the instruments. Since the primary mirror segments will not yet be aligned, the picture will still be out of focus. At 44 days after launch we will begin the process of adjusting the primary mirror segments, first identifying each mirror segment with its image of a star in the camera. We will also focus the secondary mirror.
- In the third month: From 60 to 90 days after launch we will align the primary mirror segments so that they can work together as a single optical surface. We will also turn on and operate the MIRI. By the end of the third month we will be able to take the first science-quality images. Also by this time, Webb will complete its initial orbit around L2.
- In the fourth through the sixth month: At about 85 days after launch we will have completed the optimization of the telescope image in the NIRCam. Over the next month and a half we will optimize the image for the other instruments. We will test and calibrate all of the instrument capabilities by observing representative science targets.
- After six months: Webb will begin its science mission and start to conduct routine science operations.