Unit 2, Chapter 7, Lesson 7: Ballistic Missiles and Getting into Orbit 1 ABM Treaty (cont’d) Arguments for Withdrawal Allow U.S. to develop a ballistic missile- defense system to protect against rogue nations developing weapons.

Unit 2, Chapter 7, Lesson 7: Ballistic Missiles and Getting into Orbit 2 ABM Treaty (cont’d) Arguments against Withdrawal “Fatal blow” to the Nuclear Non-Proliferation Treaty. Would lead to a world without effective legal limits on nuclear growth. Reaction from Russia and China would hinder other arms negotiations (but offsetting pluses have made that reaction milder than expected).

Unit 2, Chapter 7, Lesson 7: Ballistic Missiles and Getting into Orbit SECTION Launch Windows and Times Above: September 1997—The Shuttle Endeavour waits for launch time. Left: April 1998—The Columbia Shuttle propels off the launch pad and accelerates into orbit.

Unit 2, Chapter 7, Lesson 7: Ballistic Missiles and Getting into Orbit 4 Getting into Orbit To meet the conditions of a space mission, launch-team members need to: – Launch the spacecraft from a specific place. – Launch the spacecraft at a specific time. – Launch the spacecraft in a specific direction.

Unit 2, Chapter 7, Lesson 7: Ballistic Missiles and Getting into Orbit5 Launch Windows Definition An opportunity to launch a satellite from Earth directly into the desired orbit from a given launch site.

Unit 2, Chapter 7, Lesson 7: Ballistic Missiles and Getting into Orbit 6 Launch Windows Concept A launch window is an opportunity to launch a satellite from Earth directly into the desired orbit from a given launch site. – We can always launch into parking orbit and then perform a Hohmann Transfer to put a spacecraft in the desired orbit, but this is complicated and requires more fuel. – A launch window means getting directly into the desired orbit without having to maneuver.

Unit 2, Chapter 7, Lesson 7: Ballistic Missiles and Getting into Orbit 7 Launch Windows Concept (cont’d) Launch windows are like bus schedules. Launch windows normally cover a period of time. Launch vehicles must follow trajectory rules by Newton’s Laws of Motion.

Unit 2, Chapter 7, Lesson 7: Ballistic Missiles and Getting into Orbit 8 Launch Windows and Launch Time Endeavour clears the tower on STS-47 Columbia accelerates into orbit on STS-87

Unit 2, Chapter 7, Lesson 7: Ballistic Missiles and Getting into Orbit 9 Launch Time To determine what time a launch site passes into a launch window, we need a new definition of how to tell time.

Unit 2, Chapter 7, Lesson 7: Ballistic Missiles and Getting into Orbit 10 Orbital Racetrack  If the car is one mile past the starting line, and the pit is one-half mile past the starting line, then the car must be one-half mile from the pit.  Given the car’s speed, we can figure out how long before the car reaches the pit.  Knowing this rate allows us to determine the best time to launch a spacecraft directly into orbit, or the launch window.

Unit 2, Chapter 7, Lesson 7: Ballistic Missiles and Getting into Orbit 11 Launch Time Solar Time The spacecraft must launch in a specific direction: – The vernal equinox direction is the main direction we use to describe the motion of spacecraft that orbit the Earth. – Because the Earth and the launch site rotate, it’s an easy reference from which to measure the angular distance between the orbital plane and our launch site. – Knowing this angular distance and Earth’s rotation rate, we can figure out the best times to launch.

Unit 2, Chapter 7, Lesson 7: Ballistic Missiles and Getting into Orbit 12 Launch Time Solar Time (cont’d) Greenwich Mean Time (GMT)— is the basis for the time kept on watches to handle daily business. – GMT is the mean solar time for the Prime Meridian (Greenwich, UK). – All other time zones are GMT plus or minus a certain number of hours.

Unit 2, Chapter 7, Lesson 7: Ballistic Missiles and Getting into Orbit 13 Launch Time Solar Time (cont’d) The Royal Observatory in Greenwich, England

Unit 2, Chapter 7, Lesson 7: Ballistic Missiles and Getting into Orbit 14 Launch Time Solar Time Apparent Solar Day – An Apparent Solar Day is the time between successive passages of the Sun overhead, which is from noon one day to noon the next day. – Apparent solar day’s length varies throughout the year (Earth’s orbit around the Sun is elliptical). – Average of the lengths for one year gives us a mean solar day.

Unit 2, Chapter 7, Lesson 7: Ballistic Missiles and Getting into Orbit 15 Launch Time Sidereal Time Because there is a problem using solar time, the reference point for GMT rotates with the Earth, we seek an inertial (fixed) reference point: – We use the same reference point as the geocentric- equatorial coordinate frame—vernal equinox direction. – A sidereal day is the time between the subsequent passings of the vernal equinox direction overhead a particular longitude line.

Unit 2, Chapter 7, Lesson 7: Ballistic Missiles and Getting into Orbit 16 Launch Time Sidereal Time (cont’d) Because the inertial reference vernal equinox is so far away, the length of a sidereal day corresponds to exactly 360° of Earth rotation. A sidereal day is slightly shorter than an apparent solar day: 23 hours, 56 minutes, 4 seconds in solar time.

Unit 2, Chapter 7, Lesson 7: Ballistic Missiles and Getting into Orbit17 Launch Time Sidereal Time (cont’d) We can tell sidereal time in degrees. Earth rotates 360° in 24 hours. So, 3 a.m. (0300) is 45° of Earth’s full rotation. Similarly, noon (1200) is 180° and 6 p.m. (1800) is 270°.

Unit 2, Chapter 7, Lesson 7: Ballistic Missiles and Getting into Orbit 18 Launch Time Local Sidereal Time (LST) Time since the first point of constellation Aries was last overhead. Can be expressed in time or angle If Earth has rotated 90° since the local longitude line was aligned with the vernal equinox direction, LST is: – 90° – 0600 hrs Mission planners mathematically convert LST back to GMT for launch timing.

Unit 2, Chapter 7, Lesson 7: Ballistic Missiles and Getting into Orbit 19 Launch Time Local Sidereal Time (LST)

Unit 2, Chapter 7, Lesson 7: Ballistic Missiles and Getting into Orbit 20 Launch Time Solar Time versus Sidereal Time

Unit 2, Chapter 7, Lesson 7: Ballistic Missiles and Getting into Orbit 21 Summary Intercontinental Ballistic Missiles (ICBMs) – Understanding Ballistic Missiles and Trajectories – Conic Sections – ICBM Trajectories – ICBM Development – The Anti-Ballistic Missile Treaty Launch Windows and Times – Getting into Orbit – Launch Windows – Launch Time

Unit 2, Chapter 7, Lesson 7: Ballistic Missiles and Getting into Orbit 22 Next For some payloads, the mission requires us to get a payload into orbit and get it back to Earth from space. On the next lesson we’ll tackle the last part of a space journey: atmospheric re-entry.