1. PYTS/ASTR 206 – Exploring the Solar System from the Earth 1 l Announcements n Homework 1 due… n Make sure you give it to Kevin before he leaves n Late homeworks can be turned in class on Tuesday February 3 rd for 50% credit n Kevin has set up a facebook page w“Shane Byrne's PTYS 206 class” wCan use it to organize homework/study groups etc…

2. PTYS/ASTR 206 – The Golden Age of Planetary Exploration Shane Byrne – shane@lpl.arizona.edu Exploring the Solar System from Earth

3. PYTS/ASTR 206 – Exploring the Solar System from the Earth 3 l Telescopes and how they work n Reflectors and refractors n Resolution and magnification n Atmospheric effects l Spacecraft and how they work n Fly-bys, Orbiters & Landers n Tricks of the trade In this lecture…

4. PYTS/ASTR 206 – Exploring the Solar System from the Earth 4 l Why do we use telescopes?

5. PYTS/ASTR 206 – Exploring the Solar System from the Earth 5 l Why use telescopes? n To make things bigger wWhen light levels are high wVery nearby planets wPirate ships wSpying on your neighbors wEtc… n To make things brighter wWhen light levels are low wMost of astronomy wFar away planets wSmall objects Phoenix lander is a few m across… but a few 100km away!

6. PYTS/ASTR 206 – Exploring the Solar System from the Earth 6 l What have telescopes done for planetary astronomy? n Plenty! n Heliocentric vs. geocentric solar system n Objects visible with the naked eye wSun wMoon wMercury (if you’re lucky) wVenus wMars wJupiter wSaturn wUranus (barely) – still discovered with a telescope n Neptune n Discovery of Asteroids n Discovery of Kuiper Belt n Discovery of moons of other planets

7. PYTS/ASTR 206 – Exploring the Solar System from the Earth 7 l They take light over a wide area and put it into a small area l We can do this with either refraction or reflection l Bigger is better! n Light from distant objects comes in parallel rays n The bigger the area of the telescope the more light you can collect How do telescopes work? Human eye in daylight Plenty of light Human eye at night Iris dilates Bigger collecting area You can see fainter things The human eye is like an adjustable-size telescope

8. PYTS/ASTR 206 – Exploring the Solar System from the Earth 8 l Light travels slower in glass than air n Waves are extended – so they change direction Refractors l Flat sheets of glass produce no net effect n Refraction works in reverse when light leave the glass n Light hasn’t been concentrated onto a smaller area

9. PYTS/ASTR 206 – Exploring the Solar System from the Earth 9 l Curved pieces of glass (lenses) do produce a change n Parallel light converges on a single point – the focal point n Distance between the lens and the focal point – the focal length wDepends on the curvature of the lens and its size

10. PYTS/ASTR 206 – Exploring the Solar System from the Earth 10 l Net effect n Telescope lens much bigger than eye so more light gathered wThings are brighter n Magnifies objects wMagnification is ratio of focal lengths l A refracting telescope n First lens (objective) gathers light n Uses a second lens (eyepiece) to make the light parallel again So the human eye can use it! Magnification = Focal length of objective Focal length of eyepiece

11. PYTS/ASTR 206 – Exploring the Solar System from the Earth 11 l The problem with refractors n The amount of refraction depends on wavelength n Red light and Blue light focus in different places n Image gets blurred n Chromatic aberration

12. PYTS/ASTR 206 – Exploring the Solar System from the Earth 12 l The problem with refractors – cont. n We need big lenses to gather a lot of light n … but big lenses have long focal lengths n Telescopes rapidly get very very unwieldy!

13. PYTS/ASTR 206 – Exploring the Solar System from the Earth 13 l Pioneered by Isaac Newton n Flat mirrors don’t focus light n Use curved mirrors to concentrate light wThese mirrors also have a focal length Reflectors

14. PYTS/ASTR 206 – Exploring the Solar System from the Earth 14 l Why parabolic? n Spherical mirrors are easier to make…. n Spherical mirrors don’t focus light well n Spherical aberration fives you a blurry image n Other aberrations can also be corrected wComa wAstigmatism n Defects in the mirror surface should be small wSmaller than the wavelength of light

15. PYTS/ASTR 206 – Exploring the Solar System from the Earth 15 l Focus point is in front of the mirror – usable… but unpopular l Several designs to get light focused somewhere more convenient l Newton’s original design used a flat mirror to redirect focused light to the side l If you want to use your eye then you still need an eyepiece l Magnification is still just the ratio of the focal lengths

16. PYTS/ASTR 206 – Exploring the Solar System from the Earth 16 l Other schemes to redirect the focused light

17. PYTS/ASTR 206 – Exploring the Solar System from the Earth 17 l Benefits of reflectors n You can make the mirrors huge and the focal length short wKeck telescopes – mirrors are 10m across? (built in segments) n No chromatic aberration wAll colors behave the same l Plans for a 30m telescope – the CELT n California Extremely Large Telescope n 3 times the size means 9 times the area!

18. PYTS/ASTR 206 – Exploring the Solar System from the Earth 18 l There’s a limit to what even perfect telescopes can do l A single point of light gets spread out a little n Called the diffraction limit l Resolution – how close can two things be together without joining up. Resolution It’s easy to see that there are two separate objects here.

19. PYTS/ASTR 206 – Exploring the Solar System from the Earth 19 l As things get closer together we can no longer the individual objects l The closest angular- separation they can have and still be separate is the resolution. n Depends on size of telescope n Depends on wavelength of light l Same principle to know what the smallest feature on a planet you can see is.

20. PYTS/ASTR 206 – Exploring the Solar System from the Earth 20 l Example n Smallest object/separation that we can resolve = wavelength Size of telescope l Warning: This formula produces radians l The textbook has a formula that produces arcseconds… l Can we see the Apollo lander on the Moon? l The lander is almost 4m across l The Moon is 384,000,000m away l Angular size 10 -8 radians l Wavelength of visible light 5*10 -7 m l Size of Keck telescope is 10m Resolution of Keck 5*10 -8 radians

21. PYTS/ASTR 206 – Exploring the Solar System from the Earth 21 l Can we see the Apollo lander on the Moon? l The lander is almost 4m across l The Moon is 384,000,000m away l Angular size 1*10 -8 radians l Wavelength of visible light 5*10 -7 m l Size of Keck telescope is 10m Resolution of Keck 5*10 -8 radians We can’t resolve the lander. We’d need a telescope 50m across to be able to see the Apollo lander.

22. PYTS/ASTR 206 – Exploring the Solar System from the Earth 22 Increasing Magnification But the Same Resolution Same Magnification Increasing Resolution

23. PYTS/ASTR 206 – Exploring the Solar System from the Earth 23 l The diffraction is only a theoretical ‘best-case scenario’ l Earth’s atmosphere is a pretty turbulent place n Especially the lower atmosphere – observatories are on mountains! n Makes stars twinkle n Astronomers call this effect ‘seeing’ Atmospheric effects Good Seeing =Good images Lousy seeing =Lousy images l Typically, seeing ~ 0.5 arcsec (~2.5*10 -6 radians) l E.g. on the Moon, that’s a feature ~1km across l Equivalent to a telescope only 10cm in diameter !!!

24. PYTS/ASTR 206 – Exploring the Solar System from the Earth 24 l What’s the problem? Neptune Telescope Light-wave gets bent and warped Different parcels of air have different temperatures

25. PYTS/ASTR 206 – Exploring the Solar System from the Earth 25 l What’s the solution? n Adaptive optics - Flexible mirror Flexible mirror l Mirror deforms in a way that cancels out the n Atmosphere changes all the time n Mirror updates its shape many time per second

26. PYTS/ASTR 206 – Exploring the Solar System from the Earth 26 l How does the mirror know what to do? n You need a nearby guide-star n Starlight passes through the same patch of atmosphere as planets light n Star is supposed to be a point wWavefront sensor detects distortion… w…and figures out how to warp the mirror l Usually there’s no natural guide star n So we use a laser n Reflects of a specific layer high in the atmosphere wHigh sodium layer from meteorite burn ups w90-100km high

27. PYTS/ASTR 206 – Exploring the Solar System from the Earth 27 l We can’t use all wavelengths from ground-based telescopes n Gases in our atmosphere absorb light at many wavelengths n This time there’s no real way around the problem n Atmosphere screens out wSome infrared wavelengths wSome microwave frequencies wMost UV light – Good! wX-rays – Very Good! wGamma Rays – Very very Good!! Atmospheric effects – cont.

28. PYTS/ASTR 206 – Exploring the Solar System from the Earth 28 By-passing the Atmosphere is the best option… Hubble and its successor Infrared - Spitzer

29. PYTS/ASTR 206 – Exploring the Solar System from the Earth 29 l Fly-bys l Usually once off encounters l Can swing by multiple planets (Voyager) l …rarely the same planet multiple times l Orbiters l Usually just one destination (can’t carry the fuel needed to escape) l Long-term monitoring – missions can last years l Landers l Touch-down on solid planets l Parachute into gas giant planets l Different type of instrument Spacecraft Lots of hybrids Lander/Flyby – Deep Impact Orbiter/Flyby - Cassini

30. PYTS/ASTR 206 – Exploring the Solar System from the Earth 30 l Gravity assist l 1 st tried by mariner 10 l Common now for missions l Momentum transfer with a planet l Big effect on spacecraft velocity l Tiny effect on planet’s velocity l Narrows your range of launch dates

31. PYTS/ASTR 206 – Exploring the Solar System from the Earth 31 l Launch windows l Earth moves at 27 km s -1 l We don’t want to waste that energy l To get to Mars - Earth is in a favorable position every two years

32. PYTS/ASTR 206 – Exploring the Solar System from the Earth 32 In this lecture… Next: Exploring the solar system from the Earth l Reading l Chapter 6 to revise this lecture l Chapter 16 for next Tuesday l Telescopes and how they work n Reflectors and refractors n Resolution and magnification n Atmospheric effects l Spacecraft and how they work n Fly-bys, Orbiters & Landers n Tricks of the trade