Potensi Anomali Cuaca Pasca Letusan Merapi

Badan Penelitian dan Pengembangan Teknologi Kegunungapian (BPPTK) Yogyakarta -lembaga resmi yang memonitor gunung Merapi secara terus-menerus baik dalam kondisi aktif normal maupun di luar normal seperti saat ini- secara resmi melansir letusan Merapi 2010 ini merupakan letusan skala VEI 4. Pernyataan resmi ini mengonfirmasi dugaan sebelumnya akan dinamika Merapi dalam letusan kali ini. Namun BPPTK juga menekankan -mengutip mas Agus Hendratno- bahwa dalam skala letusan VEI 4 ini dinamika Merapi sungguh sulit untuk ditebak sehingga kapan letusan berakhir menjadi hal yang sangat sulit diprediksi. Senada dengannya, Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG) di Bandung pun melansir penyataan bahwa aktivitas Merapi masih terhitung tinggi meski intensitas letusan memang cenderung merendah, seperti telah ditulis sebelumnya. Namun masih terdapat kemungkinan intensitas letusan menaik lagi hingga mendekati atau bahkan menyamai letusan besar 3-5 November lalu. Sehingga Merapi tetap berstatus awas, dengan zonasi radius bahayanya yang spesifik per wilayah dan seharusnya dipenuhi semua pihak demi keselamatan bersama. Terlebih korban jiwa yang terenggut dalam letusan kali ini cukup banyak, diindikasikan mencapai 450-an jiwa atau terbesar dalam 80 tahun terakhir.

Menanti kapan letusan Merapi usai mungkin seperti Menunggu Godot-nya Samuel Beckett. Namun, sembari menunggu hal yang terasa tak berkesudahan itu, pihak-pihak terkait dan masyarakat luas, baik di sekitar Merapi maupun di Indonesia secara keseluruhan, sebaiknya juga mulai mempersiapkan diri akan berbagai kemungkinan pasca letusan. Dalam konteks lokal, dengan keluaran magma melebihi 100 juta meter kubik yang kini sebagian besar tertumpuk di lembah-lembah sungai yang berhulu di Merapi, dibutuhkan suatu penanganan terencana agar material tersebut tidak berdampak luas. Dalam skup Yogyakarta, sungai-sungai yang melintasi kota ini khususnya sungai Code (yang terhubung langsung dengan sungai Boyong, salah satu sungai yang menjadi jalan tol material vulkanik Merapi) harus diperhatikan, termasuk menyiapkan penduduk yang bermukim sepanjang lembahnya untuk bersiap-siap dievakuasi tatkala lahar dingin dalam volume besar meluncur melewati alur sungai. Perhatian besar terutama harus diarahkan kepada lingkungan di sekitar kelokan/tekuk sungai (slope-break), mengingat pada lokasi inilah aliran lahar dingin bisa melejit keluar dari alur sungai. Perhatian yang sama seharusnya juga diberikan pada alur sungai Krasak, Bebeng, Putih dan Pabelan yang mengarah ke Magelang. Apalagi estimasi awal memperlihatkan volume seluruh lembah sungai tersebut masih lebih kecil dibanding 100 juta meter kubik, sehingga ada ketidakseimbangan di sini.

Tambora

Secara regional (Indonesia) dan global, juga tak ada salahnya untuk mulai memprakirakan potensi anomali cuaca pasca letusan Merapi. Bahwa letusan gunung api berpotensi mengubah cuaca, tak ada yang meragukannya. Letusan Tambora 1815 adalah contoh yang paling sering diulas. Letusan yang mencapai puncaknya pada 11 April 1815 itu merupakan letusan superkolossal, memuntahkan 150 km kubik magma yang disemburkan hingga ketinggian 43 km pada skala letusan VEI 7 yang setara dengan tingkat energi 27.000 megaton TNT. Sebagai pembanding, jika seluruh hululedak nuklir yang dimiliki dua negara adidaya pada puncak Perang Dingin dikumpulkan dan diledakkan bersama-sama, energi yang dilepaskannya 'hanya' 20.000 megaton TNT, sehingga letusan Tambora masih 35 % lebih energetik. Dahsyatnya letusan Tambora bisa dilihat dari kemampuannya memenggal pucuk gunung dari yang semua berketinggian 4.300 meter menjadi hanya setinggi 2.720 meter. Pucuk yang terpenggal lantas menyisakan kawah besar (kaldera) bergaris tengah 7 km. Debu dan batu letusan diendapkan hingga 1.300 km dari gunung -di Yogyakarta pun masih setebal 1 cm-, namun mayoitas mengendap di area seluas 874 km persegi dengan ketebalan rata-rata 20 meter. Thomas Stanford Raffles -saat itu Indonesia dibawah kekuasaan Inggris- mencatat korban jiwa yang jatuh di Pulau Sumbawa saja mencapai 100 ribu jiwa.

Jejak letusan Tambora 1815 sebagaimana dicitrakan satelit pengindraan jauh NASA. Nampak kaldera bergaris tengah 7 km yang terbentuk di tubuh gunung Tambora, pasca letusan super-kolossal 1815. Inilah letusan yang mengubah wajah iklim dunia.

Sumber : NASA, 2009

Namun kedahsyatan letusan Tambora 1815 yang melegenda adalah kemampuannya menyimpangkan iklim dunia. Dari 150 km kubik magma-nya, 36 km kubik ditaburkan jauh tinggi ke lapisan stratosfer, termasuk didalamnya 65 juta ton Belerang yang lantas bereaksi dengan uap air dan oksigen di atmosfer sehingga membentuk 200 juta ton asam sulfat berbentuk aerosol. Aerosol ini tersebar ke segenap penjuru atmosfer Bumi dan berperan menghalangi sinar Matahari sehingga intensitasnya di permukaan Bumi tinggal 75 % dari normal. Padahal dalam waktu yang sama, aktivitas Matahari pun sedang cenderung menurun -ditandai dengan sedikitnya jumlah bintik Matahari di permukaannya- dalam periode yang lebih dikenal sebagai ekstrimisitas minimum Dalton. Kombinasinya dengan letusan Tambora membuat suhu global menurun 1 derajat Celcius dari normal (0,7 derajat Celcius di Eropa), yang menggiring dunia ke episode perubahan iklim tak terperi sepanjang sejarah modern.

Tahun 1816 selalu dikenang sebagai tahun tanpa musim panas (year without summer) karena inilah saat Eropa menggigil kedinginan ketika suhu rata-rata sepanjang tahun itu sama dengan suhu musim dingin. Salju menyebar dimana-mana, membuat lahan pertanian tak bisa digarap. Tiap negara pun didera problem kelaparan hebat, yang berujung pada gejolak sosial akibat melambungnya harga makanan. Hujan turun terus-menerus hampir setiap saat yang menjadikan jalanan sebagai kubangan lumpur. Ambisi Napoleon Bonaparte, kaisar Perancis yang sebelumnya tertawan namun sempat melarikan diri dari Pulau Elba, menyerbu Russia berantakan total tatkala amunisi dan perbekalan beratnya terjebak di jalanan berlumpur. Kekalahan demi kekalahan terus dideritanya dalam berbagai pertempuran, hingga akhirnya pertempuran terakhir di Waterloo memaksanya bertekuk lutut. Sementara di India, kolera jenis baru merajalela, menelan korban jiwa yang tak terhitung.

Dua Skenario

Letusan Merapi 2010 barangkali takkan berdampak sehebat skenario Tambora di atas, bahkan andaikata dikombinasikan dengan serial letusan gunung Eyjafjallajokul di Islandia (yang dikategorikan dalam skala VEI 5 atau setingkat di atas Merapi). Model perubahan iklim TTAPS (akronim dari lima ilmuwan perintisnya yakni Turco, Toon, Pollack, Ackerman dan Sagan) yang bisa diaplikasikan baik untuk memerikan dampak perang nuklir global, letusan kolossal gunung api maupun tumbukan komet/asteroid raksasa (diameter > 1 km) mengindikasikan perubahan iklim global baru akan terjadi jika energi yang dilepaskan salah satu peristiwa itu melebihi 1.000 megaton TNT. Sementara kombinasi energi letusan Merapi dan Eyjafjallajokul hingga saat ini masih dibawah 200 megaton TNT.

Namun dampak regional takkan terelakkan. Pada titik ini harus dipahami bahwa situasi atmosfer menjelang letusan Merapi 2010 hampir sama dengan skenario Tambora. Badan atmosfer dan kelautan seperti NOAA dan LAPAN telah lama memperingatkan aktivitas Matahari saat ini cenderung menurun dibandingkan normal. Sepanjang 2009 lalu permukaan Matahari bersih dari bintik Matahari selama 260 hari, rekor baru sepanjang seabad terakhir kecuali tahun 1913. Dan sejak 2004, Matahari sudah menjalani 772 hari tanpa bintik, berkebalikan dengan normalnya yang hanya 485 hari. Model-model matematis kontemporer mengindikasikan dalam siklus Matahari ke-24 ini, yang telah dimulai sejak Desember 2008, Matahari akan sangat tenang sehingga pada puncak aktivitasnya (diprediksi pada bulan April/Mei 2004 mendatang) hanya akan memiliki bilangan bintik Matahari 40. Konsekuensinya, intensitas pencahayaan Matahari pun menurun dan menimbulkan pendinginan global yang dampaknya melampar ke segenap penjuru Bumi.

Prediksi siklus ke-24 Matahari yang telah dimulai pada Desember 2008. Dibanding sebelumnya, nampak jelas bahwa siklus ini memperlihatkan Matahari yang tenang, yang memicu global cooling.

Nah, letusan Merapi 2010 (bersama Eyjafjallajokul) berpotensi kian meredupkan pencahayaan Matahari akibat tabir surya dari aerosol sulfat yang dibentuknya di atmosfer. Memanfaatkan data-data volume magma vs massa aerosol sulfat dari letusan-letusan gunung api di kawasan tropis seperti Tambora 1815, Krakatau 1883, St. Maria 1902 dan Agung 1963, diperoleh suatu hubungan linier yang mengestimasikan letusan Merapi 2010 memproduksi sedikitnya 6 juta ton aerosol sulfat di atmosfer yang akan berkorelasi terhadap penurunan suhu sekitar 0,2 derajat Celcius di kawasan regional. Merujuk pada respon lautan Indonesia terhadap global cooling, penurunan suhu tersebut mungkin tetap akan diikuti hangatnya air laut di sekitar Indonesia sehingga produksi uap air tetap tinggi dan akibatnya curah hujan yang tinggi akan tetap terjadi. Apalagi LAPAN telah mengingatkan bahwa situasi cuaca Indonesia di tahun 2011 mendatang diestimasikan akan tetap sama dengan situasi cuaca tahun ini.

Estimasi linier sulfat vs volume magma letusan gunung di daerah topis. Kiri : melibatkan Tambora 1815, kanan : tidak melibatkan Tambora 1815. Dari estimasi ini maka diindikasikan Merapi 2010 membentuk setidaknya 6 juta ton aerosol sulfat di atmosfer.

Sumber : Ma'rufin, 2010

Di sisi lain, kita juga tak bisa menafikan data empirik seperti dipaparkan Dr. Joko Wiratmo (ITB) bahwa letusan-letusan berskala tinggi gunung api (khususnya di kawasan tropis) sering diiringi fenomena El-Nino pada tahun berikutnya. Sebagai contoh, letusan El-Chichon (Meksiko) 1982 dan Galunggung 1983 yang diikuti El-Nino 1982-1983 serta letusan kolossal Pinatubo (Filipina) 1991 dan Kelut 1990 yang diikuti El-Nino 1992-1993.

Peta sebaran gas sulfurdioksida secara komposit per 4 - 8 November 2010 dari letusan Merapi. Nampak kopnsentrasi gas mayoritas berada di Samudera Hindia, pada lapisan tropopause (15 km). NASA memperkirakan jumlah gas ini sekitar 0,17 juta ton.

Sumber : NASA

Dengan demikian ada dua potensi anomali cuaca yang bisa terjadi pasca letusan Merapi ini. Yang pertama: tahun tanpa musim kering karena curah hujan berlebih. sementara yang kedua: tahun dengan sedikit musim hujan karena munculnya El-Nino. Mana yang berpeluang lebih mungkin terjadi? Biarlah ini menjadi urusan para vulkanolog dan klimatolog. Sembari menanti analisis mereka, tak ada salahnya untuk mulai mempersiapkan diri.

Hemat, Kirim Astronot Usia 60 Tahun ke Mars

Pergi ke Mars jadi impian manusia, terutama mereka para ahli yang suka menjelajah angkasa. Apalagi planet merah itu adalah planet yang mendekati kondisi bumi, adan sudah diincar jadi koloni manusia jika suatu saat Bumi makin rusak dan tak layak huni lagi.

Mewujudkan impian itu memang bukan pekerjaan mudah. Selain soal teknologi yang belum memadai, misi ini juga terkendala soal dana.

Untuk meminimalisasi biaya yang dikeluarkan, sejumlah ilmuwan menyarankan agar perjalanan ke Mars dilakukan sekali jalan. Artinya, begitu pergi ke sana, jangan kembali lagi ke bumi.

Para ilmuwan menyatakan, kolonialisasi Mars akan makin cepat diwujudkan dan murah jika para astronot berperilaku seperti para pemukim pertama di Amerika Serikat. Mereka tak pernah berharap bisa pulang.

Itu adalag, "Poin utama, agar eksplorasi Mars terwujud," kata Dirk Schulze-Makuch, profesor dari Washington State University yang menerbitkan artikel kontroversial, seperti dimuat Daily Mail, Rabu 17 November 2010.

Ia mengusulkan misi diawali pengiriman dua orang astronot berusia 60 tahunan yang akan terbang dalam dua pesawat berbeda -- pesawat ini juga berfungsi sebagai tempat tinggal mereka di Mars. Selanjutnya, kapal koloni dan perbekalan akan disusulkan kemudian.

Publikasi artikel ini kurang dari sebulan setelah NASA keceplosan sedang menginvestigasi kemungkinan kolonialisasi manusia di planet lain dalam proyek ambisius yang dinamakan 'Hundred Years Starship'.

Sementara, ilmuwan lain, Paul Davies, fisikawan dari Arizona State University berargumen, manusia harus mengkoloni planet lain untuk mengantisipasi malapetaka di Bumi.

"Sebaiknya yang dikirimkan orang berusia 60 tahun atau sekitar itu."

Alasannya, misi ke luar angkasa pasti akan mengurangi umurr seseorang -- baik akibat kurangnya perawatan medis maupun paparan radiasi dalam jumlah besar.

Radiasi juga dapat merusak organ reproduksi, sehingga pengiriman orang di usia subur bukanlah ide yang baik.

Sementara, untuk perjalanan sekali jalan, diperkirakan bisa memotong anggaran hingga 80 persen.

Namun, baik Davies maupun Schulze-Makuch menyatakan penting artinya untuk menyadari bahwa ini bukan 'misi bunuh diri'.

Tapi, "astronot di Mars akan tinggal di sana seumur hidupnya, menjadi pionir dari kolonialisasi manusia secara permanen di planet merah." Untuk itulah, profil psikologis astronot, termasuk intensitas komunikasi dengan Bumi jadi faktor penting.

Gagasan sekali jalan ini mendapat tentangan dari sejumlah pihak. Termasuk, mantan astronot Apolo 14, Edgar Mitchell. "Ini prematur. Kita belum siap untuk menjalani hal seperti ini."

NASA pun tak setuju dengan hal ini. "Kami ingin astronot bisa kembali ke Bumi."
Rekomendasikan

Lecture 1

Tuesday, July 2nd 2002. Reading: Cosmic Perspective Chapters 13 and S5

Astrobiology
What is Life?
"I'll know it when I see it" <--- Not good enough

something that ingests, metabolizes, and excretes: Cars, Candle Flames

Departure from Thermodynamic Equilibrium: Lightening, Ozone Layer

Biochemical definitions -- nucleic acids, proteins, etc.: chauvinistic

Carl Sagan's Favorite: "Life is a system capable of reproduction, mutation, and reproduction of its mutations": Impractical to apply when searching other worlds.
Life on Earth
Early History of Earth:

* primordial disk of dusty gas surrounding the protosun.
* clumps of matter stick together, gain more mass and gravitationally attract more matter: planetesimals
* Earth slowly builds up from planetesimals; molten, differentiates
* Primordial Earth collides with Mars-sized planetesimal, creates the Moon
* Earth's surface bombarded by impact from debris, surface is molten. lasts until about 3.9 billion years ago.
* Surface cools, atmosphere and oceans build up.

3.8 billion years ago Oldest Carbon evidence for life

3.5 billion years ago Oldest fossils of blue-green algae and stromatelite bacteria

***Geological Timeline*****

Biology
All life on Earth shares the same biochemistry. Carbon compounds (Organic): long chains of Carbon with Hydrogen, Oxygen, and Nitrogen attached. Same substances that Earth's surface and atmosphere are mostly made of.

Primary structural unit of chain molecules are amino acids. H,C,N,O,S all arranged into specific patterns. Amino acids form more complex chains of molecules called Proteins. Proteins give structure to living cells and provide energy to the cell.

DNA Deoxyribose Nucleic Acid. A very, very long molecule shaped like a ladder with rungs twisted into a double helix. The "rungs" are called base pairs and are a kind of molecular code. The code contains information on the construction of amino acids and proteins. The molecule can split in half and make a copy of itself. The molecule is "alive"!

That all living things on Earth share the same biochemistry (all organisms that use DNA use the same code book of base pairs) speaks to a common origin of life on Earth. A singular spark of life ~ 3.8 billion years ago leading to an unbroken chain of life. An enormous diversity of lifeforms evolve from that single spark of life!

Evolution of life is a process that leads to the diversification of life. It is not a drive toward greater complexity, it is only a drive toward greater diversity.

Lifeforms change with time due to random mutations that arise from a variety of sources: High Energy E&M radiation, Particle Radioactivity, Cosmic Rays, Chemical Reactions, and random errors in the copying of DNA (caused by the Uncertainty principle).

Most mutations are lethal to the organism (i.e. cancer), but some are beneficial. Over long periods of time species can change; those that are best suited for survival in their current environment thrive over others. In times of changing environments those species capable of rapid adaptation survive over others. This process is called Natural Selection.

Despite the increase in complexity of some organisms over time, the overwhelming majority of life on Earth is still in the form of simple one-celled organisms: bacteria. Nature likes to stick with what works.
Origin of Life
Organic molecules are very common in the Cosmos. Find them all the time in interstellar clouds. Must have been common in the early Earth atmosphere.

Miller-Urey Experiment: place compounds common in the early Earth's atmosphere in a container (nitrogen, water, carbon dioxide, etc. Note: no oxygen) and put energy into it with electrical sparks (to simulate lightening). Forms an organic sludge containing amino acids!

NOTE: Life is found practically everywhere on Earth (deep sea ocean vents, dry desserts of Antarctica, etc.). Commonality: Require liquid water. Liquid water is the best solution for organic chemistry. Life is not found to exist on Earth where there is no liquid water present.
Life on other worlds in our Solar System
We've mentioned that there are other worlds in the Solar System that either at one time or another have had (or still have) liquid water on their surfaces. Given the abundance of organic matter on these worlds might life have once existed on these worlds? Might it still exist?

Venus: Once had liquid oceans (probably water) but runaway greenhouse effect evaporated all water and cooked the planet. Definitely no life there now. fossil records from earlier epochs before the runaway greenhouse?

Mars: Liquid water on surface in past, thicker atmosphere, warmer temperatures. A world very much like Earth. Could life have started there and been extinguished? Could there still be some life hanging on? Fossils of past life?

Europa: Liquid oceans of water beneath a frozen ice crust. Heat from interior geologic activity (tidal friction). Life in those oceans?

Titan: Organic Molecules in atmosphere. Liquid Methane on surface (similar solvent properties as water). Heat from Saturn and a greenhouse effect. Life with different biochemistry?

Panspermia
Idea that life could have originated on either Mars or Earth, and meteor impacts sends rocks containing life into space that later crash land on the other planet. Cross contamination? Could we be Martians? Or could there be life on Mars that originated on Earth?

Or what if life originated on some distant world in another planetary system? Simple bacteria spores might survive the eons in the vacuum and then settle down on Earth and take root. Might we be aliens on our own world?
Other Planetary Systems
There are now upwards of 50 other known planets orbiting other stars. These planets are all being found indirectly by their gravitational influences on their parent star. Since the search for such planets is very young only the most massive planets will be found first (they have the strongest gravitational effect on their parent star). Also the first systems to be found are likely to have the big planets closer to their star than the Jovian planets are to our own Sun (because they will then have shorter orbital periods and stronger gravitational tugs on their parent stars).

Planetary scientists fully expect that in the next several decades planets like Earth will be found around many stars.

Not all stars are expected to have planets. The most massive stars (O and B stars) live so fast and die so young that planets barely have a chance to form around them before the stars blow up as supernovas.

Double stars systems also would tend to not have planets as the gravitational effects of the 2 stars would tend to eject planets from the system.

That still leaves ~200 billion stars in the Milky Way galaxy alone that could have planets.

Some of those planets must lie in a habitablity zone and have liquid water present. Given that it seems so easy to make the building blocks of life, how easy is it for life to start going and survive?




We really have no clue.

Intelligent Life in the Universe
Life has the possibility to be abundant in the Universe. In all of those living worlds, how often does intelligence arise?
UFOs, Alien Abductions, and Ancient Astronauts
If it turns out to be very possible for intelligent life to exist then, certainly there must have been many intelligent civilizations that would have evolved billions of years before us.

They would have become technologically advanced and likely become space-faring. They could build space ships that held whole generations of space explorers, and travel at relativistic speeds to make the journies between stars not so long for them (although those of us at lower relative speeds observe their journies to take aeons).

If that were the case, then where are they? Should they not have come this way already?

* Perhaps we are the first in our galaxy to reach this level. It's highly unlikely, but someone has to be first.
* Our Sun is an unremarkable star in a galaxy of 100s of billions of stars just like it. It's possible that space explorers just haven't made it out this way yet.
* Or maybe they have...

Some suggest that the phenomenon of UFOs may be extraterrestrial visitors coming to our world. Or that strange historical accounts or grand technological accomplishments of humans in the past are evidence of visits from extraterrestrials. There are those claiming to have been abducted by aliens and have terrible medical experiments performed on them.

This all falls into the realm of pseudoscience. There has never been any concrete evidence of extraterrestrials having anything to do with UFOs. UFOs could be so very many things. Why assume automatically that there is an otherworldly explanation? But those who want to believe will do so even despite evidence to the contrary.

"Extraordinary Claims Require Extraordinary Evidence" - Carl Sagan.

To this date there has never been any such evidence of an extraterrestrial connection to UFOs. The fact the the majority of the human population lives in cities now and has very little familiarity with the sky goes a long way to explaining most UFO sightings. How many of you have ever seen ball lightening, a stealth fighter or bomber in flight, swamp gas, Insect swarms flying through electric fields? All of these things are real and in the sky. Most people have no idea what they are looking at when they see them.

Alien abduction stories bare a resemblance to post traumatic stress disorder. People experience something horrible in their lives and their brains suppress the memories. The memories come back in dream-like states where the mind is highly susceptible to confabulation and fantasy.

There is such a large main-stream consciousness of the typical alien abduction story (thanks to TV and Movies) that it's no wonder that "abductees" tell stories that are remarkably similar to one another.

Their stories often involve elements that sound like a condition called sleep paralysis. When you are in REM sleep and are dreaming your body paralyzes the major voluntary muscles so that you cannot injure yourself while you dream. Sometimes we can fall into dream states before we have become completely unconscious. Our bodies become paralyzed and we can even dream in the state, but yet we are also partially awake. It's called waking dreams it's a real and studied phenomenon. Its happened to me on more than one occasion. Pretty spooky.
SETI
All of this skepticism over UFOs and alien abductions does not hinder astronomers from thinking that intelligent life could exist in the Universe. In fact most think that it can. A project now known as the Search for Extraterrestrial Intelligence (SETI) was begun back in the 1960s to search the heavens for radio signals that reach us from some other technological civilization in the Cosmos.

NASA funded the program in the 1980s but Congress pulled the plug on the project in the early 1990s. Seems the Congress felt that tax payer money shouldn't be spent on a project that they thought was more like science fiction than science.

They sought private funding for their research and have now established the SETI institute.

The idea is that radio communication is simple, inexpensive, and the Universe is relatively quiet at most radio frequencies. So the first choice for an alien civilization to attempt to make contact would be via radio communication.

So the SETI project scans billions of radio frequencies everyday from across the sky searching for some signal that would stand out as being unequivocally unnatural. They do this with supercomputers and a network of radio telescopes around the world.

To date there have been no detections. But that doesn't mean we will never find anyone out there.

Radio waves travel at the speed of light (they are light afterall). Earth has been producing radio waves for our own communication purposes for about 100 years now. Those signals escape from Earth and spread away from Earth in a sphere. So the Earth is surrounded by a sphere of about 100 light-years in radius of radio waves. Any aliens farther away than 100 light-years could not possibly know about us yet. Likewise if they have only started producing radio signals in the last 100 years we will not know about them for some time.

If a signal were ever detected it would not likely be easy to decode, unless it were specifically designed to be decoded by any species with sufficient understanding of simple mathematics. Plus 2-way communication would be excruciatingly slow. We could exchange information over periods of decades and centuries. But it would not be like a real conversation.
The Drake Equation
Can we estimate, based on the things we have learned in this class, the number of intelligent civilizations that we could contact via radio communication?

Frank Drake of UC Santa Cruz developed a simple equation which is just the multiplication of all the probabilities involved to make an estimate of this very thing.

Nc = Rs · fp · n · fl · fi · fc · L

Nc is the number of civilizations in our Galaxy with whom we can communicate.

Rs: Rate of star formation in the Galaxy
~ 40 stars/year

fp: Fraction of Stars that have planets
~ 0.5

n : Average number of planets that are habitable in a planetary system
1 - 5

fl: Fraction of habitable planets on which life arises
0.1 - 1.0

fi: Fraction of planets with life on which intelligence arises
0.01 - 1.0

fc: Fraction of intelligent civilizations that communicate via radio waves
0.1 - 1.0

L : Average lifetime of a civilization
100 - 109 years

* If you make optimistic estimates you find that the Universe is teaming with life. Billions of civilization in each galaxy.
* If you make pessimistic estimates you find that we are alone in the Universe.

Both ideas are huge!!

The Universe did at least make us. So the probability of intelligent life in the Universe is 100%.

"We are starstuff contemplating the stars" - Carl Sagan

Evaluations...