People who are interested in science, technology, engineering, and mathematics (STEM) are self-starters. They're interested in experimenting, crafting, and creating. More than anything, they're aware that the path to the future isn't given to us; we build it ourselves. Now, you can help build it with the unique skills you bring to the table, and there's no better place to put those skills to use than in the United States Space Force (USSF).

STEM skills are necessary for a wide range of jobs in the USSF. Want to protect the confidential information behind a firewall or build electrical systems that power our journey into orbit? Read on to see some of the rewarding and challenging career paths offered to STEM experts.

Whether you prefer to work in business casual or a military uniform, the USSF lets you get in on the ground floor of exciting new technology. See if it's right for you and start building our future today.

The skies may look clear from our perspective, but just out of view, there's a junkyard in our low Earth orbit (LEO), a region of space relatively close to our planet's surface. There are approximately 60,363 tracked objects in orbit, the vast majority of which is space debris. That number doesn't even include the over one hundred million bits and bobs too small to be tracked. Some speculate that there are over 8,000 metric tons of debris floating around, including non-functional satellites, discarded rocket parts, and broken pieces of spacecraft.

The working satellites in our LEO are vital to our global infrastructure, and with debris moving through space at almost seven times the speed of a bullet, even the smallest impact can create a huge problem. This is an issue we all have a stake in—here's how the U.S. Space Force is working to tackle it. 

Space may seem nearly endless, but our orbit is finite. With no international laws in place to clean up debris, the responsibility must be shared. The U.S. Space Force is doing their part by stepping up and working to keep our planet safe.

For many of us, being a Guardian in the United States Space Force (USSF) is a dream job that involves a lot of unique and fascinating tasks, like monitoring space for potentially dangerous debris and working with prototypes of new aerospace systems. The USSF has fewer personnel than any other U.S. military branch, so competition for positions is fierce. They expect the absolute best. And with only five years of history, this branch needs innovative problem solvers with STEM skills in order to take a collective giant leap forward. Guardians make that happen.

Opportunities are waiting. Whether you want to be involved as an enlisted member, an officer, or a civilian, your expertise will be put to good use ensuring space is secure, stable and accessible for future generations.

This year marks the fifth anniversary of the founding of the United States Space Force, but many are still unfamiliar with its mission. If you're one of them, check out this deep dive into the USSF's mission and why its work is so critical to our day-to-day lives.

Whether it's providing early warning systems for dangerous weather or managing GPS satellites, the United States Space Force has its eye on the sky, preparing to handle whatever comes next. Learn more at https://www.spaceforce.com/

Stars speckle the expanse of our ever-expanding universe. While our closest star is the sun that we orbit in our little corner of the cosmos, we can peer out to see what stellar neighbors live nearby.

The nearest stars to Earth are three stars that lie about 4.37 light-years away in the Alpha Centauri triple-star system. The closest of these stars, Proxima Centauri, is just about 4.24 light-years away. (for reference, one light-year is approximately equivalent to 5.88 trillion miles (9.46 trillion kilometers)). 

Suffice to say, "close" in space is far different from our Earthly definition.

Related: Does every star have planets?

Of all the stars closer than 15 light-years, only two are the same type as our sun: a G-type main-sequence star. G-type stars like our sun, also called yellow dwarf stars, typically have about 0.9 to 1.1 solar masses with surface temperatures that can range from about 9,080 degrees Fahrenheit (5,026 degrees Celsius) to 10,340 degrees F (5,726 degrees C). 

The only two other G-type stars in our neighborhood are Alpha Centauri A and Tau Ceti. The majority of nearby stars are M-type stars, also known as red dwarfs, the most common stars in the universe.

Only nine of the stars in this area are bright enough to be seen by the naked human eye from Earth. These bright stars include Alpha Centauri A and B, Sirius A, Epsilon Eridani, Procyon, 61 Cygni A and B, Epsilon Indi A and Tau Ceti.

Barnard’s Star, a red dwarf 5.96 light-years away, has the largest proper motion of any known star. This means that Barnard’s Star moves rapidly against the background of more distant stars, at a rate of 10.3 seconds of arc per Earth year.

Sirius A is the brightest star in Earth’s night sky, due to its intrinsic brightness and its proximity to us. Sirius B, a white dwarf star, is smaller than Earth but it has a mass 98 percent that of our sun.

In late 2012, astronomers discovered that Tau Ceti may host five planets including one within the star’s habitable zone. Tau Ceti is the nearest single G-type star like our sun (although the Alpha Centauri triple-star system also hosts a G-type star and is much closer).

The masses of Tau Ceti’s planets range from between two and six times the mass of Earth.

This page was updated by Space.com senior writer Chelsea Gohd in Jan. 2022.

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NASA's Parker Solar Probe mission, which is scheduled to launch in July 2018, will come within 3.9 million miles (6.2 million kilometers) of the sun — seven times closer than any other spacecraft ever has. 

The specially shielded Parker Solar Probe will have to endure temperatures up to 2,500 degrees Fahrenheit (1,370 degrees Celsius) and solar radiation intensities 475 times higher than we're used to here on Earth.

If all goes according to plan, the Parker Solar Probe will zoom close to the sun 24 times between 2018 and 2025, gathering a variety of data about the sun's structure and magnetic and electric fields, as well as the energetic particles cruising near and away from Earth's star. This information could help researchers solve two longstanding mysteries: How the solar wind is accelerated and why the sun's outer atmosphere, or corona, is so much hotter than the solar surface, NASA officials have said.

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On Jan. 31, 1958, the United States launched its first successful satellite: Explorer 1. It was the American answer to the Soviet Union's Sputnik 1, which kicked off the Space Age when it launched in 1957.

Explorer 1 launched on a U.S. Army Juno rocket, also known as Jupiter-C, and marked the first space mission to carry a scientific instrument into space. The satellite weighed 30 lbs., 18 lbs. of which was science gear like cosmic ray detectors, temperature sensors and a microphone to hear micrometeorites that might hit the satellite.

Explorer 1: The First American Satellite in Pictures

The U.S. used Explorer 1 as its contribution to the International Geophysical Year (which ran from 1957 to 1958). The satellite was originally slated to launch on U.S. Navy's Vanguard rocket, but it exploded moments after launch, garnering nicknames like "Kaptunik" in media headlines. The Army's Jupiter-C, developed as a ballistic missile.  

The Jupiter-C rocket delivered Explorer 1 into an orbit that ranged between 220 miles to 1,563 miles above Earth. It beamed data to Earth for four months, going silent on May 23. The satellite re-entered Earth’s atmosphere on March 31, 1970 and burned up.

Explorer 1's main instrument was a cosmic ray detector designed by James Van Allen of the State University of Iowa. The experiment discovered evidence of radiation belts around Earth, now called Van Allen Belts, that marked the first scientific discovery in space.

The Van Allen Belts are doughnut-shaped regions of high-energy particles held in place by Earth’s magnetic field. They serve as a buffer, preventing cosmic rays from bombarding Earth, and may have played a role in making Earth habitable for life, NASA has said.

In 2012, NASA launched the Van Allen Probes to study the Van Allen radiation belts in detail.

Icy Water Worlds That Might Host Life

Alien life may be lurking right in Earth's cosmic backyard. Some of the icy moons of Saturn and Jupiter are known to harbor subsurface oceans that could provide habitable environments.

In the dark, cold sections of the Earth's ocean floor, communities of life-forms survive on the heat and nutrients from hydrothermal vents. Under the ice of Antarctica, scientists have found rich microbial ecosystems.

These discoveries have opened up the possibility that life could also survive in extreme environments on other worlds.

There are five icy moons in our solar system that could potentially host extraterrestrial life.

EUROPA

This icy moon of Jupiter is thought to harbor a liquid-water ocean more than twice the volume of all Earth's oceans. The subsurface sea, which lies underneath a thick layer of subsurface ice, likely remains a liquid because of tidal heating, which (similar to tides on Earth) comes from the gravitational pull of Jupiter.

Other geologic activity in the moon's rocky core could create an additional heat source for life-forms. The ice layer on Europa is more likely at least 10 to 15 miles (15 to 25 km) thick, so getting a look at those life-forms would be extremely challenging. There may be, however, isolated lakes at shallower depths.

NASA has green-lit a mission to orbit Europa and learn more about this potentially habitable world.

TITAN

Saturn's moon Titan might appear hospitable at first glance, because it is covered in rivers, lakes and oceans. Unfortunately, all of them are flowing with liquid ethane and methane, and all known life-forms need water to survive. In addition, the surface temperature on Titan is about minus 292 degrees Fahrenheit (minus 180 Celsikus) — far too cold for life as we know it.

But the active chemistry on Titan has led some scientists to hypothesize about how life could arise there. Living organisms create most of Earth's methane supply, but it's unclear where Tian's methane comes from. The source could be an underground ocean, where temperatures might be warmer.

ENCELADUS

The bluish-gray surface of Enceladus looks too frigid to host life, but under its surface lies a vast ocean. Just like on Europa, it's possible the underground ocean contains a suitable environment for life. Over 100 geysers on the moon's surface vent material from that ocean up and away from the satellite. Analysis of the plumes by the Cassini probe revealed water, ammonia, salts and organics (molecules that contain carbon, the building block for life on Earth).

A proposed mission to Enceladus would send a probe to collect samples from those plumes and analyze them in situ.

GANYMEDE & CALLISTO

Like their siblings, Europa and Enceladus, the Jovian moons Ganymede and Callisto may have subsurface, liquid oceans. But in these cases, the underground seas would be buried under at least 60 miles (100 km) of rock.

These moons are less likely to support life than icy worlds like Enceladus, according to NASA. But the European Space Agency is planning a mission to study the buried oceans of the Jupiter system, with particular emphasis on Ganymede. 

When NASA launched astronaut John Glenn into orbit on Feb. 20, 1962, the U.S. joined the realm of orbital spaceflight and never looked back. Seven years later, the first Americans would land on the moon. See how NASA made the leap into orbital spaceflight with Glenn's historic Friendship 7 spaceflight in the SPACE.com inforgrahic above.

 John Glenn died on Dec. 8, 2016 at age 95. Read our full obituary here.

• In Photos: John Glenn, First American in Orbit
• Astronaut John Glenn: An American Hero's Greatest Moments Remembered
• Video: This is Friendship 7: America's First Orbital Spaceflight at 50
• Project Mercury: Photos of NASA's 1st Manned Spaceflights
• Giant Leaps: Top Milestones of Human Spaceflight
• America's First Spaceship: Project Mercury (Infographic)

On Sept. 15, 2016, China launched its uncrewed Tiangong-2 space lab to Earth orbit. Learn all about the spacecraft, which China views as a key step toward building a bona fide space station, in this infographic.

Other Tiangong-2 resources:

• China Launches Tiangong-2 Space Lab to Prep for 2020s Space Station
• China Launches Shenzhou-11 Astronauts to Tiangong-2 Space Lab
• Chinese Astronauts Dock with Tiangong-2 Space Lab
• Gallery: Tiangong-1, China's First Space Laboratory
• Shenzhou-10 Explained: Chinese Astronauts Head to Space Lab (Infographic)
• How China's First Space Station Will Work (Infographic)

Follow us @Spacedotcom, Facebook or Google+. Originally published on Space.com.

In August 2016, astronomers announced that a potentially Earth-like planet orbits Proxima Centauri, the closest star to the sun. Learn all about this possibly habitable world, known as Proxima b, in this infographic.

Other Proxima b resources:

• New Planet 2016: Complete Coverage of Proxima b's Discovery
• Proxima b: Closest Earth-Like Planet Discovery in Pictures
• Alien World 'Proxima b' Around Nearest Star Could Be Earth-Like | Video
• Proxima Centauri's Alien Planet Closer Than You Think - With Right Spacecraft | Video
• Found! Potentially Earth-Like Planet at Proxima Centauri Is Closest Ever
• Aliens Next Door: Does Proxima b Host Life?
• How We Could Visit the Possibly Earth-Like Planet Proxima b
• Proxima b By the Numbers: Possibly Earth-Like World at the Next Star Over
• Why Did It Take So Long to Find Proxima b?
• What's It Like On Our Neighbor, Proxima b?

Follow us @Spacedotcom, Facebook or Google+. Originally published on Space.com.

On May 9, 2016, Mercury will pass directly between the Earth and sun in a rare solar transit. Mercury transits occur just 13 times every 100 years. 

The Mercury transit of 2016 will begin at 7:16 a.m. EDT (1116 GMT) and last more than seven hours, ending at 2:38 p.m. EDT (1838 GMT). 

Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.

After landing on Dec. 25, 2003, the ESA's Beagle 2 spacecraft didn't phone home. Its fate remained a mystery until Jan. 16, 2015, when the ESA announced the probe had been found in photos taken by NASA's Mars Reconnaissance Orbiter.

Full Story: UK's Lost Beagle 2 Mars Lander, Missing Since 2003, Found in NASA Photos

Beagle 2 landed in Isidis Planitia, a basin near the equator of Mars. Photos taken from orbit show that only two or three of the lander's four solar panels appear to have deployed. The tiny 3-foot (1 meter) spacecraft's post-landing software began executing but it is not known why it stopped. Because of the partial deployment of the robot's mechanisms, its radio transmitter was blocked from contacting Earth.

UK's Beagle 2 Mars Lander Mission in Photos

On April 26, 2016, scientists with the University College London's Mullard Space Science Laboratory revealed that new images from the Mars Reconnaissance Orbiter, which they billed as the sharpest views of Mars ever, revealed even better views of Beagle 2 on the surface of the Red Planet. 

Supersharp Mars Photos Show UK's Long-Lost Beagle 2 Lander

• Mars: The Spacecraft Graveyard
• Mars Explored: Landers and Rovers Since 1971 (Infographic)
• The Best (And Worst) Mars Landings in History

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Billionaire investor Paul Allen has unveiled a new company, Stratolauch Systems, to provide private rocket launches into Earth orbit from a giant aerial launch platform made up of the biggest airplane in history. The novel launch system will use a giant twin-boom airplane with a wingspan longer than a football field and launch a rocket and space capsule designed by the private spaceflight company SpaceX. See how the Stratolaunch system stacks up in the SPACE.com infographic above. 

• Stratolaunch Images: Paul Allen's Giant Rocket-Launching Plane
• Billionaire Paul Allen's New Space Project Has Deep Roots
• Video: Operation: Stratolaunch - Dream Team Announces New to Space Venture

The Breakthrough Starshot initiative intends to develop, within a generation, the capability to launch tiny laser-propelled space probes to the nearest star.

Stephen Hawking Helps Launch Project 'Starshot' for Interstellar Space Exploration

The ongoing miniaturization of electronics has made it feasible to create a chip weighing about the same as a paper clip (1 gram, or 0.035 ounces) that could include the communications lasers, cameras, nuclear battery, computer and other devices needed for an interstellar space probe. The "Starchip" would be about the size of a postage stamp.

Breakthrough Starshot in Pictures: Laser Sail Nanocraft to See Alpha Centauri

Many Starchips would be launched at a time, propelled by a 100-gigawatt laser blast from a ground-based light-beamer array. This is about the same amount of energy required to launch a space shuttle. The probes would be accelerated to 20 percent of the speed of light in about two minutes (an acceleration of 60,000 times that of the Earth's gravity). This velocity would get the probe past the orbit of Pluto in three days and to the nearest star in 20 years.

Stephen Hawking: 'Transcending Our Limits' With Breakthrough Starshot | Video

Many solutions must be found before Starchips could be launched to other solar systems. Present challenges include:

• Building and cooling a ground-based light-beamer array
• Overcoming atmospheric interference on the laser beams as they exit Earth's atmosphere
• Precise aiming of the probes at an exoplanet
• Integrity and stability of the sail under thrust
• Fast travel through the interstellar medium (dust, gas, cosmic rays)
• Maintaining fuctionality over decades in space
• Precision aiming of cameras at target
• Precision aiming of transmitter at Earth
• Transmitting images using a laser as a transmitter and the sail as an antenna
• Power generation and storage
• Policy issues

How Interstellar Space Travel Works (Infographic)

Gallery: Visions of Interstellar Starship Travel

Star Trek's Warp Drive: Are We There Yet? | Video

The Top 10 Star Trek Technologies

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The Japan Aerospace Exploration Agency's Hitomi X-ray observatory, also called ASTRO-H, launched into on Feb. 17, 2016 on a mission to study black holes, star explosions and other objects in deep space.

Japanese Astronomy Satellite Hitomi Malfunctions, Generates Debris

The $273 million Hitomi mission suffered a major blow on March 26, 2016, when Hitomi stopped communicating with its operations center. 

Ground tracking shows Hitomi broke into several sections before March 26. Radio contact with the probe was lost on that date.

'Hitomi' X-Ray Astronomy Satellite Launched By JAXA | Video

Hitomi was intended to occupy Earth orbit at an altitude of 357 miles (575 kilometers), inclined 31 degrees to the equator. It completes one orbit every 96 minutes, maintaining its attitude toward the target for continuous observations lasting up to several days. Hitomi's four telescopes and two gamma-ray detectors all face the same direction, and were designed for simultaneous observations.

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Bigelow Aerospace is designing a plug-in module to expand living space on the International Space Station. Larger expandable modules could someday become free-flying space stations themselves.

The Bigelow Expandable Activity Module (BEAM) will be carried into orbit by SpaceX’s Falcon 9 rocket, stowed in the cargo trunk of a Dragon capsule. A robot arm will dock BEAM to Node 3 of the International Space Station.

BEAM is 13 feet long (4 meters) and 10.5 feet in diameter (3.2 m). The module weighs 3,000 pounds (1,360 kilograms)

A larger inflatable module called BA 330 is being developed for use as a stand-alone space station. Larger than the International Space Station's existing Destiny habitation module, each BA 330 can house up to six astronauts. Bigelow plans a two-module outpost called Alpha Station which could be orbited after 2016.

The BA 330’s internal volume is 11,654 cubic feet (330 cubic meters). The length is 45 feet (13.7 m) and its diameter is 22 feet (6.7 m)

Further in the future, inflatable modules could enhance the living volumes of deep-space stations, lunar bases or even Mars expeditions.

The inflatable space station concept dates to the 1960s. Kevlar, the material used for bullet-proof vests, inspired NASA to take another look at inflatable space modules in the 1990s. NASA’s module, called Trans-Hab, never flew and was officially canceled in 2000.

FollowSPACE.com on Twitter @Spacedotcom. We're also on Facebook & Google+. 

Photos: Bigelow's Inflatable Space Station Idea

Gallery: Private Space Stations of the Future Imagined

Robert Bigelow: Lessons, Visions, Realities (Video)

For a full description, read: The Moon's Phases: Lunar Cycle Completely Explained

See how moon phases work, from the full moon to the new moon and everything in between in this Space.com infographic. The moon is illuminated by light from the sun, which observers on Earth see reflected off the lunar surface. As the moon moves around Earth, the amount of illumination it receives from the sun changes, creating the lunar phases. The moon completes a single lunar cycle in about one month.

• Moon Master: An Easy Quiz for Lunatics
• How the 'Supermoon' Looks (Infographic)
• The Moon: 10 Surprising Facts

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p

Exobiology on Mars (ExoMars) is an ambitious mission being undertaken by the European Space Agency and its international partners. In 2016, the Trace Gas Orbiter (TGO) and Schiaparelli lander will be launched on a Russian Proton rocket. In 2018, the ExoMars rover will be launched.

TGO's mission is to sniff Mars' atmosphere for evidence of methane, a gas with implications for the existence of life on the Red Planet. Schiaparelli's main purpose is to demonstrate Mars-landing technology. It is expected to survive only a few days on the surface, running off of its internal batteries. The little lander is 5.4 feet (1.65 m) in diameter and weighs just 1,323 lbs. (600 kg).

Set to be launced on a Russian Proton rocket in 2018, the golf-cart-size ExoMars rover will spend six months searching for signs of present or extict life on Mars.

The rover's onboard biology laboratory looks for molecules indicative of life, while its 6.6-foot (2.2 m) surface drill brings up samples for analysis. A tall mast carries the panoramic camera system (PanCam) with twin lenses for stereoscopic imaging. Ground-penetrating radar looks for ice under the surface.

The British Skylon single-stage-to-orbit space plane would take off from a runway and fly on air-breathing hydrogen-fueled rocket engines for much of its ascent through the atmosphere. When the air becomes too thin, Skylon switches to onboard liquid oxygen. 

Watch Our Skylon Space Plane Video Show

Skylon's payload bay carries both passengers and cargo, although the craft itself is controlled remotely and has no onboard pilots.

Crew: None (remotely controlled from the ground)

Passengers: None (up to 30 in optional passenger module)

Payload: 33,000 lbs. (15,000 kilograms)

Length: 273 feet (83 meters)

Wingspan: 88 feet (26.8 m)

Loaded weight: 717,000 lbs. (325,000 kg)

Maximum speed: air-breathing Mach 5.14, rocket Mach 27.8

Orbital altitude: 373 miles (600 kilometers)

The Skylon Space Plane in Pictures

The Skylon Personnel / Logistics Module (SPLM) could be installed in Skylon's cargo bay for carrying a combination of passengers and supplies to orbital stations. If carrying passengers only, it could support up to 30 people.

Lynx is XCOR’s planned suborbital passenger space plane. The craft is designed to use conventional kerosene fuel, take off and land on a standard airport runway and make up to four flights per day.

Lynx carries a pilot and one passenger to an altitude of about 62 miles (100 kilometers), where they experience about 5 minutes of weightlessness.

The Dark Ages Radio Explorer (DARE) is a proposed moon-orbiting radio probe that would seek to answer many questions about the early universe:  When did the first stars light up? What kind of stars were they? When were the first black holes created? How massive were these black holes? How did the first galaxies start to form? When did the universe transform from opaque to transparent?

As the universe cooled and expanded, particles were able to combine to form neutral atoms. Space became filled with dense, neutral hydrogen gas, through which light could not pass.

As the first stars ignited, their energy reionized the hydrogen gas. Bubbles of transparency began to form in the opaque universe, and radiation began to travel freely.

DARE's instruments would study the frequency of radio waves emitted by hydrogen atoms in the early universe. The spacecraft's radial antennas span 24.6 feet (7.5 meters) when fully extended. The probe weighs 2,640 lbs. (1,198 kilograms) when fully fueled.

DARE's 2-hour science orbit would make use of the moon to shield the probe from the heat of the sun and the radio noise from the Earth. Around the time of a full moon, DARE is doubly shielded and can make optimal observations.

On Jan. 20, 2016, scientists announced the discovery of what could be a giant planet in the Kuiper Belt at the outer edge of our solar system. Nicknamed "Planet Nine," the world about 10 times the mass of the Earth and would take between 10,000 and 20,000 years to orbit the sun. Such mini-Neptunes may actually outnumber most other planets in our galaxy, scientists say.

Discovery Story: 'Planet Nine' May Exist: New Evidence for Another World in Our Solar System

The Evidence for 'Planet Nine' in Our Solar System (Gallery)

Planet X May Be Real - Evidence Mounting For 9th Planet | Video

A New Giant Planet In Our Solar System? Evidence Mounts | Video

NASA's space shuttle Challenger accident was a devastating tragedy that killed seven astronauts and shocked the world on Jan. 28, 1986. Killed in the accident were Challenger commander Dick Scobee, pilot Michael Smith, mission specialists Judy Resnik, Ronald McNair and Ellison Onizuka, payload specialist Gregory Jarvis and Christa McAuliffe, who was set to become the first teacher in space.

Remembering Challenger: NASA's 1st Shuttle Tragedy (Photos)

Here's a look at how the Challenger accident occurred:

An inspection of the launch pad revealed large quantities of ice collecting due to unusually cold overnight Florida temperatures. NASA had no experience launching the shuttle in temperatures as cold as on the morning of Jan. 28, 1986. The coldest temperature of a previous launch was 20 degrees warmer.

Columbia & Challenger - Astronaut Jerry Ross Remembers | Video

Morton Thiokol, the builder of the solid-rocket boosters, advised NASA that they believed the O-ring seals in the solid-rocket boosters would perform adequately in the cold.

To make each solid-rocket booster, the Morton Thiokol factory built four hull segments filled with powdered aluminum (fuel) and ammonium perchlorate (oxidizer).

At the launch site, the fuel segments were assembled vertically. Field joints containing rubber O-ring seals were installed between each fuel segment.

The O-rings were never tested in extreme cold. On the morning of the launch, the cold rubber became stiff, failing to fully seal the joint.

Space Travel: Danger at Every Phase (Infographic)

As the shuttle ascended, one of the seals on a booster rocket opened enough to allow a plume of exhaust to leak out. Hot gases bathed the hull of the cold external tank full of liquid oxygen and hydrogen until the tank ruptured.

At 73 seconds after liftoff, at an altitude of 9 miles (14.5 kilo- meters), the shuttle was torn apart by aerodynamic forces.

 The two solid-rocket boosters continued flying until the NASA range safety officer destroyed them by remote control.

The crew compartment ascended to an altitude of 12.3 miles (19.8 km) before free-falling into the Atlantic Ocean.

NASA Remembers Challenger | Video

Columbia Space Shuttle Disaster Explained (Infographic)

'Challenger Disaster: Lost Tapes' Includes McAuliffe Lesson Plan Rehearsal | Nat Geo Video

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On Jan. 20, 2016, scientists announced the discovery of what could be a giant planet in the Kuiper Belt at the outer edge of our solar system. Nicknamed "Planet Nine," the world is a behemoth - about 10 times the mass of the Earth. It would take the planet between 10,000 and 20,000 years to orbit the sun.

The Evidence for 'Planet Nine' in Our Solar System (Gallery)

Evidence for Planet Nine was unveiled by researchers Mike Brown and Konstantin Batygin at the California Institute of Technology in Pasadena, California. The scientists used mathematical modeling and computer simulations to make the find, but have not actually observed the planet directly.

Planet X May Be Real - Evidence Mounting For 9th Planet | Video

The planet's existence is inferred by the orbits of six other, smaller objects in the Kuiper Belt. The orbits of those smaller objects lined up in a way that suggested the gravitational influence of another much more massive object, which Brown and Batygin dubbed Planet Nine.

A New Giant Planet In Our Solar System? Evidence Mounts | Video

NOVA: A white dwarf star pulls matter off of a companion red giant star until a powerful nuclear fusion explosion occurs on the dwarf’s surface. The star is not destroyed and additional explosions can occur, a phenomenon called a recurrent nova.

SUPERNOVA: Much more brilliant than a nova, a supernova can shine brighter than an entire galaxy for a brief time.

Astronomers identify two major types of supernovas: 

Type I Supernova: A white dwarf star pulls matter from a companion star until the dwarf’s dead core re-ignites in a thermonuclear explosion that destroys the star. This is similar to a nova but the explosion is much more powerful. A Type I supernova has no hydrogen in its spectrum. 

Type II Supernova: A star several times more massive than the sun runs out of nuclear fuel and collapses under its own gravity until it explodes. A Type II supernova has hydrogen in its spectrum.

SUPERLUMINOUS SUPERNOVA (Hypernova): A burst 5 to 50 times more energetic than a supernova. A hypernova may or may not be associated with a powerful burst of gamma radiation.

Ridley Scott’s film of the Andy Weir novel “The Martian” boasts a wide variety of space hardware, both real and extrapolated from current NASA plans.

BUY "The Martian" on Blu-ray or DVD >>

Unrealistically large by NASA standards for a Mars vessel, the Hermes appears to be the size of the International Space Station. Hermes uses low-thrust ion engines to ferry Ares crews from Earth to Mars and back. When one crew returns home, the next crew boards Hermes to prepare for another mission.

Hermes has a rotating wheel, or centrifuge, to provide artificial gravity for the crew. Spaceships in “2001: A Space Odyssey” and “Mission to Mars” were similarly equipped.

Although it is not shown in the film, the astronauts would have used a Mars descent vehicle (MDV) to get down from Hermes to the surface. SpaceX’s upcoming Dragon manned capsule would be one option for an MDV.

In Andy Weir’s novel “The Martian,” the Ares III habitat is described as an inflated canvas structure. The habitat and its provisions were prepositioned on Mars by unmanned rocket launches.

To get back to Hermes in Mars orbit, the Mars ascent vehicle (MAV) is used. MAV resembles an Apollo or Orion capsule mounted on booster rockets and landing legs.

To get from Earth to the Hermes, crews ride the Orion space capsule. By 2035, the year in which the film is set, NASA’s new Space Launch System should be available. The film’s producers used footage of Orion’s first unmanned test flight in 2014 to represent the launch. The test flight used a Delta IV Heavy booster.

NASA launches supplies to Watney on Mars using an Atlas V rocket.

From planetary meet-ups to eclispes of the moon and the sun, the year 2016 has a lot to offer for skywatchers. Here's a look a the year's most promising events to help you plan. 

For the latest on what's up in the night sky, visit our Skywatching section here. 

You can also see the current month's night sky events here: 
Night Sky: Visible Planets, Moon Phases & Events

Announced to the pubic in August 1965, the Manned Orbiting Laboratory (MOL) was described as a technology-development platform that would aid manned and unmanned space efforts. The project also had a top-secret mission: MOL was to be the first U.S. manned spy satellite. The program was terminated in 1969, before any operational flights had occurred.

MOL was to be 71.9 feet (21.9 meters) from the nose of its Gemini-B command module to the tail of its spy satellite cargo. The craft was to be 10 feet (3.0 m) in diameter and would have weighed 31,910 lbs. (14,470 kilograms).

The rear section of MOL would have housed a Keyhole-10 spy satellite code-named “Dorian.”  This space telescope was designed to be pointed down at Earth, not out toward the stars. Mission planners hoped to use MOL’s 72-inch (1.8 m) optical mirror to capture high-resolution images of the Soviet Union from a polar orbit of Earth.

A planet close to its parent star is hot like Venus, but rotates quickly and could have a substantial atmosphere. 

FULL STORY: Closest Earth-size Alien Planet Found, May Be a Venus Twin

Called GJ 1132b, the exoplanet is located 39 light-years from Earth and is the closest Earth-size planet of its kind, scientists say. In fact, the planet may be a potential twin of Venus. 

GJ 1132b is about 9,200 miles (14,800 km) wide and has a mass that is abotu 1.6 times that of the Earth. It circles its planet superfast, completing a single "year" in about 1.6 Earth days. Its orbit brings it closer to its parent star than Mercury is to our sun.

VIDEO: Alien Planet GJ 1132b Explained

The average temperature of GJ 1132b is about 440 degrees Fahrenheit, or about 227 degrees Celsius. The exoplanet is tidally locked, so that one side is always facing its parent star. 

The Ares III mission was supposed to last 31 sols (a sol is a 24.5-hour Martian day). Just to be safe, NASA sent 68 sols’ worth of food, for six people. For Watney alone, that will last 300 sols, extended to 400 if he rations.

Watney needs 1,500 calories every day. He has 400 sols’ supply of food already. So how many calories does Watney need to generate per day during the entire time period to stay alive for 1,400 days total?

Fortunately, Watney has a supply of potatoes. He can use his own feces as fertilizer on the sterile Martian soil, to grow the vegetables.

Soil needs 0.3 gallons of water per cubic foot (40 liters per cubic meter). Watney needs a total of 163.26 gallons (618 liters) of water. The Hab has only 79.25 gallons (300 liters) available.

Water can be manufactured by burning hydrogen and oxygen.

The ambitious Cassini orbiter and Huygens lander mission achieved the first landing of a probe on another planet’s moon. For more than 11 years, Cassini has orbited the ringed planet, taking photos and making observations that have greatly expanded our understanding of the Saturnian system. 

As of Oct. 28, 2015, Cassini has completed 220 orbits of Saturn. Cassini’s path is often adjusted to allow the probe to make close flybys of Saturn’s satellites. Since 2004, Cassini has made 147 of these so-called “targeted” flybys, including 114 of Titan. Cassini has also made 604 “untargeted” flybys, or flybys that do not require a rocket engine burn.

In 2005, the Huygens lander revealed the surface of Titan is a wasteland of water ice and frozen hydrocarbons (photo, right). Despite Titan’s lack of liquid water, some scientists believe the moon may support life, now or in the distant future when the sun’s heat increases.

Cassini photos revealed massive plumes of water ice spewing from cracks in the surface of Saturn’s moon Enceladus. Recently, researchers discovered that Enceladus has a giant world-circling subsurface ocean beneath its icy crust. The presence of massive quantities of water makes Enceladus a prime target in the search for other life in the solar system.

Far from being a dead boulder field in the sky of Saturn, Cassini has discovered that the ring system is a chaotic, ever-changing system and a laboratory for studying how small bodies collide to form larger moons.

Despite an average surface temperature of minus 290 degrees Fahrenheit (minus 179 degrees Celsius), Saturn’s moon Titan turns out to have quite a bit in common with Earth. Both worlds have rain, rivers, lakes and seas – of liquid methane, not water.

The action of flowing liquid carves the surface of Titan, allowing the formation of bodies of standing liquid. This makes Earth and TItan two of only a very few such worlds in the solar system.

Titan’s smoggy atmosphere is filled with some of the most chemically complex organic molecules in the solar system.

For more than 300 years, astronomers have known that Iapetus has a strange surface, with one hemisphere jet black and the other bright white. Cassini’s observations proved that Iapetus sweeps up dark, reddish dust in its path as it orbits Saturn. The darkened hemisphere absorbs solar energy and becomes warmer, while the snow-white, trailing hemisphere remains cooler.

One of Saturn’s remaining mysteries is a well-defined, hexagonal jet stream at the north pole of the planet. In the remaining years of Cassini’s mission, scientists hope to learn more about the hexagon’s properties.

Stars come in different types, and most stars will change types throughout their lifetimes. Stars are often organized using what's known as the Hertzsprung-Russell (H-R) diagram.  

Stars like Earth’s sun that burn hydrogen into helium are in a group called the "main-sequence."

The sun is currently a type G, yellow, main-sequence star. In the future, the sun will run out of hydrogen, leave the main-sequence, and become a red giant. After it explodes into a nova, the sun will become a tiny white-dwarf star, surrounded by a planetary nebula.

The search for extraterrestrial intelligence (SETI) got a boost in July 2015, when investor Yuri Milner and physicist Stephen Hawking (left) announced a new $100 million SETI initiative called Breakthrough Listen.

The first serious, scientific attempt to listen for alien radio signals was Project Ozma in 1960, conducted by astronomer Frank Drake.

Since the invention of radio in 1900, researchers have occasionally detected unexplained signals that have led them to wonder about the possibility of life on other planets. In the 1960s, robotic probes revealed that the other planets of this solar system are not compatible with advanced civilizations. The many other planets and moons in the Milky Way galaxy and in billions of other galaxies in the universe still provide hope for the discovery of alien civilizations.

Popular culture often depicts unidentified flying objects (UFOs) as alien spacecraft and bizarre ancient artifacts as evidence that alien beings visited the Earth in the distant past. Scientists, however, have never found evidence that either of these things have ever occurred and do not take alien UFOs and ancient astronauts seriously.

Astronomer Frank Drake created an equation to estimate the number of intelligent, communicating civilizations currently living in the galaxy.

Detecting tiny planets amid a star's glare, from many light-years across the galaxy, is very challenging. Direct imaging, or resolving planets as pixels in a photograph, is only possible today for large, Jupiter-class and above, planets. 

Scientists define time as one of the four dimensions of "space-time," a term used to describe the totality of the universe. 

Related: Time Travel Theories

The fictional Ares 3 mission in Andy Weir's novel "The Martian" is based on an actual NASA plan for exploration of the Red Planet. When he is stranded after his astronaut team leaves without him, Mark Watney must struggle to survive. Matt Damon stars as Watney in Ridley Scott’s film of "The Martian."

'The Martian' Movie and NASA: Full Coverage

If you were left on Mars without a spacesuit, you would immediately freeze and choke, then die. The air on Mars is toxic: It’s more than 95 percent carbon dioxide with just a trace of oxygen (0.13 percent). Air pressure on Mars is equivalent to that at about 21 miles altitude (38 kilometers) on Earth. It's cold: planetwide, about minus 67 degrees Fahrenheit (minus 55 degrees Celsius), although it may get up to almost room temperature at the equator, on the hottest summer day. 

The Ares mission strategy is to land an empty habitation module (Hab) with supplies and a Mars Ascent Vehicle (MAV) first, before sending humans. Automatic equipment chemically breaks down Martian air into breathing oxygen and fuel for the return trip. Only when the tanks are full does the first expedition leave Earth. This is called in-situ resource utilization, or "living off the land."

The Hab module in the film carries only enough supplies to support Watney for about 300 days, but it will be years before a rescue from Earth is possible. His equipment can make breathable air from the local atmosphere, but food is a problem. Watney, a botanist, figures out how to grow crops on Mars to keep himself alive.

Starting in 2014, real-life astronauts on the International Space Station used the "Veggie" plant growth system to grow edible greens in space.

A proof-of-concept experiment called MOXIE (Mars Oxygen In-Situ Resources Utilization Experiment) will ride aboard NASA's Mars 2020 rover (inset, below). Carbon-dioxide-rich Martian air is flowed over a sandwich of anode and cathode plates. A process of solid oxide electrolysis splits the air into oxygen and waste carbon monoxide gas, which can be dumped back into the atmosphere.

Some of the Ares mission's equipment is powered by a radioisotope thermoelectric generator (RTG). This nuclear generator releases 100 watts of power by means of the heat generated by the radioactive decay of plutonium. Similar generators were used by the New Horizons Pluto probe and on the moon by Apollo astronauts.

The Ares 3 habitat is located on Mars' Acidalia Planitia, a vast plain in the Martian northern hemisphere. Although author Weir describes the plain as flat and easily traversed, satellite photos taken recently by NASA have revealed terrain on the real Acidalia Planitia to be rugged and hard to navigate.

Watney's Ares 3 camp is about 500 miles (800 km) north of the robotic Mars Pathfinder lander that landed in 1997. The one vehicle that is capable of taking Watney off the planet is at the Ares 4 site, 2,000 miles (3,200 km) away in the crater Schiaparelli. Once in orbit, Watney would still be stranded because the Hermes, the Ares 3 mission's mother ship, had already departed.

Watney's Ares 3 base is equipped with two pressurized, electric-powered rovers. Actual NASA plans include a very similar rover, the 14.7-foot-long (4.5 meters) Space Exploration Vehicle (SEV).  

The rover's pressurized cabin can hold up to four astronauts in shirtsleeves. A side hatch allows the SEV to dock to another rover or to a habitat module. Two "suit port" hatches allow two astronauts to slide into their spacesuits from the rear.

The wheeled chassis can be used by itself as an unpressurized, stand-up roving vehicle. 

The Mars program depicted in the film and in the book features a reusable, International-Space-Station-size mother ship, the Hermes. 

In the film, the Hermes is an ion-drive interplanetary spacecraft powered by a nuclear reactor. An external carousel spins to produce artificial gravity for the crew. Solar panels turn sunlight into electricity to run shipboard systems.

A NASA design for a Mars ship uses a bimodal nuclear thermal rocket. "Bimodal" means that the nuclear engine is used for both propulsion and electric power generation. The crew resides in a hab module at the front of the spacecraft.  The entire ship is rotated end over end to provide artificial gravity.

The proper alignment for an energy-efficient flight between Earth and Mars occurs every 2.13 years. 

Six astronauts are launched from Earth in an Orion crew vehicle. Their capsule intercepts Hermes in "parking orbit" around Earth.

Hermes' ion-drive engines use electricity to propel argon atoms out of the back of the vehicle to create forward thrust. The acceleration is tiny, but the engines fire continuously all the way to Mars, a trip of 124 days.

After arriving in Mars orbit, the crew transfers to a Mars Descent Vehicle (MDV). The crew lands near a Hab module containing supplies prepositioned by previous unmanned missions. Two surface exploration vehicles (SEVs, or rovers) are available for wide-ranging exploration of the Martian surface.

The crew can stay on Mars either 30 days or 500 days, depending on the mission plan. The Ares 3 mission depicted in "The Martian" is a 30-day "short stay" mission.

When the alignment between Earth and Mars is again correct, the crew boards a Mars Ascent Vehicle (MAV) and blasts off. In orbit, the crew catches up to the Hermes and powers up its ion engines for the return to Earth.

When Hermes returns to Earth orbit, the crew disembarks. Another crew boards the Hermes to prepare the ship for another trip to Mars.

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Discovery Story: Salty Water Flows on Mars Today, Boosting Odds for Life

The announcement that liquid, salty water has been found flowing on the surface of Mars came as no real surprise to astronomers. Mars is full of water, mostly in the form of ice at the poles and under the surface. Also, Martian air is surprisingly humid: up to 100 percent humidity on cold winter nights.

The clearest evidence for liquid water on Mars comes from dark streaks known as recurring slope linnea (RSL). These streaks form in the spring and disappear later in the Martian year.

Flowing Water on Mars: The Discovery in Pictures

Spectroscopic analysis showed that the streaks are hydrated perchlorate, a briny liquid of perchlorate salt with water trapped in its crystals. 

The salt causes water to remain liquid at much lower temperatures than on Earth. Perchlorate brine can stay liquid down to minus 94 degrees Fahrenheit (minus 70 degrees Celsius). The brine boils at 75 degrees F (24 degrees C), close to the highest summertime temperature on the surface of Mars.

Scientists estimate that 4.3 billion years ago, Mars had a huge ocean covering its entire northern hemisphere. This ocean would have contained more water than Earth’s Arctic Ocean.

The Search for Life on Mars (A Photo Timeline)

Indications are that some of Mars' water is still there, frozen beneath the surface. A giant slab of ice as big as California and Texas combined, and as deep as a 13-story building, lies beneath the surface of Mars between the planet's equator and north pole, researchers say.

The Mars rover Curiosity weather station shows that the thin Martian air is surprisingly humid. Curiosity's measurements range from about 5 percent humidity on summer afternoons to up to 100 percent (saturation point) on autumn and winter nights.

Water on Mars Could Help Put Astronaut Boots on Red Planet

Mars Gets More Habitable with Water Discovery, Scientists Say

7 Biggest Mysteries of Mars

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Editor's note: To find out more about the rare supermoon lunar eclipse of Sept. 27-28 and how to see it, visit: Supermoon Lunar Eclipse 2015: Full 'Blood Moon' Coverage. Such an event won't happen again until 2033.

The word "eclipse" means to obscure. When the moon passes between the Earth and the sun, it's called a solar eclipse. When the Earth passes between the sun and the moon, it is a lunar eclipse.

The Earth casts its shadow far out into space, beyond the orbit of the moon. Once in a while the moon passes through the shadow, and an eclipse occurs. [Amazing Total Lunar Eclipse Photos]

Four Blood Moons: Total Lunar Eclipse Series Not a Sign of Apocalypse

Lunar eclipses occur in pairs with solar eclipses, two weeks apart. This is because the Earth, sun and moon must be aligned for an eclipse to occur, and the alignment can only happen twice during the moon's month-long orbit.

A lunar eclipse lasts for hours as the moon slowly orbits through the Earth's shadow.

If the moon passes through the edge of Earth's shadow, it's called a partial eclipse. Passage through the dense center of the shadow is a total lunar eclipse.

Four Blood Moons: A Tetrad Of Lunar Eclipses Explained | Video

A sequence of four total lunar eclipses including no partial eclipses is called a tetrad. In 2014-2015, four lunar eclipses will occur. This sequence features eclipses on April 15, 2014; Oct. 8, 2014; April 4, 2015 and Sept. 27, 2015.

Earth's shadow is red at the edges for the same reason a sunset is red: When sunlight is scattered by passing through Earth's atmosphere, the other colors of the spectrum are removed.

In a solar eclipse, the moon casts its shadow on Earth. The darkest part of the moon's shadow – the umbra — is no more than 166 miles wide (267 kilometers) on the surface of the Earth.

Because the orbit of the moon is tilted relative to the orbit of Earth, eclipses can't happen at every new and full moon. The shadow cones pass “above” or “below” the moon and Earth most of the time. Only at two points during the year do the sun, Earth and moon line up properly to allow for eclipses.

A lunar eclipse is visible to anyone who can see the moon, which usually means half of the Earth at a time. In a total solar eclipse, the moon's shadow passes directly over only a narrow strip on the Earth's surface. Observers in a larger area can see the sun partially eclipsed.

• The Moon: 10 Surprising Lunar Facts
• Moon Master: An Easy Quiz for Lunatics
• Blue Moon Photos: August Full Moon of 2013 in Pictures

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Editor's note: To find out more about the rare supermoon lunar eclipse of Sept. 27-28 and how to see it, visit: Supermoon Lunar Eclipse 2015: Full 'Blood Moon' Coverage. Such an event won't happen again until 2033.

A supermoon is a spectacular sight in which the full moon of a given month occurs at the same time the moon is at perigee – the point in its orbit that brings it closest to Earth. During these events, the moon can appear up to 30 percent brighter and 14 percent bigger to skywatchers on Earth. See how the supermoon, or "perigee moon," works in the SPACE.com infographic.

• 'Supermoon' Science: Why Saturday's Full Moon is Biggest of 2012
• 5 Moon Mysteries to Ponder During Saturday's Supermoon
• Amazing Supermoon Photos from 2011
• Moon Master: An Easy Quiz for Lunatics
• Saturday's 'Supermoon' Won't Destroy Earth
• Why the 'Supermoon' Will Look Largest Near the Horizon

Sungrazing comets are comets that pass very close to the sun, sometimes to within a few thousand miles of its surface.

The Solar and Heliospheric Observatory (SOHO) is a joint NASA/ESA mission to probe the corona, or outer layer of the sun's atmosphere. The spacecraft became operational in May 2006. Owing to its constant study of the sun, SOHO is the leader in spotting sungrazing comets, having recorded around 3,000 as of Sepp. 13, 2015.

Comets are called sungrazers when they pass within 850,000 miles (1.38 million km) of the sun’s surface.

Small sungrazers can be shattered or totally evaporated by the sun’s heat and tidal forces. Other sun-

grazers can survive many close passes. 

Eighty percent of comets captured by SOHO's instruments travel along the Kreutz path, a single orbit that takes 800 years to complete. These Kreutz comets are fragments from a single large comet that was shattered thousands of years ago. The far end of the Kreutz path lies 160 times farther from the sun than the orbit of Earth.

New Shepard, named after Mercury astronaut and Apollo moonwalker Alan Shepard, is Jeff Bezos’ scheme for high-altitude, near-space tourism. A propulsion module (rocket) lobs the crew to an altitude of 307,000 feet (93,573 meters) – 

well above the height required to earn NASA astronaut wings. The rocket returns to its launch site and lands, while the crew capsule descends on a parachute.

The six-person crew capsule has an interior volume of 530 cubic feet (15 cubic meters). The capsule has six big observation windows that the company boasts are the largest-ever windows on a spacecraft.

There are three types of lunar eclipses — total, partial and penumbral — with the most dramatic being a total lunar eclipse, in which Earth's shadow completely covers the moon.

Here's how it works: Earth casts two shadows that fall on the moon during a lunar eclipse: The umbra is a full, dark shadow. The penumbra is a partial outer shadow. The moon passes through these shadows in stages. When the moon is in the penumbral shadow, the eclipse is not so noticeable. But when the moon is in the umbral shadow, the apparent change in color is far more dramatic.

Editor's note: If you capture an amazing lunar eclipse photo and would like to share it with Space.com for a story or gallery, send images and comments to managing editor Tariq Malik at spacephotos@space.com. 

• Lunar eclipse guide: When, where & how to see them 
• Eclipse glossary: Solar eclipses, lunar eclipses and their terms 
• Under a Blood Moon: A look at famous lunar eclipses in history 
• Longest lunar eclipse of the century dazzles skywatchers

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Solar eclipses are one of the cosmic wonders of our solar system. They occur when the new moon blocks part or all of the sun as seen from the surface of the Earth. Check out the SPACE.com Infographic above to see how solar eclipses work.

When the moon passes in front of sun, as viewed from Earth, the eclipse that occurs is visible from a narrow path on Earth that corresponds to the location of the moon's shadow. During a total solar eclipse, this path is known as the path of totality. WARNING: Never look directly at the sun during an eclipse with a telescope or your unaided eye. Severe eye damage can result and scientists use special filters to safely view the sun.

There are several other types of solar eclipses.

In addition to total eclipses of the sun, the moon can block part of the sun's disk (a partial solar eclipse), or leave only an outer ring of the sun visible in a so-called annular solar eclipse. A hybrid solar eclipse occurs when the tip of the moon's shadow lifts off the surface of the Earth at some point, allowing some observers to see a total eclipse while others witness an annular eclipse.

• Amazing Total Solar Eclipse Photos
• CAUTION! - How to SAFELY Observe the Sun (Video)
• Venus Transit of 2012: Complete Coverage

Astronomers have directly photographed a planet not unlike Jupiter orbiting 51 Eridani, a sunlike star 96 light-years from Earth.

Main Story: Found! 'Young Jupiter,' the Smallest Exoplanet Directly Seen by Telescope

The star is young, only 20 million years old (compared with the sun's 4.6 billion years). The planet, 51 Eridani b, still glows with the heat of its formation.

Jupiter-Like Exoplanet With Methane Atmosphere Found | Video

Scientists used the Gemini Planet Imager, an instrument on the Gemini South telescope in Chile, to make the discovery. The Gemini Planet Imager is an instrument designed specifically for directly imaging exoplanets around distant stars.

Related: 'Young Jupiter' 51 Eridani b: Why Directly Imaging an Exoplanet Is Big (Kavli Q+A)

More resources:

7 Ways Scientists Find Alien Planets

Alien Planet Quiz: Are You an Exoplanet Expert?

Planets Large and Small Populate Our Galaxy (Infographic)

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On Nov. 12, 2014, 317 million miles (510 million kilometers) from Earth and 14 miles (22.5  km) from comet 67p, the European Space Agency's Rosetta probe releases the Philae lander.

Full Story: Surprising Comet Discoveries by Rosetta's Philae Lander Unveiled

Video: Philae Lander Seen Post-Touchown By Rosetta? + New Descent Pics

Related: Will Europe's Philae Comet Lander Make Another Comeback?

A comet nucleus has very low gravity, so the lander relied on harpoons, hold-down thrusters and ice screws to secure itself to the surface. When these mechanisms all failed, the lander bounced back into space for a 1 hour and 50 minute ballistic flight. Due to Comet 67P's low gravity, Philae weighed about the same as a paper clip. On its first rebound, Philae ascended with a speed of 15 inches (38 centimeters) per second. Escape velocity from the comet is 19.7 inches (50 cm) per second. 

A grazing collision with rim of crater Hatmehit caused Philae to begin tumbling. The lander's second impact with the comet was followed by about 7 more minutes of drifting.

The closest view taken by Philae, just before its first impact, reveals a granular soil called regolith, littered with blocks up to 16.4 feet (5 meters) in size. Scientists estimate the regolith to be between zero and 6.4 feet (2 m) deep.

Announced on July 23, 2015, planet Kepler-452b is larger than Earth and orbits a star nearly identical to the sun, but older and therefore hotter. The planet's orbit is nearly the same as Earth's, and its year is about 20 days longer than Earth's. Kepler-452b is 1,400 light-years away.

NASA Finds Closest Earth Twin Yet in Haul of 500 Alien Planets

Between 15 and 25 percent of sunlike stars seem to have Earth-size planets. Smaller rocky planets like Earth (as opposed to gas-giant planets and other types) appear to be common in the Milky Way galaxy.

Exoplanet Kepler 452b: Closest Earth Twin in Pictures

The instruments on the Kepler Space Telescope search a wedge-shaped area of the Milky Way about 3,260 light-years long and oriented along the spiral arm in which our solar system is located. Another space probe, called Gaia, is tasked with precisely measuring the position, distance and motion of 1 billion stars in the Milky Way galaxy. Also to be measured will be the orbits and masses of 1,000 extrasolar planets. Gaia's first report is expected in 2016.

Earth-Size Planet Kepler-186f, a Possibly Habitable Alien World (Gallery)

Alien Planet Quiz: Are You an Exoplanet Expert?

Exoplanet Kepler-186f: Earth-Size World Could Support Oceans, Life (Infographic)

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After the end of the Vietnam War in 1975, the tense relationship between the United States and the Soviet Union was begining to improve. America’s moon landing program was over, and the Apollo spacecraft was due to be retired to make way for the space shuttle program. A joint U.S./Soviet space mission, first proposed in the 1960s, was now diplomatically possible.

The final Apollo spacecraft to fly was commanded by Tom Stafford, a space veteran who flew the lunar module on Apollo 10 as a rehearsal for the landing. Deke Slayton, an astronaut who had been grounded in 1962 for an abnormal heart rhythm, had served as chief astronaut on the Apollo program. At 51, he was the oldest person to fly in space up to that time. Rookie astronaut Vance Brand would go on to command three space shuttle flights. Soviet cosmonaut Alexei Leonov was world famous as humanity’s first spacewalker. His co-pilot, Valeri Kubasov, had flown before, on Soyuz 6.

After a successful Soyuz 19 launch, a Saturn 1B rocket lifts Apollo into orbit. After a series of maneuvers, the Apollo and Soyuz dock and the two crews celebrate their historic meeting. Soyuz 19 landed on July 21; Apollo remained in orbit until July 24.

Soyuz 19 was launched at 8:20 a.m. EDT on July 15, 1975, followed by Apollo seven-and-a-half hours later. After a series of orbital changes to bring the two spacecraft closer together, Apollo and Soyuz docked on July 17.

Although most of the planet candidates cataloged by NASA's Kepler space telescope have diameters in the range of 2 to 4 times that of the Earth, little Kepler-138 b is only about the size and mass of Mars.

Full Story: Alien Planet Is Smaller Than Earth and Surprisingly Light

Kepler-138 b is one of three planets orbiting the red dwarf star Kepler-138, which is located 200 light-years from Earth in the constellation Lyra. The gravity of Kepler-138's three planets pulls them into slightly different positions than they would occupy otherwise. This enables scientists to calculate the mass of the planets.

The innermost planet is Kepler-138 b, which has a density about the same as the rocky planet Mars. The middle planet, Kepler-138 c, has a density about the same as Earth, and the outermost planet (Kepler-138 d) is less than half as dense, indicating a large proportion of light materials such as water and hydrogen. [The Smallest Known Alien Planets in Pictures]

• 7 Ways to Discover Alien Planets
• The Search For Another Earth | Video
• Gallery: A World of Kepler Planets

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Researchers studying data from the LADEE (Lunar Atmosphere and Dust Environment Explorer) spacecraft have discovered that the airless moon is surrounded by a thin, asymmetrical dust cloud caused by the high-speed impacts of dust particles from comets.

Full Story: The Moon's Puzzling Dust Cloud Finally Yields an Answer

The lunar cloud's density increases during annual meteor showers. Dust density peaks in the direction of the moon's orbital motion.

Apollo astronauts reported a "horizon glow" around the moon, but the new dust discovery is not dense enough and is too close to the surface to explain those observations.

• The Moon: 10 Surprising Lunar Facts
• Moon Facts: Fun Information About the Earth's Moon
• Moon Master: An Easy Quiz for Lunatics

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