Deimos
The Forgotten Twin
Mars's tiny outer moon that rises in the EAST and crosses the sky slowly over 2.7 days
🌙 Mind-Blowing Fact
While Phobos zips overhead 3 times per day rising in the west, Deimos creeps across the sky taking 2.7 Martian days to cross from east to west - like a very slow Moon!
What is Time on Deimos?
What is Time on Deimos?
Deimos is Mars's outer moon and Phobos's smaller twin. While Phobos races across the Martian sky rising in the west three times per day, Deimos is the opposite - slow, normal, almost boring. But from a TIME perspective, Deimos shows what "normal" lunar time looks like.
The 30-Hour Day
- One Deimos orbit = 30.3 hours (1.26 Earth days)
- Mars's rotation = 24.6 hours
- Result: Mars rotates just 1.23 times per Deimos orbit
- From Mars: Deimos takes 2.7 Martian days to cross the sky
The Anti-Phobos
Deimos is everything Phobos is NOT:
| Feature | Phobos | Deimos |
|---------|--------|--------|
| Orbit time | 7.7 hours | 30.3 hours |
| Rises in | WEST (wrong!) | EAST (normal!) |
| Crosses sky in | 4 hours | 2.7 days |
| Times visible per day | 3 | 0.4 (once every 2.7 days) |
| Fate | Doomed (spiraling in) | Safe (stable orbit) |
| Apparent size | Half the size of our Moon | Looks like a bright star |
The Slowest Thing in the Martian Sky
From Mars's surface, Deimos appears to barely move:
- Rises in the east (like Earth's Moon - normal!)
- Takes 2.7 Martian days to cross from horizon to horizon
- Appears as a bright star, not a disk
- So slow you can't see it move with naked eye
- Hangs in sky for 60 hours before setting
The Tiny Potato
Deimos is incredibly small:
- Dimensions: 15 × 12 × 11 km (smaller than most cities!)
- Mass: Only 2×10¹⁵ kg
- Gravity: 0.003 m/s² (0.0003 g)
- Escape velocity: 5.6 m/s (12 mph) - you could throw a baseball into space!
The Distant View
At 23,460 km from Mars:
- Mars appears 2x larger than our Moon from Earth (still small!)
- You can see Phobos racing past Mars below you
- Phobos orbits 3.4 times per Deimos day
- Watch your twin zip around while you drift slowly
The Safe Orbit
Unlike Phobos (doomed to crash in 50 million years):
- Deimos is slowly drifting away from Mars (like our Moon from Earth)
- Stable orbit: Will exist for billions of years
- Safe: No tidal forces tearing it apart
- The survivor: While Phobos crashes, Deimos endures
A Day in the Life
The Slow Moon
You've just arrived at Deimos Station after six months on Phobos. The contrast is immediate and surreal.
The Opposite Experience
"Welcome to Deimos," says Station Chief Alvarez. "The boring moon."
You laugh, but she's serious. After Phobos - where Mars loomed enormous and rotated visibly, where sunrises happened every 2.6 hours, where everything was frantic and fast - Deimos is...
Slow. Quiet. Normal.
The Distant View
You look at Mars through the observation dome. It's... small. About twice the size of the Moon from Earth. Nothing like the massive, overwhelming globe you saw from Phobos.
"We're 23,000 km out," Alvarez explains. "Seven times farther than Phobos. Mars looks like a planet, not a ceiling."
You can see the whole thing - north pole, south pole, Olympus Mons, the canyon systems. It's beautiful but distant.
The 30-Hour Day
"One Deimos day is 30 hours," Alvarez says. "Mars rotates just 1.2 times during that period. Much slower pace than Phobos."
Hour 0: Deimos sunrise
Hour 15: Halfway through day
Hour 30: Sunset
Hour 30-60: Night (another 30 hours)
It's almost Earth-like. Almost reasonable.
Watching Phobos Race By
"Look down," Alvarez points.
Below you, between Deimos and Mars, you can see Phobos. It's orbiting much closer to Mars, moving fast.
"Phobos completes 3.4 orbits during our one orbit," Alvarez explains. "Every 9 hours, it zips past below us. Watch."
You watch. Phobos creeps into view... moves steadily past Mars... disappears around the other side. Nine hours later, it reappears.
"It's like watching the clock hand," says Alvarez. "Phobos is our hour hand. Every time it comes around, 9 hours have passed."
The Forgotten Moon
"From Mars," Alvarez continues, "Deimos looks like a bright star. It takes 2.7 Martian days to cross from east to west. Most Martian colonists barely notice us."
"Phobos races across the sky three times per day - everyone sees Phobos. But Deimos? We're just a slow, dim point of light that takes days to move."
She sounds almost... bitter?
"We're the forgotten moon," she admits. "Phobos gets all the attention. But we'll be here long after Phobos crashes."
The Stable Orbit
You ask about Deimos's fate.
"Unlike Phobos, we're slowly drifting AWAY from Mars," Alvarez says proudly. "Phobos is doomed - it'll crash in 50 million years. But Deimos? We're stable. We'll orbit Mars for billions of years."
"We're the survivor," she adds. "The tortoise, not the hare."
The Tiny World
Outside, you realize how SMALL Deimos is. You can see the curve of the horizon just a few hundred meters away.
"The entire moon is only 12 kilometers across," Alvarez explains. "You could walk around it in a day. Well, bounce around it - gravity here is 0.0003 g. You weigh less than a smartphone on Earth."
You jump. You drift upward for several seconds before slowly settling back down.
"Don't jump too hard," Alvarez warns. "Escape velocity is only 12 mph. If you throw a baseball hard enough, it'll orbit Deimos or escape completely."
Life in Slow Motion
The station routine is relaxed:
Hours 1-12: Work shift during Deimos morning
Hours 12-18: Rest/recreation
Hours 18-30: Work shift during Deimos afternoon/evening
Hours 30-60: Night shift (minimal crew)
"After Phobos's frantic 7.7-hour days," Alvarez says, "Deimos feels almost lazy. We have time. We can think. We can plan long-term."
The Phobos Refugees
You meet several crew members who transferred from Phobos.
"I couldn't handle the pace," admits Dr. Chen. "On Phobos, everything was intense. Mars filled the sky. Sunrises every few hours. You felt like you were falling toward Mars constantly."
"Here," she continues, looking at the distant red disk of Mars, "it's calm. We're far enough away that Mars feels like a planet, not a doom. The 30-hour day is almost Earth-normal. I can sleep without Mars looming overhead."
The Long Crossing
That "night" (hour 35 of the current Deimos day), you watch Mars through the telescope.
From the Martian surface, Deimos is barely visible - a bright star that takes 2.7 days to cross the sky. So slow that Martian colonists don't even notice it moving.
"Phobos races overhead three times per Martian day," Alvarez says. "Everyone sees Phobos. But Deimos? We're invisible. Forgotten."
"But," she adds with a smile, "we'll still be here when Phobos is dust."
The Survivor's Perspective
You ask Alvarez why she prefers Deimos.
"Phobos is exciting," she admits. "But Deimos is real. We're far enough from Mars to have perspective. We're stable - not spiraling to our doom. We have time - not frantic hours."
She pauses, looking at Mars.
"Plus, from here, we can see the whole system. Mars. Phobos racing below us. The asteroid belt above. We're not falling toward Mars - we're orbiting it. Properly."
Time's Opposite Twins
As you settle into life on Deimos, you realize: Phobos and Deimos are a study in contrasts.
Phobos:
- Too close, too fast
- Doomed, falling
- Frantic, overwhelming
- Unique, spectacular
Deimos:
- Far, slow
- Stable, enduring
- Calm, contemplative
- Normal, forgettable
Both are Mars's moons. Both are potato-shaped captured asteroids. Both are tidally locked. But their TIME experiences are opposite.
"We're the forgotten moon," Alvarez says again. "But we'll be here billions of years from now, long after Phobos is gone."
"Time," she adds, "favors the patient."
Thought Experiments
Could you see Phobos and Deimos at the same time from Mars?
YES! Since they orbit at different distances, you could see Phobos zipping across the sky while Deimos hangs nearly motionless. From Mars, Phobos would race overhead (4-hour crossing) while Deimos would barely move (2.7-day crossing). It would be like watching a fast clock hand (Phobos) and a very slow clock hand (Deimos) simultaneously. During one Deimos crossing, Phobos would cross the sky ~16 times!
If you played baseball on Deimos, what would happen?
With escape velocity of only 12 mph (5.6 m/s), a professional pitcher could throw a ball into ORBIT or into space! Gravity is 0.0003 g - you weigh about 200 grams (weight of a phone on Earth). If you threw a ball at 90 mph (40 m/s), it would: 1) Escape Deimos completely, 2) Possibly hit Phobos, 3) Or orbit Mars! "Home runs" would literally leave the moon. Baseball would be impossible - you'd lose every ball to space!
What would a "Deimos day" feel like to a Martian colonist?
From Mars, Deimos takes 2.7 Martian days (66 hours) to cross the sky from east to west. It would: 1) Rise in the east (normal), 2) Move so slowly you can't see it move, 3) Be visible for ~60 hours continuously (multiple day-night cycles), 4) Look like a bright star, not a disk, 5) Finally set in the west 2.7 days later. Most Martian colonists would barely notice it. It would be like Earth having a very slow, very dim "second moon" that most people ignore!
The Science of Time on Deimos
The Science of Time on Deimos
Orbital Mechanics vs Phobos
The key difference between Mars's two moons:
Phobos (inner moon):
- Orbit: 7.7 hours
- Sub-synchronous: Orbits FASTER than Mars rotates
- Result: Rises in WEST
Deimos (outer moon):
- Orbit: 30.3 hours
- Super-synchronous: Orbits SLOWER than Mars rotates
- Result: Rises in EAST (normal)
The dividing line is Mars's synchronous orbit (17,032 km). Inside this: rise in west. Outside: rise in east.
Why Deimos Is Stable
Unlike Phobos (doomed to crash):
- Beyond synchronous orbit: Tidal forces push it AWAY from Mars
- Recession rate: ~1.8 cm/year (similar to Earth's Moon)
- Stable long-term: Will orbit for billions of years
- No stress cracks: Unlike Phobos, which shows fractures
The Captured Asteroid Theory
Both Phobos and Deimos likely are:
- Captured asteroids: Not formed around Mars
- D-type composition: Dark, carbon-rich (similar to outer asteroid belt)
- Different orbits: Captured at different times/locations
- Circularized orbits: Tidal forces over billions of years
Evidence: Spectroscopy matches D-type asteroids, not Martian material.
Deimos's Unusual Features
Despite being tiny, Deimos has:
- Smooth regions: Less cratered than expected
- Regolith depth: 50-100 m of loose dust
- Albedo: 0.068 (very dark - darker than asphalt!)
- No large craters: Either resurfaced or young surface
Apparent Size from Mars
From Mars's surface:
- Phobos: ~0.14° (about 1/4 size of our Moon from Earth)
- Deimos: ~0.02° (looks like Venus from Earth - a bright "star")
- Comparison: Deimos is 7x smaller-looking than Phobos
This is why Deimos is "forgotten" - it's barely visible!
The 2.7-Day Crossing
From Mars, Deimos's motion:
- Rises in east: Like normal moon
- Moves: 0.016° per hour (imperceptibly slow)
- Visible for: ~60 hours per crossing
- Sets in west: 2.7 Martian days after rising
During this time, Mars experiences 2.7 day-night cycles while Deimos creeps across the sky.
Tidal Forces and Orbital Evolution
Deimos experiences:
- Tidal acceleration: Mars's rotation transfers angular momentum
- Outward spiral: Moves ~1.8 cm/year away from Mars
- Future fate: Eventually could escape Mars's orbit entirely (billions of years)
- Unlike Phobos: Which spirals IN and will crash
Gravity and Escape Velocity
On Deimos:
- Surface gravity: 0.003 m/s² (0.0003 g)
- Your weight: If you're 70 kg on Earth, you're 21 grams on Deimos!
- Escape velocity: 5.6 m/s (12.5 mph)
- Walking speed: 5 km/h (3 mph) is safe, but 20 km/h (12.5 mph) and you fly away!
Deimos as a Future Base
Advantages for human base:
- Stable orbit: Won't crash like Phobos
- Low gravity: Easy to land/launch
- Low delta-v: Only 5.6 m/s to reach orbit
- View of Mars: Complete hemisphere visible
- Refueling depot: Process ice → fuel for Mars missions
Why So Small and Irregular?
Deimos is small because:
- Never grew large: Either formed small or is a captured fragment
- No self-gravity reshaping: Too small to become spherical
- Irregular shape: 15 × 12 × 11 km (potato-shaped)
- Differentiation: Not enough mass/heat to separate into layers
Time Dilation Effects
Minimal but calculable:
- Orbital velocity: 1.35 km/s (much slower than Phobos's 2.14 km/s)
- Mars's gravity well: Weak compared to Jupiter/Saturn
- Time dilation: ~0.0000001% slower than deep space
- Practical effect: Completely negligible
Time on Deimos represents "normal" lunar time - slow, stable, patient. The opposite of Phobos's frantic, doomed rush. In the race between Mars's twin moons, Deimos is the tortoise that wins by simply surviving.