Abby Sunderland, 16, is feared lost at sea today in her attempt to become the youngest sailor ever to circumnavigate the globe.
A support crew lost contact with Abby, who was in heavy seas in the Southern Indian Ocean, early this morning.
Sunderland's brother, Zac Sunderland, said his sister's boat was clearly in trouble.
"The boat is most likely not completely submerged because there's another alarm that sends off a signal if it goes 15 feet underwater," Zac Sunderland said in an interview with Ron Kilgore of KNX radio in Los Angeles. "So yeah, she's pretty banged up out there right now, and [we're] just trying to get the rescue teams out."
Abby's mother, MaryAnne Sunderland, told ABC News that Abby manually activated two emergency beacons sometime before 6 a.m. Pacific Time today.
A plane from Qantas air, based in Australia, planned to fly over Sunderland's location at first light Friday -- or late Thursday evening Eastern Time.
Watch the full story Friday on "20/20" at 10 p.m. ET
Abby was in 20-25 foot waves at the time of last contact, with 35-knot winds, said MaryAnne Sunderland, who is due to give birth at the end of the month. She was shaken but focused on trying to get a rescue effort together.
The closest land to Abby's boat was Reunion Island, which is east of Madagascar. The nearest ship was 400 miles away. Rescuers were trying to contact the ship.
Zac Sunderland told KNX that the support crew was "still trying to figure out the rescue situation."
"There's two boats headed out to her position. One of them's an estimated 40 hours, the other is 48. So right now we're just trying to figure out if there's any way faster. She's in the middle of nowhere pretty much, in the Southern Indian Ocean, there's nothing closer.
"We're just hoping everything's all right out there. There's nothing we can really know for sure out there right now."
A note posted to a blog tracking Abby's progress by her support team Thursday detailed her survival gear.
"Abby has all of the equipment on board to survive a crisis situation like this," the posting said. "She has a dry suit, survival suit, life raft, and ditch bag with emergency supplies. If she can keep warm and hang on, help will be there as soon as possible."
Click HERE to watch Abby give a tour of her boat.
Jeff Casher, an engineer on Sunderland's support team, told ABC News that he last spoke with the 16-year-old sailor before 6 a.m. PDT, after she had been knocked down twice during the night because of strong winds -- meaning that her sail had touched the water.
One of those knock-downs, Casher said, ripped the radar off the boat. She had been speaking with Casher on a satellite telephone earlier because of engine problems and was in the process of fixing those problems when she told Casher she'd call right back.
She has not been heard from since, except for the distress signals.
Abby is approximately 500 miles north of the Antarctic Islands on her bid to become the youngest to circumnavigate the globe in a sailboat, solo.
Abby herself last posted a blog entry from her boat, "Wild Eyes," on Wednesday night.
"It was a nice day today with some lighter winds which gave me a chance to patch everything up," she wrote. "Wild Eyes was great through everything but after a day with over 50 knots at times, I had quite a bit of work to do."
The weather was getting rough again, she noted.
"The wind is beginning to pick up. It is back up to 20 knots and I am expecting that by midnight tonight I could have 35-50 knots with gusts to 60 so I am off to sleep before it really picks up," she wrote.
Abby's goal at the outset of her trip, to become the youngest sailor to pull off a solo nonstop circumnavigation, ended in April, when she was forced to stop in South Africa because her autopilot malfunctioned.
She still was bidding to become the youngest sailor to perform a solo circumnavigation.
How Young Is Too Young?
In a June 2 report titled "How Young Is Too Young?," "20/20" described Sunderland's circumnavigation attempt.
"I am definitely nervous," Abby told "20/20." "People say you shouldn't be nervous if you are really ready to do this. But I understand [the] ocean, and I understand how dangerous what I am doing actually is, and I understand how careful I need to be out there."
Late last year, Abby began talking publicly about her plan to circumnavigate the globe by herself in a 40-foot boat. It would be, she said, the fulfillment of a dream she sprang on her parents when she was 13.
Laurence and MaryAnne Sunderland, Abby's parents, helped their daughter to prepare for the attempt.
"Could there be a tragedy?" MaryAnne Sunderland said. "Yeah, there could be. But there could be a tragedy on the way home tonight, you know, or driving with her friends in a car at 16. You minimize the risks."
"You arm them with the coping skills," Laurence Sunderland said. "And then you pray."
Not everyone who heard of Abby's plan has been as supportive.
"Child abuse. Child endangerment," said T.J. Simers, a sports columnist for the Los Angeles Times and father of two grown daughters. "I just don't understand the idea of risking life. This kid's going to be out there all by herself. Death is a possibility. Bad weather. Are you kidding me? Who's responsible for this? She's a kid."
Laurence Sunderland answers his critics by saying they don't know his daughter and her abilities.
"When any of my kids come to me with an ambition, I don't laugh at it," said Abby's father, Laurence Sunderland. "I like to listen to my kids. There was a part of me that was even excited, because she's jumped into an arena where I know a little bit about it."
Sunderland schooled Abby in seamanship, testing her, he said, with tougher and tougher solo scenarios through her teens.
"When things went wrong, I'd bring [it to] her attention," Sunderland said. "It was a particularly rough day and her boat was rocking from gunnel to gunnel. ... I knew she was freezing cold, tired and hungry, and we'd been at it for, you know, 20 hours at that stage. I said, 'So Abby, are you ready to sail around the world now?' To which she replied, 'Where is my boat?'"
MaryAnne Sunderland, Abby's mom, joined the story.
"She ended up getting a sponsor," she said. "All she needed was approval, you know, from us."
Laurence Sunderland said the easiest thing for them to have said as parents would have been "No." But then he thought of his daughter and all her talents.
Abby is not the first talented teenager to dream of sailing around the world solo. In 1999, a 17-year-old named Jesse Martin accomplished just that. Yet while at sea, the teen struggled with loneliness and life-threatening storms that left him emotionally devastated.
But Martin witnessed some unforgettable sights along the way -- and made it safely. His adventures were chronicled in the documentary "Lionheart."
"If I never came back it would not have been a tragedy ... a tragedy would be someone who dies at 80 and spent 80 years not being satisfied," Martin said in the documentary. "I was out there doing what I wanted."
Abby Sunderland: Setting Sail
Abby set sail Jan. 23 from Marina del Rey, Calif. Her parents watched as their teenage daughter faded into the horizon.
Later, via a Skype interview from her boat, Abby would recall her emotions that day.
"It was completely overwhelming," she said. "I mean, there was people everywhere, and boats, and all this noise and commotion and stuff. And then all of a sudden it just leaves, and you're the only person out there."
But Abby wasn't completely alone. While at sea, she talked with her mother twice a day, blogged her latest news and kept up her page on Facebook. One of her friends was Jessica Watson, a 16-year-old Aussie also in the process of sailing solo around the world.
"I just love, you know, going out and doing something and having to rely on yourself -- you know, it's up to me," Jessica said in an ESPN documentary.
As Abby got under way, Jessica was about to sail into history as the youngest ever to circle the globe, alone, nonstop. But Abby was five months younger, and so hoped to take that world record for herself.
"She set out to achieve -- a goal as being the youngest person to solo circumnavigate the world nonstop," said Laurence Sunderland.
Abby's route, crafted to avoid any threat of pirates, took her past Chile, where she was unaffected by the 8.8 magnitude earthquake; around Cape Horn and into the South Atlantic. When "20/20" talked with her, she'd been at sea for 101 days.
Abby Sunderland: 'I'm Happy to Do It Alone'
We asked what it was like to go for so long and not see any sign of civilization.
"I think it actually might be more fun if there was somebody else on board," Abby said. "But -- I'm happy to do it alone too."
We also asked her what was the toughest hit her boat had taken, and whether she got scared.
"I got hit by a rogue wave," she said. "I did get knocked down. ... I'd be happy if that didn't happen again."
Abby admitted to having been scared a few times.
She'd also dealt with disappointment. Because when she spoke with "20/20" she knew that her boat needed to stop in Cape Town for repairs -- ending her dream of a solo circumnavigation nonstop, but not ending her voyage.
When asked why she didn't go back home when she encountered problems, her father answered: "I think Abigail set out to sail around the world and she will accomplish that."
Guinness World Records says it will no longer recognize "youngest ever" sailing records, because they are so risky. But still-younger sailors are hoping to get their yo-ho-ho on.
A Dutch family court barred 13-year-old Laura Dekker from raising her mainsail alone. But she may finally get her chance later this year.
Her quest completed after 210 days at sea, Jessica Watson pulled into Sydney Harbor early this spring to a huge welcome -- and a classic sailor's response.
"Jessica, you are our new Australian hero," Australian Prime Minister Kevin Rudd said.
"I am actually going to disagree with what our prime minister just said," Jessica told the crowd of well-wishers. "I don't consider myself a hero. I am an ordinary girl who believed in a dream. You don't have to be someone special to achieve something amazing. You just have to have a dream, believe in it and work hard."